Title of Invention	A BOUNDARY LAYER MICRO PULSE LIDAR SYSTEM
Abstract	ABSTRACT "A boundary layer micro pulse lidar system" The invention relates to a boundary layer micro pulse lidar system comprising a laser head to generate a laser source. A beam expander is provided to expand the laser beam output to achieve eye safety, the expanded laser beam being projected into the atmosphere through mirrors positioned at 45° angles A cassegrain telescope is provided only to receive the backscattered light A Photo Multiplier Tube (PMT) is provided as a detector system for single photon counting. A data acquisition unit and analyzer for acquiring and processing the signal from the detector to profile aerosols/particulates and atmosphere clouds is provided. The said laser head comprises a laser diode, a Nd doped Yttrium-Aluminium-Garnet crystal (Nd:YAG) which generates the laser, an acoustic opto-modulator (AOM), potassium tri-phosphate (KTP), which is a crystal that generates wavelength at the second harmonic of YAG resonating wavelength and other output optics. (Fig 4)

Title of Invention

A BOUNDARY LAYER MICRO PULSE LIDAR SYSTEM

Abstract

ABSTRACT "A boundary layer micro pulse lidar system" The invention relates to a boundary layer micro pulse lidar system comprising a laser head to generate a laser source. A beam expander is provided to expand the laser beam output to achieve eye safety, the expanded laser beam being projected into the atmosphere through mirrors positioned at 45° angles A cassegrain telescope is provided only to receive the backscattered light A Photo Multiplier Tube (PMT) is provided as a detector system for single photon counting. A data acquisition unit and analyzer for acquiring and processing the signal from the detector to profile aerosols/particulates and atmosphere clouds is provided. The said laser head comprises a laser diode, a Nd doped Yttrium-Aluminium-Garnet crystal (Nd:YAG) which generates the laser, an acoustic opto-modulator (AOM), potassium tri-phosphate (KTP), which is a crystal that generates wavelength at the second harmonic of YAG resonating wavelength and other output optics. (Fig 4)

Full Text	FIELD OF INVENTION The present invention relates to a boundary layer lidar system incorporating the micro pulsing technique. BACKGROUND OF THE INVENTION Lidar is an acronym for Light Detection and Ranging. It is the optical counter part of the familiar radar technique for remote sensing, which uses laser light and it can be thought of as Laser Radar. A lidar system transmits a pulse of light in to the atmosphere and analyzes the backscatter signal for intensity as a function of time. Because the laser pulse travels at the speed of light, it is possible to convert time in to range and consider the lidar signal to be backscattered intensity as a function of range. Optical probing of the atmosphere through elastic scattering actually predates the invention of the laser (Schawlow and Townes, 1958). It was, however, the development of Q-switching by McCIung and Heliwarth (1962) that made possible the generation of very short, single pulses of laser energy and thereby range-resolved measurements. The energy compression afforded by Q-switching also gave rise to laser power densities of such magnitude that remote measurements that based on inelastic scattering from specific molecular could be considered for certain applications. Fiocco and Smullin (1963) working at MIT were the first to use laser for atmospheric studies. In 1962, using a 0.5 Joule Ruby laser, they obtained Rayleigh scattered signals from the atmosphere up to 50 km altitude and also detected dust layers in the atmosphere. Ligda in 1963 made the first lidar measurements of cloud heights at tropospheric heights. Recently, NASA (LITE) and ESA (GLAS) have undertaken the development of a series of space-borne Lidar missions to measure atmospheric dynamics, atmospheric aerosol and cloud properties and water vapor. Over the last forty years, lidar instruments have been developed to study the structure of the atmosphere by means of the elastic and inelastic scattering of light from 2 constituents of the atmosphere such as air molecules and aerosols. The backscattered energy is approximately proportional to the aerosol and molecular content of the observed air mass. Based on this information, a backscattering lidar can provide information on the PBL structure, mixed-layer depth, entrainment zones and convective cells structure, aerosol distribution, clear air layering, condensation levels, cloud-top altitudes, cloud depolarization properties, middle atmospheric temperature, atmospheric transport, low-level jets, diffusion processes, and other inferences of air motion. Elastic backscatter lidars are, by far, the most common and simple remote sensing instruments in the world today and this will continue to be true for the foreseeable future. Elastic backscatter lidar measurements exhibit all the desirable characteristics of remote sensing techniques. The measured values represent spatial averages over volumes that can be varied by proper design of the lidar system. Such spatial averaged values arc usually more representative of the overall state of the turbulent atmosphere than the single point in situ measurement. By using aerosols as the tracers of atmospheric dynamics, lidar can identify several dynamical parameters of the ABL such as boundary layer top and entrainment zone depth in real-time with high temporal and spatial resolutions. However, the traditional lidar instruments used so far during the last three decades have typically been high-energy pulse, low repetition rate systems. Recent advances in the solid-state lasers, detectors and data acquisition systems have enabled the development of a new generation lidar technology that uses micro pulsing technique. These lidars employ high pulse repetition rate, low pulse power transmitter approach. Concept of Micro pulse Lidar A micro lidar employs a diode-pumped Nd-YAG laser system, a co-axial transceiver for transmitting the laser pulses and detecting the collected photons, a dedicated acquisition system, and a computer control and interface system. Pulses of light energy (at green wavelength) are transmitted from the telescope into the atmosphere. As the pulse propagates, part of it is scattered by molecules, water droplets, ice crystals, dust and haze aerosol in the atmosphere. A small portion of the light that scattered back is 3 collected by the telescope and then detected. The distance to the particle layers is inferred based on the time delay between each outgoing transmitted pulse and the backscattered signal. The special approach of the micro pulsing technique is to use expanded micro Joule pulse energies to obtain eye safety and reliability. The detected signal is stored in bins according to how long it has been since the pulse was transmitted, which is directly related to how far away the backscatter occurred. The collection of bins for each pulse is called a profile. A number of articles have reported on the applications of micro pulse lidar to environment studies (Chen et al., 2001; Parikh and Parikh, 2002; Legrosos et al., 2004; Bhavani Kumar, 2006). The temporal evolution of the boundary layer height and boundary layer aerosol can also be studied (Parikh and Parikh, 2002; Legrosos et al., 2004; Bhavani Kumar, 2006;Bhavani Kumar et al., 2007a and b). The micro pulse approach has several advantages over to conventional lidar systems. These advantages made them more useful for field use, than limited to research field. The advantages are given below The laser pulse repetition rate is much higher and the pulse energies are much lower. So it is a low heat generation system, hence there is no thermal runway problem. The laser employed is a diode pumped rather than flashlamp pumped. Hence the reliability and stability of the system is high compared to the big pulse lasers. The system employs photon counting detection. Usually the photon counting technique is far superior compared to the conventional analog method of detection, because it is more sensitive to the long range echoes. Though photon counting has a drawback of limited dynamic range in signal acquisition compared to analog method, the use of high repetition rate of laser operation overcomes this limitation. APPLICATIONS OF BOUNDARY LAYER LIDAR 4 The health of the planet Earth and its inhabitants is highly dependent upon the health of the atmosphere. The boundary layer lidar (BLL) has a significant role in the atmospheric research, because it facilitates the autonomous monitoring of atmospheric clouds and aerosol scattering. The following are the potential application areas where BLL systems can be made use. Atmospheric and climate research: Continuous observation of aerosol distribution and atmospheric cloud profiling, thereby building the aerosol climatology for future climate prediction. Environment monitoring: The evolution of the concentration of particulate pollutants is important requirement in air pollution control for public health and environmental safety. Meteorological application: Correct treatment of the physical parameter such as the atmospheric boundary layer depth is necessary for weather forecasting and numerical simulations of climate change. Visible range or slant height: It is of practical importance in aircraft landing and marine navigation. A simple slope method assumes that the asymptotic decrease in the range compensated backscattering intensity is proportional to the round trip laser beam transmittance, in clear atmosphere. The boundary layer (BL) height from lidar profiles is defined differently by numerous researchers. Sasano et al. [1985] identified the BL height as the height where the signal backscatter begins to decrease from a relatively higher value to lower region. Endlich et al [1979] described BL height as the height at which a maximum negative gradient of laser backscatter in vertical direction occurs. Flamant et al [1997] also mentioned the BL height as a zone of minimum in the vertical gradient of the backscatter as defined by Endlich etal. [1979]. 5 An automated lidar system (Micro Pulse Lidar (MPL) system disclosed in US 5,241,315 (Spinhirne) was originally developed at Goddard Space Flight Centre (GSFC) - National Aeronautical and Space Administration (NASA), USA and was granted a US Patent No. 5,241,315. The MPL was constructed for the purpose of unattended measurements of cloud and aerosol height structure. This system is an eye-safe, solid-state lidar whose transmitter is a diode pumped u.1 pulse energy, high repetition rate Nd:YLF laser. The receiver uses a photon counting solid state Geiger mode avalanche photodiode detector. Data acquisition uses a single card multi-channel scalar. Fig 1 shows the schematic diagram of an MPL system of Spinhirne. The Spinhirne developed MPL system design is such that the same telescope is employed for both transmitting and receiving the laser beam, and hence a small portion of the transmitted laser energy is always reflected inside the telescope and may often damages the detector. This configuration provides narrow FOV and eye safety, but disadvantage is always some amount of laser power enters into the detector and cause damage to the receiver and also increases receiver noise. The Spinhirne -MPL system employs Avalanche Photodiode (APD) as detector operating in photon counting mode (Geiger mode of operation) for the reason being its high quantum efficiency. However, APDs are not suitable for visible wavelengths because they are noisier at these wavelengths. Moreover, the APD mode of operation (Geiger mode -single photon detection) requires that the diode should be operated above its breakdown voltage (Kovalev and Eichinger, 2004). Since prolonged operation of APD in the breakdown region can potentially damage the diode, hence it requires avalanche quenching circuitry. This is an issue in the Spinhirne-MPL system. As there are many optical components such as beam collimator, half-wave plate, beam steering mirrors, depolarizer are used in the transmission and as well in receiving path of the Spinhirne-MPL system, makes the overall optical efficiency of the system is less. Moreover, use of many optical components make the system more complex and cumbersome for future augmentations such as addition of another receive channel for depolarization measurements. There is also no possibility of maintenance or augmentation at the site. The cost of the Spinhirne system is about 3 to 4 times the cost of the system of the 6 present invention, since it employs many optical components and also several custom built parts. SUMMARY OF THE PRESENT INVENTION The present invention provides a novel micro pulse system known as Boundary Layer Lidar (BLL) for atmospheric profiling. The system comprises a laser source and transmitting optics, receiving optics, detector unit, and data acquisition unit. Laser source and Transmitting Optics The lidar transmitter system employs a Russian pulsed laser M/s Laser Export made, model LCS-DTL-314QT system. It is a microchip (all-in-one laser cavity) Nd:YAG laser employing second harmonic output. It is laser diode pumped and acoustic switched. The output pulse energy is adjustable in the range between 2 and 20 uJ depending on the repetition rate. Usually the output pulse energy is set at 10uJ at 2500 Hz repetition rate in the boundary layer lidar according to this invention. The laser beam diameter is 0.4 mm and its divergence is less than 1.5mrad. The laser beam was expanded to 3 mm in diameter and collimated to have the beam divergence of about 200 urad. The eye safety is achieved with this beam divergence for the heights above 150m from ground. The maximum permission emission (MPE) at 532 nm is 0.5 uJ/cm2 The light output was linearly polarized with the degree of polarization being greater than 99%. The laser beam is sent into the atmosphere using two mirrors kept at 45 °angles. Receiving Optics A monoaxial configuration was employed in the lidar system according to this invention. The laser backscattered light was received by a classical -cassegrain 7 telescope, whose diameter and F-value were 15 cm and 9, respectively. The geometrical form factor for a coaxial lidar having no apertures (other than the objective lens or mirror of the telescope) or obstructions is unity, provided the divergence angle of the laser beam is less than the opening angle of the telescope (Measures, 1984). Hence an iris diameter (pin-hole) of 0.5 mm was used to obtain a receive FOV of about 400 urad. A narrowband interference (IF) filter was positioned in front of the photomultiplier tube (PMT). An IF filter, whose center-wavelength and bandwidth were 532 nm and 0.5 nm, respectively, was used to reduce background light. The filter used in the system is not temperature controlled, hence as specified by manufacturer the unit is to be maintained at a temperature of about 22 °C. For this reason the lidar system is kept in a temperature controlled cubical. Detector unit The detector system employed in the lidar was a high gain PMT (Hamamatsu R3234-01). It is a head-on type PMT with 10 mm aperture. This was selected for use in the system based on two important specifications such as low dark current (about 1 nA) and good pulse generation characteristics (after pulse effect is 0.6 %). More over, its rise time is very good, about 2 nsec. It has a reasonable quantum efficiency of about 10. The PMT out put pulses are amplified and discriminated using a Phillips make discriminator (Model 6908), which is a 300 MHz BW system. The discriminator outputs are the current source type with two pairs of negative bridged and two complements. The current source type output (NIM standard) is used for data acquisition system for recording the data. Data Acquisition Unit The data acquisition system used in the lidar is a PC based multichannel analyzer (EG&G Ortec model MCS-pci). A Multi-Channel Scalar (MCS) records the counting rate of events as a function of time. When a scan is started, the MCS begins counting input events in the first channel of its digital memory. At the end of the pre-selected 8 dwell time, the MCS advances to the next channel of memory to count the events. This dwell and advance process is repeated until the MCS has scanned through all the channels in its memory. A display of the contents of the memory shows the counting rate of the input events versus time. In repetitive measurements, where the start of the scan can be synchronized with the start of the events, multiple scans can be summed to diminish the statistical scatter in the recorded pattern. The MCS-pci can profile counting rates versus time, or it can function as a multiple-stop time digitizer for measuring flight times of multiple photons or particles. Typical applications are in time-resolved single-photon counting. It can count rates up to 150 MHz or 65,536 time bins (channels). Dwell times selectable from 100 ns to 1300 s/channel. It can do single scan once, or average repeated scans. There is no dead time between channels and no double counting. There is no end-of-scan dead time. In the lidar the instrumental bin width was normally set at 200 ns, which corresponding to an height resolution of 30 m. Usually 300000 shot integrated photon count profile constitutes a raw data profile that corresponds to a time resolution of 120 sec. The overall specifications of the Boundary Layer Lidar System according to the present invention are given in table 1. Accordingly, a portable coaxial configured prototype laser radar system using a visible pulsed laser has been realized for ranging the atmospheric boundary Layer (ABL), for profiling atmospheric aerosols and real-time detection of clouds with very high range resolution at a lower cost. The lidar system uses a. A coaxial (monoaxial) configuration, which made the advantage of full overlap height near to the ground. b. A mechanical arrangement incorporating the lidar transmitter and receiver such that, it is possible to orient the lidar system at any angles of azimuth and elevation. c. A diode pumped, high repetition rate Nd:YAG laser operating at micro joule energies as laser transmitter. The laser beam is initially expanded and then collimated to send in to the atmosphere. d. A high gain, low dark count PMT as photon counting device for detecting the laser-backscattered photon returns. Good dynamic range (in terms of range) is achieved by employing high repetition rate low pulse energy concept with PMT based photon counting technique. e. A single card PC based multi-channel scaler, which provides high range resolution height profiles. At present 30m (200 nsec is the dwell time) is range resolution is employed in the prototype lidar system. It is also possible to operate the system with 15 m range resolution (100 nsec is the dwell time). f. A narrow receiver field-of-view achieved with fine focal aperture, which made the possibility of operating the lidar system even during daytime on continuous basis in an unattended way. Statement of Invention A boundary layer micro pulse lidar system according to the present invention comprises a laser head to generate a laser source, a beam expander to expand the laser beam output to achieve eye safety, the expanded laser beam being projected into the atmosphere through mirrors positioned at 45" angles, a telescope to receive the backscattered light, a Photo Multiplier Tube (PMT) as a detector system for single photon counting, a data acquisition unit and analyzer for acquiring and processing the signal from the detector to profile aerosols/particulates and atmosphere clouds, wherein the said laser head comprises a laser diode, a Nd doped Yttrium-Aluminium-Garnet crystal (Nd:YAG) which generates the laser, an acoustic opto-modulator (AOM), potassium tri-phosphate (KTP), which is a crystal that generates wavelength at the second harmonic of YAG resonating wavelength and other output optics. BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 shows the schematic diagram of Micro Pulse Lidar (MPL) system of the prior art. Figure 2 shows the schematic diagram of the Micro Pulse Lidar (MPL) system of the present invention. Figure 3 laser output laser pulse energy variations with respect to repetition rate of laser pulse. Figure 4 shows the detailed schematic block diagram of the Boundary layer lidar system Figure 5 shows a Schematic diagram of the boundary layer Figure 6 shows the BLL observation of atmospheric backscatter profile recorded at 08:51 LT (during daytime) over Gadanki site. Figure 7 shows the Diurnal variation of range-squared signal shows the typical ABL evolution Figure 8 shows the temporal evolution of Entrainment zone depth Figure 9 shows the BLL data showing atmospheric oscillations at the top of the ABL. Figure 10 shows the diagram of the formation of gravity waves Figure 11 shows height profiles of lidar extinction, water vapor mixing ratio, 12 atmospheric temperature, wind speed and wind direction Figure 12 shows BLL observation of elevated multiple layers of aerosol on two separate dates Figure 13 shows Typical height profile showing (a) Basic photon count observed from BLL system (b) Corresponding range squared corrected signals indicating backscattered signals from (a) cloud and (b) aerosols. Figure 14 shows Derived height profiles of aerosol backscattering coefficient from BLL lidar backscatter returns. The height profiles provide total backscatter, molecular and aerosol backscatter coefficients. Figure 15 shows Typical height profile of aerosol observed from BLL system over Gadanki. The shaded lines show the variability in the observed aerosol profiles Figure 16a shows BLL observation of long range transported aerosol Figure 16b shows A 7 day HYSPLIT back trajectories showing the air mass advection from Far East Figure 17a shows Lidar derived mean height profiles of aerosol extinction observed over Gadanki during a certain period. Figure 17b shows A comparison of aerosol extinction and water vapor mixing ratio profiles showing the hygroscopic nature of boundary layer aerosol Figure 18 shows MODIS satellite observation of dust storm over Indus-Valley Figure 19 shows ARIES-NARL Lidar observation of dust storm above Manora peak, Nainital. a) Temporal variation of range squared signal b) Lidar derived backscatter ratio during the period 2200 - 2300 LT Figure 20 shows BLL system observation of low-level cloud during a light rain period. Figure 21 shows BLL system observation of low-level cloud during a light rain time over Gadanki. Figure 22 shows BLL observation of middle level cloud over Gadanki site Figure 23 shows BLL observation of high altitude clouds over Gadanki. Figure 24 shows BLL system observations of high altitude clouds during winter period over Gadanki Figure 25 shows Wave generated K-H bellows in cirrus layers observed by BLL over 13 Gadanki site. Figure 26 shows BLL observation of multiple clouds occurrence in the atmosphere Figure 27 shows Comparison of BLL cloud top and MODIS cloud top in terms of pressure DETAILED DESCRIPTION OF THE INVENTION Figure 1 shows the conventional MPL system developed by Spinhirne. This system comprises a laser head (100), which is a high repetition rate, low pulse energy laser source operating at green wavelength. A laser head controller (10!) is provided to controls the output green laser wavelength repetition rate and energy. A transmitting optics arrangement (102) is provide to subject the laser source output beam to pass through several optical elements like beam expansion optics, half wave plate, beam steering optics, focusing lens, depolarizer (103) and finally achieves eye safety and delivers through telescope into the atmosphere. A telescope (104) is a 200mm diameter cassegrain telescope. The filter (105) is an optical narrow band filter and the detector (106) is an avalanche photo diode (APD) detector. A digitizing/MSC unit (107) is provided for digitizing the APD signal for range gating. A computer is provided for processing data archived and for further processing. The above-mentioned prior art MPL system developed by Spinhirne has drawbacks, which were mentioned earlier. The present invention overcomes these drawbacks by providing a novel MPL system that will be described below with reference to Figures 2 to 4. The MPL system according to this invention comprises a laser head (200) situated on one side of a telescope (201). The laser head (200) comprises a laser diode (LD), a Nd doped Yttrium-Aluminium-Garnet crystal (Nd:YAG) which generates the laser, an acoustic opto-modulator (AOM), Potassium tri-phosphate (KTP), which is a crystal that generates wavelength, the second harmonic of YAG resonating wavelength and other output optics. The laser is guided into the atmosphere using two mirrors (M) kept at 45° angles. A beam Expander (202) is provided to expand the laser beam output to achieve 14 eye safety. The laser beam diameter is 0.4 mm and its divergence is less than 1.5mrad. The laser beam was expanded using the beam expander (202) to 3 mm in diameter and collimated to have the beam divergence of about 200 urad. The eye safety is achieved with this beam divergence for the heights above 150m from ground. A thenno electric cooling device (206) is provided adjacent to the laser diode (LD) to control the heat dissipated from the laser head (200). The laser backscattered due to contact with molecules, water droplets, ice crystals, dust and haze aerosol in the atmosphere is received by a telescope, preferably, a cassegrain telescope (203) as shown in Figure 2 and 4. The diameter of the telescope is 150mm and its F-value is 9. The geometrical form factor for a coaxial lidar having no apertures (other than the objective lens or mirror of the telescope) or obstructions is unity, provided the divergence angle of the laser beam is less than the opening angle of the telescope (Measures, 1984). Therefore, an iris (204) having a diameter (pin-hole) of 0.5mm is used to obtain a field of view of about 400// rad. A narrow band interference filter (205), whose center-wavelength and bandwidth were 532mm and 0.5nm respectively, was used to reduce background light. The filter used in this system is not temperature controlled, hence as specified it needs to be maintained at a temperature of about 22°C. For this reason, the lidar system is kept in a temperature controlled cubical. In the system according to the present invention, a Photon Multiplier Tube (PMT) is used as a detector system. The PMT is positioned behind the Interference filter (205). In the prior art systems, Avalanche Photodiode (APD) were used as detector operating in photon counting mode. Since prolonged operation of APD in the breakdown region can potentially damage the diode, hence it requires avalanche quenching circuitry. However, the PMT used in the present invention are simple systems for use in the single photon counting applications. Because of their inherently high gain and fast response it makes them ideal for single photon counting. Moreover, their noise performance is much better compared to APDs. Also, the PMTs do not require any pulse effect corrections. 15 The data acquisition unit used in the lidar system of the present invention is a PC based multichannel analyzer (EG & G Ortec model MCS pci). A Multi Channel Scalar (MCS) records the counting rate of events as a function of time. When a scan is started, the MCS begins counting input events in the first channel of its digital memory. At the end of the pre-selected dwell time, the MCS advances to the next channel of memory. At the end of the pre-selected dwell time, the MCS advances to the next channel of memory to count the events. This dwell and advance process is repeated until the MCS has scanned through all the channels in its memory. A display of the contents of the memory shows the counting rate of the input events versus time. In repetitive measurements, where the start of the scan can be synchronized with the start of the events, multiple scans can be summed to diminish the statistical scatter in the recorded pattern. The MCS-pci can profile counting rates versus time, or it can function as a multiple-stop time digitizer for measuring flight times of multiple photons or particles. Typical applications are in time-resolved single-photon counting. It can count rates up to 150 MHz or 65,536 time bins (channels). Dwell times selectable from 100 ns to 1300 s/channel. It can do single scan once, or average repeated scans. There is no dead time between channels and no double counting. There is no end-of-scan dead time. In the lidar the instrumental bin width was normally set at 200 ns, which corresponding to an height resolution of 30 m. Usually 300000 shot integrated photon count profile constitutes a raw data profile that corresponds to a time resolution of 120 sec. Application to atmospheric boundary layer studies The boundary layer is that part of the troposphere that is directly influenced by the presence of the earth's surface, and responds to surface forcings with a timescale of about an hour or less (Stull, 1988). These forcings include frictional drag, evaporation, heat transfer, pollutant emission, and terrain induced flow. The processes occurring in this region greatly affect the lives of mankind. In this region, man spends most of his life in. Figure 2.1 shows the schematic diagram of the boundary layer (Piiromen, 1994). The boundary layer formed from sunrise to sunset is characterized by the formation of thermal plumes from solar heating. These plumes carry with them moisture that 16 contributes to the formation of convective clouds. Solar heating is also responsible for the transport of heat and aerosols and thus mixing occur in this time frame. From sunset to sunrise, the boundary layer is characterized by a stable layer formed when the solar heating ends and radiative cooling starts to occur in the lowest part of the atmospheric boundary layer. Above the stable layer is the residual layer, which is the remnant of the daytime boundary layer. The Atmospheric Boundary Layer (ABL) thickness is quite variable in space and time, ranging from hundreds of meters to a few thousand meters. It is practice in air pollution meteorology to use the term mixed layer (ML) since pollutants that are emitted into the ABL become gradually dispersed and mixed through the action of turbulence. The mixed layer depth (MLD) is the height of the top of the ML and is an important parameter to characterize the ABL and its structure. In some papers, the MLD is also called the mixing height (Seibert et al., 1998; 2000). Measurements, parameterizations and predictions of the MLD have many theoretical and practical applications such as the prediction of pollutant concentrations, in numerical weather prediction and climate modeling (Seibert et al., 1998; 2000) and the study of turbulence in ABL. Different mathematical methods were reported to determine or estimate the MLD (Endlich et al., 1979; Melfi et al., 1985; Boers et al., 1988; Hooper and Eloranta, 1986; Senff et al., 1996; Hayden et al., 1997; Flamant et al., 1997; Menut et al., 1999; Cohn and Agevine, (2000); Brooks, 2003). During daytime, radio soundings are the most common data source to retrieve the MLD based on wind, temperature and pressure profiles but in most stations regular launches are made only twice a day at predetermined synoptic times (0000 and 1200 UTC). Consequently, radio soundings can often be used as a reference for comparison with modeled ABL heights only around midnight and noon. However, nighttime measurements of ABL are still a gap region to modelers, which can be compensated from lidar soundings. Active remote sensing systems such as lidars use aerosols as tracers of the ABL dynamics (Bhavani Kumar, 2006). The optical power measured by a lidar device is proportional to the aerosol content of the atmosphere. The lidar signal shows a strong backscattering within the ML, which decreases through a transition zone and becomes weak in the free troposphere (FT). 17 These contrasts are the basis for the lidar estimations of the MLD. Under convective conditions, the ABL can be divided into three different layers: the surface layer (SL), the mixed layer (ML) and the entrainment zone (EZ). The latter represents a transition zone between the ML and the stable FT above. Usually, the EZ is defined in a horizontal average sense (Deardorff et al, 1980). It is important to distinguish between the instantaneous (or local) MLD that varies between the EZ top and the middle of the ML and the average MLD which is the middle of the EZ (Stull, 1988). Ground-based aerosol lidars give a high-resolution picture of the instantaneous ML top that is marked by a large contrast between the backscatter signal from aerosol-rich structures below and cleaner air above. Method of Interpretation of Lidar data The Lidar System according to the invention emits a short laser-pulse at given wavelength and given duration into the atmosphere. The laser pulse is absorbed and scattered by atmospheric molecules through Rayleigh-scattering and by particles through Mie-scattering. The backscattered fraction of light within the field of view of the coaxial telescope is collected and transmitted to the detector. The use of the lidar technique for boundary layer ranging relies on the altitude resolved measurement of atmospheric backscatter intensity from outgoing laser radiation. The backscatter intensity measured by lidar is proportional to the signal backscattering by particles and air molecules present in the atmosphere. The expression that relates laser energy output (Eo ) and the backscattered signal P(r), in the case of a backscatter single wavelength lidar is given by (Bhavani Kumar et al., 2007) P(r) = he, [j3{r)\^T:{r)\nYNHRAt (1) where P(r) = Expected number of photons detected in the range interval (r-Ar/2, r+Ar/2) E0 - Laser energy (Joules) h = Plank's constant (6.63 x 10 34 J s-1) 18 c = Velocity of light (3 x 108 m s"1) X = Lidar operating wavelength (532 rnn) p(r) = backscatter cross-section (m 1 s r 1) AR = Receive telescope area R = laser pulse rate (s"1) Ta(r) = One-way transmission of atmosphere rj = Efficiency of lidar system N/j = Expected photon count per range bin per pulse due to background noise and dark counts from detector A / = Integration time (s) The first term in Eq (1) gives the number of transmitted laser photon within the time bin length of At. The second term provides the probability that a transmitted photon backscattered by scatterers in to a unit solid angle at angle of it (180°). The term refers to the volume backscatter coefficient of the atmosphere, which relates to the amount of photons that would be backscattered when an atmospheric thickness is crossed by the laser pulse length of r and is given in units of'nf'sr1'. This is equal to the product of the effective backscatter cross-section of scatterer, their number density and the scattering layer thickness. The third term indicates the probability that a scatter photon collected by the receiving telescope i.e., by the receive aperture to the scatter. The term —f- is the r' solid angle of the receiving telescope aperture relative to the scattering source. This term gives the geometric fraction of the surface of a sphere with radius r for which the backscattered photons can be colleted by the telescope with the aperture area Q?AR units. The fourth term informs the amount of atmospheric transmission during the light transmission from laser source to distance r and from distance r to receiver. The term Ta(r) is the one-way transmittance of the atmosphere. The fifth term provides the optical efficiency of overall lidar components e.g., steering mirror, collimating lens, optical IF filters etc. The sixth term is the expected photon count per range bin per unit due to background and detector noise. It is the number of photons detected from the background including any light sources other than the emitted laser light such as 19 airgiow emission, star light, and photo detector dark counts. The dark count is caused by spontaneous emission of photoelectrons from the cathode in the PMT. The expression (/) can be simplified as: P(r) = Ps (r)+PN (2) where the measured signal intensity P(r), given in terms of photon counts, corresponds to signal counts Ps(r) due to the atmospheric backscatter (comes from air molecules and aerosols) and counts P^fr) due to sky background signal. Usually the lidar signal is usually background signal corrected and transformed into a variable that removes the range square —- dependence X(r) [Fernald, 1984] or its logarithm, S(r) [Klett, 1985]. r~ X{r) = [p{r)-PN]xr2 (3) S(r) = In X(r) - (4) Identification Of ABL Height Front Lidar Data A general reliable method than that of the existing art involves the direct comparison of the backscatter signal with a fitted model Rayleigh backscattering profile. The boundary layer height can then be defined as the first altitude point for which the measured backscatter profile exceeds the Rayleigh model profile by some fixed amount, ij>. Following Melfi et al. [1985], $ is chosen to be 25%, although it is noted that the boundary layer height retrieved by this method is not particularly sensitive to reasonable values of >. The lidar backscatter profiles recorded are shown in Figure 6a and 6b respectively. The Rayleigh model is fitted between the indicated altitudes. APPLICATION TO ABL STUDIES To demonstrate the performance of the lidar system for ABL evolution, a continuous 15-day observation was conducted at Gadanki site (dry season) during clear sky period. 20 The variation of range corrected signal S(r) over a diurnal period (24-hour) is shown in Fig. 7. Fig 7 provides the time evolution of lidar range squared signal S(r). Over land surfaces, the boundary layer has a well-defined structure that evolves with the diurnal cycle [Stull, 1988]. From Fig. 7, one can observe the mixed layer (ML) development from the lidar signal enhancement during 06:00 to 15:00 LT. The mixed layer (ML) or Convective Boundary Layer (CBL) is formed by the convection that arises from solar heating of the earth's surface and is associated with organized thermal transport due to highly developed vertical motion. Soon after the sunset buoyant (eddy) production ceases and the atmosphere changes to a near neutral condition. A weak residual layer containing the remnants of the daytime mixed layer is observed in the background after 18:00 LT. In the late night hours, as the surface gets cooled the elevated eddies are directed downwards. This causes formation of a stable boundary layer (SBL) near to the surface. This is seen at late evening hours over land surfaces. The intensified signal returns in late evening hours clearly show the formation of SBL. The SBL has been observed to form in the height range of 200 to 600 m during dry season due to low night temperatures. Thus, the portable lidar system has been successfully used to study the structure, dynamics and the evolution of Atmospheric Boundary Layer using aerosols as atmospheric tracers. Using the portable lidar system, the internal sub-layers of ABL such as Mixed Layer (ML), Residual Layer (RL) and Stable Boundary Layer (SBL) or Nocturnal Boundary Layer (NBL) have been identified clearly over the tropical rural site Gadanki. This result shows the feasibility of the low-pulse energy lidar system application in atmospheric boundary layer measurements. ENTRAINMENT ZONE (EZ) DEPTH The top of the BL is generally associated with a transition region from rather polluted air in the ABL to mostly clean air in the free troposphere, hence a strong negative gradient in lidar signal is observed at the transition region. The vertical gradient D(r) of the lidar signal [Endlich et al., 1979] is expressed as DW=±iLM] (5) dr dr 21 Figure 8 indicates the temporal variation of lidar signal S(r) and its first derivative D(r) from 06:00 to 18:00 Hrs LT on 6 Jan 2005. Figure 8 demonstrates that this transition region is clearly detected from lidar signals, with high temporal and spatial resolutions. The method of using D(r) provides a precise description of the boundaries between air masses identified by difference in the aerosol content. Positive gradients of X(r) correspond to increasing backscatter with altitude, and vice versa. A strong decrease with altitude in the aerosol concentration is expected at the entrainment zone [Endlich et al., 1979]. The largest negative vertical gradient in the lidar signal, associated with a sharp transition from "polluted" to "clean" air, can be used to identify this layer. The Entrainment Zone is the region of statically stable air at the top of the ML, where there is entrainment of free-troposphere (FT) air downward and the overshooting thermals upward. From the Figure 2.4 (b), one can observe that the entrainment is clear during early hours of morning and also late afternoon hours. But during strong convective period, from 09:00 to 15:00 hrs LT, the Entrainment zone appeared to be patchy. This is clear that during this time the upward thermals are strong and thus preventing air entering from FT in to BL. OSCILLATION IN BOUNDARY LAYER Figure 9 shows an example of an oscillating aerosol layer observed by the BLL system. The oscillation that produced the buoyancy waves was observed to start from 17:00 to 21:00 local time. In this time range, the period of oscillation is found to be equal to 12 min. THEORY BEHIND THE ATMOSPHERIC OSCILLATIONS A stable environment supports buoyancy waves. Atmospheric oscillations or sometimes called gravity waves are a common occurrence in the atmosphere and are formed due to shear instability, as shown in Fig 10 frontal acceleration, air flow over orographic regions, or geostrophic adjustments (Hertzog et al., 2001). For example, when clouds are present in the atmosphere, they act as obstructions to the flow of air. The waves are generated when air flows over the top of these obstacles, which is, in this case, a mountain. Clouds too are obstacles and can trigger gravity waves.. In the presence of fronts, waves can occur in pre- and postfrontal zones (Stull, 1988). Waves 22 are also formed within the entrainment zone. Hooke and Jones (1986) observed these undulations by acoustic echo sounders. These waves have wavelengths that are equal to the diameter of the thermals and are quickly absorbed in the mixing layer. OBSERVATION OF AN ELEVATED BOUNDARY LAYER At nighttimes the PBL tends to be lower in thickness while during the day it tends to have a higher thickness. The two reasons for this are the wind speed and thickness of the air as a function of temperature. Usually, the formation of PBL is dominated more by advection and thermal energy budgets than levels above the PBL. The earth gains most of its energy and losses most of its energy from the surface. It is warmed through solar heating and cooled through long-wave radiation emissions. Strong wind speeds allow for more convective mixing. This convective mixing will cause the PBL to expand. It can warm up significantly during the day and cool at night while the rest of the atmosphere stays at a fairly uniform temperature. At nighttime, the PBL contracts due to a reduction of rising thermals from the surface. Cold air is denser than warm air, therefore the PBL will tend to be shallower in nighttimes. An interesting observation of elevated boundary layer during nighttime has been observed over Gadanki on the night of 23 July 2006. This observation was supported by lidar and GPS data. Usually the determination of the depth of the PBL is done using the radiosonde data. The top of the PBL is often marked with a temperature inversion, a humidity gradient, and change in wind speed and/or a change in wind direction. However, there will be an abrupt change in air mass at the top of the boundary layer. This information will be obtained from the lidar data. The Fig 11 shows the lidar extinction profile, GPS sonde derived temperature, water vapor, wind speed and wind direction. It can be seen from Figure that the structure of aerosol distribution is closely related to the structure of atmospheric water vapor. It shows clearly that the distribution of aerosol is hygroscopic in nature. MULTIPLE LAYERING PHENOMENA Multiple layers are easily identified in lidar scans by the horizontal stratification that is always present. The figure shows a thin stratified aerosol layer above NBL. The Above 23 the thin aerosol layer a constant haze layer is also seen. Fig. 12 shows the layers of aerosol layers formed at height between land 3 km. Each aerosol layer represents a different aerosol concentration. The existence of these layers shows primarily that the atmosphere is stable. It can also be seen that the layers are oscillating in time. APPLICATION TO AEROSOL STUDIES INTRODUCTION Our atmosphere consists of two different types of microscopic scatterers, namely the molecular (Rayleigh scatterers) and the particulates or aerosols (Mie scatterers), based on the optical effects observed in the atmosphere. Rayleigh scatterers have dimensions that are small compared to the interactive wavelength of light, while Mie scatterers have dimensions that are comparable or even greater than the wavelength of the interactive light. On the average, Rayleigh scatterers have dimensions of the order In the field of laser remote sensing of atmosphere, the most common type of lidar system is the elastic Mie scattering lidar. These lidars rely on the fact that Mie scattering have large scattering cross section of about —-s 10~8cm2sr~x. This means that even low concentrations or changes in the concentrations of dusts or aerosols can easily be detected (Measures, 1984). This makes this system ideal for continuous atmospheric monitoring. In general, Mie scatter lidars are utilized to monitor aerosols and to detect cloud base height and diurnal variations of the atmosphere. With a fine temporal resolution, a continuously operated lidar can easily detect real-time aerosol dynamics in the atmosphere. It is well known that vertical layering of aerosols is vital in the atmospheric remote sensing [Franke et al., 2003]. The present emphasis is on the observation of physical 24 properties of the atmospheric boundary layer aerosols and elevated aerosol layers which are most relevant to radiative forcing applications. In the lower troposphere, aerosols come from a number of sources and they cause visibility degradation when they are located in the boundary layer. Widespread surface sources include biogenic sources, volcanoes, deserts, oceans and fresh water, crustal and cryospheric aerosols, and biomass burning. This is commonly experienced during dust events (Husar et al., 2001; Sugimoto et al., 2003). Biofuels such as wood fuel, dung, and crop waste are the primary contributors of aerosols in rural areas of India (Habib 2004). In addition to ubiquitous soil and sea salt aerosols (coarse particles), urban aerosols have anthropogenic origins such as dust from paved and unpaved road (coarse particles), primary or secondary particles from exhaust of automobile combustions and industrial processes (fine particles). Aerosol emissions in urban regions result from fossil fuel sources such as diesel and kerosene mixed-fuel vehicles, industrial processes, and coal-fired power generation (Dickerson 2002, Prasad 2006). Industrialization and population expansion with agricultural and fuel-use practices induce high aerosol loading over the NIS (Lelieveld 2001, Prasad 2006). However, the anthropogenic influence is not entirely responsible for persistent aerosols. Meteorological phenomena induced by local geography (i.e., Himalayan Mountains and the Indian Ocean) exacerbate smog conditions by prolonging the lifetime of aerosols and clouds to further impact climate. Since the beginning of the age of industrialization, the increase of human and industrial activities has raised the production of aerosols released to the atmosphere. In the context of urban air pollution, the terminology of suspended particulate matter (SPM) is used to specify aerosols causing human health problems. These are solid or liquid particles with varying sizes (less than 10 mn) and shapes whose origins are often ascribed to industrial activities. Exposures to these fine particles, especially particulate matter smaller than 2.5 um (PM 2.5), can result to serious respiratory disorders (Abbey et al. (1999), Samet et al. (2000), Pope et al. (2002)). Atmospheric aerosols can also impact the local and regional radiation heat budget. Black carbon aerosols absorb incident solar radiation and heat the atmosphere more effectively than dust, sulfates, and organic carbons, which reflect more radiation and cool the atmosphere. However, the scientific understanding of the aerosol radiative forcing magnitude remains very 25 low (Albritton and Filho 2001). Although the lifespan of aerosols is about one week, persistent meteorological phenomena and anthropogenic activities can provide an environment of greater climate impact (Rasch 2001). As part of understanding the effects of aerosols on human lives and the environment, it is also equally important to study the optical properties and dynamics of aerosols in the boundary layer so that pollution transport and local atmospheric variation can be clearly revealed. Because aerosols play an important role in radiation budget of the atmosphere, quite a number of studies were conducted on this area (Holben et al. (1998), Dammann et al. (2002), Rajeev and Ramanathan (2002), Birmili et al. (1999) and Langmann et al. (1998)). By knowing the optical properties of the aerosols, we will be able to understand whether aerosols aid in the cooling or heating process (Kandel, 1999) of the atmosphere. To achieve this, altitude profiling of aerosol are required on long-term basis. It is possible with the elastic backscatter lidars. However, to obtain aerosol height distribution on long-term basis in an un-attended way needs Micro pulse lidars (Spinhirne, 1993; Bhavani Kumar, 2006), because big pulse lidars cannot be operated for longer periods. The Figure 13 shows a typical backscatter and its range squared signal from micro pulse based lidar system such as BLL, which is capable of providing the altitude distribution of aerosol and cloud information through the troposphere from ground up to tropopause within a time sampling of 10 minute. LIDAR SIGNAL INVERSION - HEIGHT PROFILES OF AEROSOL The lidar equation for return signals due to elastic backscatter by air molecules and aerosol particles can be, in its simplest form, written as (Fernald et al., 1972) () PkhPtA^faMzV^e z2 ■ — ' 2 l\a(z)ck (6) where P(z), is the lidar signal received from a range z at a wavelength A, PL is the emitted laser power , A0 is the telescope receiving area, ^(z)is the receivers spectral transmission factor, /3(z) is the atmospheric volume backscattering coefficient, £" (z) is the overlap factor between the field of view of the telescope and the laser beam, a (z) 26 is the extinction coefficient from atmospheric molecules and particles, c is the speed of light and r, is the laser pulse length. The lidar data have been processed with the algorithm described by Fernald [1984] and Klett [1985], and assuming to know the molecular (or Rayleigh) contribution to the signal from the CIRA 1986 standard Atmosphere. According to the Fernald - Klett formulation, the solution of the lidar equation is given by the equation (7) M = exp{-fe)-S(z)]} (7) -p[7)+l ]exPB".M(z)]} where, the total backscattering coefficient is the sum of Rayleigh and particulate contribution given by /?(z )=/?om, (z ) + /?„,„, (z), the /?(z0)is the boundary condition set on /?(z)at the reference far-end range. The detail of the numerator is given in equation (8). The typical height profile of aerosol backscatter derived is shown in Figure 14 S(z0)-S{z)= ln[z()2 * P(z0)]-ln[z2 * p(z)]-± £?*Mt (8) To derive the above equation, several assumptions were made (a) The backscatter-to-extinction ratio is known, (b) The reference range (z-ref) is taken in a region where the lidar profile followed the molecular atmosphere (generally between 6 and 8 km). (c) The value of /?(z) at the reference range was accordingly chosen as been equal to pm(z) (no aerosol contribution). (d) The value of C was chosen as been equal to 0.035, which is an average value among those suggested for rural, urban and maritime aerosols (Kovelev, 1993). OBSERVATION OF BOUNDARY LAYER AEROSOL OVER GADANKI Using the BLL system, the height profiles of lower troposphere aerosol, at 532 nin wavelength, have been derived up to an altitude of 7 km using the standard Klett 27 technique from BL lidar nighttime data. These are the first observations of the lower troposphere aerosols taken from the Gadanki site. The following observations are bought out from the preliminary analysis of the data. (1) Most of the time, a thick aerosol layer will always be present in the lowermost troposphere which top 1.5-2.0 km above ground level, as shown in Figure 15. We consider that the layer correspond to the local mixing layer. Parameswaran et al. [1997] from an observational study of night time aerosol concentrations in the lower atmosphere at a tropical coastal station reported that accretion of aerosol occurs in a stable atmosphere sandwiched between two turbulent regions which are normally observed within the boundary layer. OBSERVATION OF LONG-RANGE AEROSOL It is well recognized that the lidar measurements are valuable in aerosol studies in that they potentially supply vertical information on aerosol optical properties. Lidar profiles sampling the air mass advected from the Indian sub-continent show multiple layers present over the Indian ocean as a result of convection and long range transport of aerosol originating from arid and semi arid regions of the world [Muller et al., 2001; Ramanathan et al., 2001]. At times the air mass shows 3 km deep pollution layer above the boundary layer [Ansmann et al., 2000]. Such elevated aerosol plumes were also observed over the northern India and also over southern India, particularly during winter season due to dry convective lifting of pollutants at distant sources and subsequent horizontal upper air long range transport [Ratnana et al., 2004; Niranjan et al., 2007]. Figure 16(a) shows the aerosol backscatter coefficient as a function of altitude for. It may be seen that a thin aerosol layer was observed from an altitude of 2 to 3 km indicating high altitude aerosol layers. The aerosol layers found above the boundary layer could be transported several thousands of km without significant removal and can contribute significantly to the column aerosol optical depth, at times more than the boundary layer [Franke et al., 2003]. The observation shown above indicate that the air mass advecting over the eastern coast of India into Bay of Bengal not only contains the anthropogenic contribution from the 28 Indian sub-continent but also at times contains the plumes advecting from far east regions with significant contribution of biomass aerosol. Figure 16(b) indicates a 7 day HYSPL1T back trajectories showing the air mass pathway from far east region. It is considered that these layers were on the way of long-range transport. Such type layers are ejected in to the free troposphere by distant sources and get transported globally. HYGROSCOPIC NATURE OF AEROSOL The lidar system detects the laser-backscattered radiation from air molecules and aerosols up to about 3 km during daytime and up to about 17 km during night. Using the system, temporal variation in the vertical profiles of aerosols over Gadanki is being studied. Fig. 17(a) shows the average monthly aerosol extinction profiles measured over Gadanki. During winter months the aerosol height profiles are limited to 2 km altitude region where as one can observe an increase in aerosol amount during summer months, pre-monsoon period, in the 2 to 4 km altitude region. This shows a rise in local boundary layer during summer months. This increase could be probably due to increased hygroscopic growth of aerosol particles in the 2 to 4 km altitude region. This was confirmed by a study of comparison with the GPS sonde derived water vapor mixing ratio with that of the lidar derived aerosol extinction profiles. Figure 17(b) shows the comparison profile, which clearly indicate that the aerosols in the lower atmosphere are hygroscopic in nature. OBSERVATION OF SCAVENGING OF ATMOSPHERIC AEROSOL BY ARIES-NARL LIDAR AT A HIGH-ALTITUDE STATION, NAINITAL Recently a portable lidar system was set up at Manora Peak, Nainital (29° 22' N, 79°27' E, 1960 m MSL) by National Atmospheric Research Laboratory (NARL), Gadanki under a joint scientific collaborative programme between ARIES (Aryabhatta 29 Institute for Observational Sciences), Nainital and NARL, Gadanki. The lidar system was operated on two days on 16 and 17 May 2006. During the observations the lidar system has collected backscatter returns from the lower atmospheric aerosol and also high altitude cloud such as cirrus. These are the first observations taken from a high altitude location in the northern part of India. The height distribution of aerosol was seen up to the height of 3 km above the ground level. The high altitude clouds were observed in the height ranging between 8 and 11 km. During this period, an interesting feature was noticed that there was a drastic reduction in the height distribution of aerosol particles due to scavenging process caused by heavy rain on 17 May 2006, consequently a significant reduction in AOD was also observed. The AOD obtained on 16 May 2006 was about 0.208 at ^.=0.532 urn, which showed a significant reduction to a value of 0.08 on 17 May 2006 due to the scavenging effect in the height range of 0.2 to 3.0 km. OBSERVATION OF DUST STORM IN THE INDUS VALLEY A large dust storm blew through the Indus Valley, along the border between Pakistan and India, on June 12, 2006. The Moderate Resolution Imaging Spectroradiometer (MODIS) flying onboard NASA's Aqua satellite captured this image the same day. In this picture (Figure 18), the dust heads toward the Himalaya Mountains in the top right corner of the image. In the lower-left corner of the image, sprays of clouds appear to blow in the same direction as the dust, away from the Arabian Sea and toward the northeast. Once the dust reaches the mountains, it changes direction and blows along their southern edge. Aerosol emissions originating from the Northern Indian Subcontinent (NIS) region have a significant effect on local and regional climate. Using ARIES-NARL lidar an aerosol analysis was conducted over the Manora peak , Nainital during June 2006 (Figure 19). The lidar analysis shows the dust layer appears above the local boundary layer and peaks at a height of about 1 km AGL. The height distribution extends up to 4 km. 30 This analysis found that 500nm daily average aerosol optical depth (AOD) values as high as 1.6 AOD consisted primarily of fine mode particles during the dry monsoon phase, while the wet monsoon phase produced much lower 500nm daily average values near 0.1 AOD with coarse mode particles. Satellite imagery and retrieved data, back trajectory analyses, and model output provided supporting information on aerosol source regions, transport, and chemistry. PROFILING ATMOSPHERIC CLOUDS INTRODUCTION Research in the last few years has shown that clouds are a major variable in Earth's climate system. The climate is very sensitive to small (10-30 percent) changes in clouds. This sensitivity is very important in trying to figure out what man-made changes, such as increasing carbon dioxide, will do to Earth's climate. Depending how these changes affect clouds, their influence on climate could be either enhanced or damped out. We all know that the energy to the Earth comes from the sun. An equal amount of energy must go back into space or Earth's temperature will change. The picture shows that clouds make the biggest contribution to reflected energy (think about how bright clouds can appear ~ this is reflected energy), and also to emitted heat (clouds act like radiators in the atmosphere ~ though much colder than a radiator in a building). You can think of clouds as one thermostat that sets Earth's temperature. If. for example, you increase the average thickness of low clouds a little (i.e., make them more reflective), the Earth's temperature will decrease a little. The effect is as if you turned down the thermostat a little. We have learned that clouds can act to either warm or cool the Earth. High clouds are often thin and not very reflective. They let lots of the sun's warmth in. They also sit high in the sky, where the air temperature is quite cold; so they do not emit a lot of heat. On balance, high clouds tend to warm the Earth. Low clouds are often quite thick and reflect lots of sunlight back to space. They are also lower in the atmosphere where the air is warmer so they emit more heat. On balance, low clouds tend to cool the Earth. 31 Hence, it essential to build a regional or global cloud climatology to properly access the affect of clouds on the Earth's radiation budget, in order to do it so, we need instruments on different platforms. One of the potential applications of lidar is in the studies of characterization of clouds. Usually strong backscattering arises from clouds due to relatively large scattering cross-sections of cloud particles and huge number density of scatterers. Lidar has the capability to delineate the position and spatial structure of clouds in the atmosphere. Because of its high spatial resolution, it can be used to locate cloud base and its top with a good precision that is not possible with any other remote sensing technique. This feature of lidar has attracted more researchers recently in investigations of clouds at various height regions as clouds play a critical role in the radiation budget of the earth atmosphere (Liou 1986). The recent reports on backscatter lidar employed in studies of upper tropospheric clouds (Nee et al. 1998; Winker and Trepte 1998; Piatt et al. 1998; Boehm and Verlinde 2000; Bhavani Kumar et al. 2001; Roumeau 2000) indicate the potential application of the instrument in cloud research.. OBSERVATION OF RAIN CLOUD USING BLL The formation of clouds comes from rising air parcels. Air rises because of orographic lift, convective lift, and convergent lift. Orographic lifts are formed when air is forced to move upward as it encounters a cooler, denser body of air or when it meets high landforms such as a mountain. Convective lift occurs when air encounters a warm surface, heats up and becomes less dense compared to the surrounding air. This usually occurs during daytime when the sun drives the convective circulation on the ground. Convergent lift occurs when air is whirled toward the center of a cyclone colliding with itself and is forced to move upward. This is the mechanism that drives air upward during storms or tornados. As air parcels rise, they cool down and condense. Rain clouds occur when the clouds start to produce moisture. Examples of these types of clouds are the nimbostratus and the cumulonimbus. Nimbostratus clouds are layered, uniform, rain clouds. These clouds are generally dark, and associated with large areas of continuous rain. In the case of cumulonimbus clouds, they are formed by the upward movement of warm air currents. These types of clouds are accompanied by 32 compensating downdrafts of cold air and are common in warm and humid weather (Wallace and Hobbes, 1977). For raindrops to occur there must be particles in the air, such as dust or aerosols, at temperatures above freezing. When particles are cooled below the freezing temperature, water condenses on them. As this happens, the particles become heavy and start to fall. Rain clouds can also be formed locally when air rising over a moist area causes the formation of cumulus clouds. As the moisture condenses, the clouds begin to grow darker. The movement of these rain clouds can be tracked using radar (Crane, 1996). Figure 20 and 21 show the streaks formed from falling raindrops. By measuring the vertical displacement of the streak and the time for the streak to cover the vertical displacement, the average speed of the streaks can be estimated. The value of the average speed can then be used to approximate the raindrop size. OBSERVATION OF MIDDLE LEVEL CLOUD The occurrence of clouds above 6 km and below 10 km is very rare. Here is a typical observation of cloud appearing in height range between 6 and 10 km, depicted in Figure 22. These clouds generally formed from the remnants of rainy cloud tops, which appear at heights between 3 and 5 km. DETECTION OF HIGH LEVEL CLOUDS The appearance of deep cumulus clouds during convective periods is a common phenomenon at tropical latitudes. Usually the upper portion of convective cumulous clouds constitutes ice that extends in the fibrous anvil, which at a later stage takes the form of cirrus. The formation of high altitude clouds such as cirrus in the tropics plays a particularly important role in the Earth-atmosphere radiation budget. Tropical cirrus develops in a variety of forms, ranging from optically thick anvil cirrus that is closely associated with deep convection (Houze 1993) to optically thin cirrus layers frequently observed near the tropopause (Boehm et al. 1999). Much of the cirrus in the atmosphere, especially in the tropics, arises from mesoscale convective activity (Houze 1993). Moreover, the tropical tropopause being a very cold region with temperatures often reaching as low as 190 K, forms a favorable situation for nucleation and 33 condensation processes, leading to the formation of optical thin cirrus in the vicinity of tropopause (Jenson et al. 1996). It is well established that lidar has the capability to delineate the position and spatial structure of clouds in the atmosphere. Because of its high spatial resolution, it can be used to locate cloud base and its top with a good precision that is not possible with any other remote sensing technique. This feature of lidar has attracted more researchers recently in investigations of clouds at various height regions as clouds play a critical role in the radiation budget of the earth atmosphere. The appearance of deep cumulus clouds during convective periods is a common phenomenon at tropical latitudes. Usually the upper portion of convective cumulous clouds constitutes ice that extends in the fibrous anvil, which at a later stage takes the form of cirrus. The formation of high altitude clouds such as cirrus in the tropics plays a particularly important role in the Earth-atmosphere radiation budget. Tropical cirrus develops in a variety of forms, ranging from optically thick anvil cirrus that is closely associated with deep convection to optically thin cirrus layers frequently observed near the tropopause. A typical lidar observation of cirrus system in the night of 11-12 September 2005 is shown in Figure 23. Figure 23 represents the temporal variation of backscatter ratio as a function of altitude for the night observation made on 11-12 September 2005. Similar type clouds appear during winter period over Gadanki is also shown in Figure 24. The generation mechanism of this type clouds are different from the monsoon generated cirrus clouds. OBSERVATION OF K-H BELLOWS IN HIGH LEVEL CLOUDS Another "specialty" cloud is one that can develop due to Kelvin-Helmholtz (K-H) instability waves and sub-harmonic resonance with other waves in the atmosphere. A typical observation of K-H bellows in cirrus clouds observed by BLL system is shown in Figures 25. Atmospheric gravity waves are responsible for formation of such type clouds in the upper troposphere. This can result in an intertwined or spiral cloud pattern as shown. K-H instability is the result of strong wind shear. K-H clouds that form in 34 early stages can resemble well-organized waves that appear to be breaking like ocean waves. The high spatial and temporal resolution measurements made the possibility of observing such type of instability structures in the upper troposphere clouds. Large-scale shear in the upper troposphere could be the probable reason for development of ambient gravity-wave activity. OBSERVATION OF MULTI-LAYER CLOUDS Atmospheric clouds appear at different altitudes. Appearance all three different heights of clouds are shown in figure 26. A high level cirrus is seen around 11 km , a middle level cloud is appeared around 6 km height and a low level cloud occurred on the top of the boundary layer is shown clearly in Figure 26. These clouds occur at multiple altitudes and generally appears above Southern Indian Ocean. MODIS -BLL CLOUD HEIGHT COMPARISON A study was carried out jointly by Indian Institute of Technology, Kharagpur and National Atmospheric Research Laboratory, Gadanki for cloud height comparison using BLL system at NARL, Gadanki and MODIS satellite pass data over Gadanki. The MODIS satellite passes over Gadanki at 16:00-17:00 time and also 04:00-05:00 LT. Using the satellite data and the BLL detected cloud height is compared. It is found that the comparison was highly correlated and matched with the MODIS detected cloud heights with BLL detected cloud heights. From the study it is found that for high-level clouds the difference in cloud top pressures is less than 100 hPa in 80% of the cases and for low-level clouds the difference is less than 100 hPa in 86% of the cases. Linear Regression analysis shows that the slope of the linear fit is very close to unity (0.9975), which means that overall there is only a slight underestimation of cloud top pressure by MODIS. We Claim 1. A boundary layer micro pulse lidar system comprising a laser head to generate a laser source, a beam expander to expand the laser beam output to achieve eye safety, the expanded laser beam being projected into the atmosphere through mirrors positioned at 45° angles, a telescope to receive the backscattered light, a Photo Multiplier Tube (PMT) as a detector system for single photon counting, a data acquisition unit and analyzer for acquiring and processing the signal from the detector to profile aerosols/particulates and atmosphere clouds, wherein the said laser head comprises a laser diode, a Nd doped Yttrium-Aluminium-Garnet crystal (Nd:YAG) which generates the laser, an acoustic opto-modulator (AOM), potassium tri¬phosphate (KTP), which is a crystal that generates wavelength at the second harmonic of YAG resonating wavelength and other output optics. 2. The system as claimed in claim 1 wherein the said telescope is a classical cassegrain telescope with a diameter of 150 mm and a F-value of 9. 3. The system as claimed in claim 1, wherein an interference filter is provided in front of the photo Multiplier tube to reduce the background light. 4. The system as claimed in claim 1, wherein an iris with diameter of at least 0.5mm is provided in between the telescope and the interference filter to receive a field of view of 400 urad. 5. The system as claimed in claim 1, wherein a thermo electric cooling unit is provided adjacent the laser diode to control the heat dissipated from the laser head. 36 (>. The system as claimed in claim 1, wherein the data acquisition unit is a Multi Channel Scalar analyzer connected to a personal computer. 7. The system as claimed in claim 3, wherein the said interference filter is a narrow band interference filter, whose center-wavelength and bandwidth were 532mm and 0.5nm respectively.

Full Text

FIELD OF INVENTION
The present invention relates to a boundary layer lidar system incorporating the micro pulsing technique.
BACKGROUND OF THE INVENTION
Lidar is an acronym for Light Detection and Ranging. It is the optical counter part of the familiar radar technique for remote sensing, which uses laser light and it can be thought of as Laser Radar. A lidar system transmits a pulse of light in to the atmosphere and analyzes the backscatter signal for intensity as a function of time. Because the laser pulse travels at the speed of light, it is possible to convert time in to range and consider the lidar signal to be backscattered intensity as a function of range.
Optical probing of the atmosphere through elastic scattering actually predates the invention of the laser (Schawlow and Townes, 1958). It was, however, the development of Q-switching by McCIung and Heliwarth (1962) that made possible the generation of very short, single pulses of laser energy and thereby range-resolved measurements. The energy compression afforded by Q-switching also gave rise to laser power densities of such magnitude that remote measurements that based on inelastic scattering from specific molecular could be considered for certain applications. Fiocco and Smullin (1963) working at MIT were the first to use laser for atmospheric studies. In 1962, using a 0.5 Joule Ruby laser, they obtained Rayleigh scattered signals from the atmosphere up to 50 km altitude and also detected dust layers in the atmosphere. Ligda in 1963 made the first lidar measurements of cloud heights at tropospheric heights. Recently, NASA (LITE) and ESA (GLAS) have undertaken the development of a series of space-borne Lidar missions to measure atmospheric dynamics, atmospheric aerosol and cloud properties and water vapor.
Over the last forty years, lidar instruments have been developed to study the structure of the atmosphere by means of the elastic and inelastic scattering of light from
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constituents of the atmosphere such as air molecules and aerosols. The backscattered energy is approximately proportional to the aerosol and molecular content of the observed air mass. Based on this information, a backscattering lidar can provide information on the PBL structure, mixed-layer depth, entrainment zones and convective cells structure, aerosol distribution, clear air layering, condensation levels, cloud-top altitudes, cloud depolarization properties, middle atmospheric temperature, atmospheric transport, low-level jets, diffusion processes, and other inferences of air motion. Elastic backscatter lidars are, by far, the most common and simple remote sensing instruments in the world today and this will continue to be true for the foreseeable future. Elastic backscatter lidar measurements exhibit all the desirable characteristics of remote sensing techniques. The measured values represent spatial averages over volumes that can be varied by proper design of the lidar system. Such spatial averaged values arc usually more representative of the overall state of the turbulent atmosphere than the single point in situ measurement. By using aerosols as the tracers of atmospheric dynamics, lidar can identify several dynamical parameters of the ABL such as boundary layer top and entrainment zone depth in real-time with high temporal and spatial resolutions. However, the traditional lidar instruments used so far during the last three decades have typically been high-energy pulse, low repetition rate systems. Recent advances in the solid-state lasers, detectors and data acquisition systems have enabled the development of a new generation lidar technology that uses micro pulsing technique. These lidars employ high pulse repetition rate, low pulse power transmitter approach.
Concept of Micro pulse Lidar
A micro lidar employs a diode-pumped Nd-YAG laser system, a co-axial transceiver for transmitting the laser pulses and detecting the collected photons, a dedicated acquisition system, and a computer control and interface system. Pulses of light energy (at green wavelength) are transmitted from the telescope into the atmosphere. As the pulse propagates, part of it is scattered by molecules, water droplets, ice crystals, dust and haze aerosol in the atmosphere. A small portion of the light that scattered back is
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collected by the telescope and then detected. The distance to the particle layers is inferred based on the time delay between each outgoing transmitted pulse and the backscattered signal. The special approach of the micro pulsing technique is to use expanded micro Joule pulse energies to obtain eye safety and reliability. The detected signal is stored in bins according to how long it has been since the pulse was transmitted, which is directly related to how far away the backscatter occurred. The collection of bins for each pulse is called a profile. A number of articles have reported on the applications of micro pulse lidar to environment studies (Chen et al., 2001; Parikh and Parikh, 2002; Legrosos et al., 2004; Bhavani Kumar, 2006). The temporal evolution of the boundary layer height and boundary layer aerosol can also be studied (Parikh and Parikh, 2002; Legrosos et al., 2004; Bhavani Kumar, 2006;Bhavani Kumar et al., 2007a and b).
The micro pulse approach has several advantages over to conventional lidar systems. These advantages made them more useful for field use, than limited to research field. The advantages are given below
The laser pulse repetition rate is much higher and the pulse energies are much lower. So it is a low heat generation system, hence there is no thermal runway problem.
The laser employed is a diode pumped rather than flashlamp pumped. Hence the reliability and stability of the system is high compared to the big pulse lasers.
The system employs photon counting detection. Usually the photon counting technique is far superior compared to the conventional analog method of detection, because it is more sensitive to the long range echoes. Though photon counting has a drawback of limited dynamic range in signal acquisition compared to analog method, the use of high repetition rate of laser operation overcomes this limitation.
APPLICATIONS OF BOUNDARY LAYER LIDAR
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The health of the planet Earth and its inhabitants is highly dependent upon the health of the atmosphere. The boundary layer lidar (BLL) has a significant role in the atmospheric research, because it facilitates the autonomous monitoring of atmospheric clouds and aerosol scattering. The following are the potential application areas where BLL systems can be made use.
Atmospheric and climate research: Continuous observation of aerosol distribution and atmospheric cloud profiling, thereby building the aerosol climatology for future climate prediction.
Environment monitoring: The evolution of the concentration of particulate pollutants is important requirement in air pollution control for public health and environmental safety.
Meteorological application: Correct treatment of the physical parameter such as the atmospheric boundary layer depth is necessary for weather forecasting and numerical simulations of climate change.
Visible range or slant height: It is of practical importance in aircraft landing and marine navigation. A simple slope method assumes that the asymptotic decrease in the range compensated backscattering intensity is proportional to the round trip laser beam transmittance, in clear atmosphere.
The boundary layer (BL) height from lidar profiles is defined differently by numerous researchers. Sasano et al. [1985] identified the BL height as the height where the signal backscatter begins to decrease from a relatively higher value to lower region. Endlich et al [1979] described BL height as the height at which a maximum negative gradient of laser backscatter in vertical direction occurs. Flamant et al [1997] also mentioned the BL height as a zone of minimum in the vertical gradient of the backscatter as defined by Endlich etal. [1979].
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An automated lidar system (Micro Pulse Lidar (MPL) system disclosed in US 5,241,315 (Spinhirne) was originally developed at Goddard Space Flight Centre (GSFC) - National Aeronautical and Space Administration (NASA), USA and was granted a US Patent No. 5,241,315. The MPL was constructed for the purpose of unattended measurements of cloud and aerosol height structure. This system is an eye-safe, solid-state lidar whose transmitter is a diode pumped u.1 pulse energy, high repetition rate Nd:YLF laser. The receiver uses a photon counting solid state Geiger mode avalanche photodiode detector. Data acquisition uses a single card multi-channel scalar. Fig 1 shows the schematic diagram of an MPL system of Spinhirne.
The Spinhirne developed MPL system design is such that the same telescope is employed for both transmitting and receiving the laser beam, and hence a small portion of the transmitted laser energy is always reflected inside the telescope and may often damages the detector. This configuration provides narrow FOV and eye safety, but disadvantage is always some amount of laser power enters into the detector and cause damage to the receiver and also increases receiver noise. The Spinhirne -MPL system employs Avalanche Photodiode (APD) as detector operating in photon counting mode (Geiger mode of operation) for the reason being its high quantum efficiency. However, APDs are not suitable for visible wavelengths because they are noisier at these wavelengths. Moreover, the APD mode of operation (Geiger mode -single photon detection) requires that the diode should be operated above its breakdown voltage (Kovalev and Eichinger, 2004). Since prolonged operation of APD in the breakdown region can potentially damage the diode, hence it requires avalanche quenching circuitry. This is an issue in the Spinhirne-MPL system. As there are many optical components such as beam collimator, half-wave plate, beam steering mirrors, depolarizer are used in the transmission and as well in receiving path of the Spinhirne-MPL system, makes the overall optical efficiency of the system is less. Moreover, use of many optical components make the system more complex and cumbersome for future augmentations such as addition of another receive channel for depolarization measurements. There is also no possibility of maintenance or augmentation at the site. The cost of the Spinhirne system is about 3 to 4 times the cost of the system of the
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present invention, since it employs many optical components and also several custom built parts.
SUMMARY OF THE PRESENT INVENTION
The present invention provides a novel micro pulse system known as Boundary Layer Lidar (BLL) for atmospheric profiling.
The system comprises a laser source and transmitting optics, receiving optics, detector unit, and data acquisition unit.
Laser source and Transmitting Optics
The lidar transmitter system employs a Russian pulsed laser M/s Laser Export made, model LCS-DTL-314QT system. It is a microchip (all-in-one laser cavity) Nd:YAG laser employing second harmonic output. It is laser diode pumped and acoustic switched. The output pulse energy is adjustable in the range between 2 and 20 uJ depending on the repetition rate. Usually the output pulse energy is set at 10uJ at 2500 Hz repetition rate in the boundary layer lidar according to this invention. The laser beam diameter is 0.4 mm and its divergence is less than 1.5mrad. The laser beam was expanded to 3 mm in diameter and collimated to have the beam divergence of about 200 urad. The eye safety is achieved with this beam divergence for the heights above 150m from ground. The maximum permission emission (MPE) at 532 nm is 0.5 uJ/cm2 The light output was linearly polarized with the degree of polarization being greater than 99%. The laser beam is sent into the atmosphere using two mirrors kept at 45 °angles.
Receiving Optics
A monoaxial configuration was employed in the lidar system according to this invention. The laser backscattered light was received by a classical -cassegrain
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telescope, whose diameter and F-value were 15 cm and 9, respectively. The geometrical form factor for a coaxial lidar having no apertures (other than the objective lens or mirror of the telescope) or obstructions is unity, provided the divergence angle of the laser beam is less than the opening angle of the telescope (Measures, 1984). Hence an iris diameter (pin-hole) of 0.5 mm was used to obtain a receive FOV of about 400 urad. A narrowband interference (IF) filter was positioned in front of the photomultiplier tube (PMT). An IF filter, whose center-wavelength and bandwidth were 532 nm and 0.5 nm, respectively, was used to reduce background light. The filter used in the system is not temperature controlled, hence as specified by manufacturer the unit is to be maintained at a temperature of about 22 °C. For this reason the lidar system is kept in a temperature controlled cubical.
Detector unit
The detector system employed in the lidar was a high gain PMT (Hamamatsu R3234-01). It is a head-on type PMT with 10 mm aperture. This was selected for use in the system based on two important specifications such as low dark current (about 1 nA) and good pulse generation characteristics (after pulse effect is 0.6 %). More over, its rise time is very good, about 2 nsec. It has a reasonable quantum efficiency of about 10. The PMT out put pulses are amplified and discriminated using a Phillips make discriminator (Model 6908), which is a 300 MHz BW system. The discriminator outputs are the current source type with two pairs of negative bridged and two complements. The current source type output (NIM standard) is used for data acquisition system for recording the data.
Data Acquisition Unit
The data acquisition system used in the lidar is a PC based multichannel analyzer (EG&G Ortec model MCS-pci). A Multi-Channel Scalar (MCS) records the counting rate of events as a function of time. When a scan is started, the MCS begins counting input events in the first channel of its digital memory. At the end of the pre-selected
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dwell time, the MCS advances to the next channel of memory to count the events. This dwell and advance process is repeated until the MCS has scanned through all the channels in its memory. A display of the contents of the memory shows the counting rate of the input events versus time. In repetitive measurements, where the start of the scan can be synchronized with the start of the events, multiple scans can be summed to diminish the statistical scatter in the recorded pattern. The MCS-pci can profile counting rates versus time, or it can function as a multiple-stop time digitizer for measuring flight times of multiple photons or particles. Typical applications are in time-resolved single-photon counting. It can count rates up to 150 MHz or 65,536 time bins (channels). Dwell times selectable from 100 ns to 1300 s/channel. It can do single scan once, or average repeated scans. There is no dead time between channels and no double counting. There is no end-of-scan dead time. In the lidar the instrumental bin width was normally set at 200 ns, which corresponding to an height resolution of 30 m. Usually 300000 shot integrated photon count profile constitutes a raw data profile that corresponds to a time resolution of 120 sec.
The overall specifications of the Boundary Layer Lidar System according to the present invention are given in table 1.

Accordingly, a portable coaxial configured prototype laser radar system using a visible pulsed laser has been realized for ranging the atmospheric boundary Layer (ABL), for profiling atmospheric aerosols and real-time detection of clouds with very high range resolution at a lower cost. The lidar system uses
a. A coaxial (monoaxial) configuration, which made the advantage of full overlap
height near to the ground.
b. A mechanical arrangement incorporating the lidar transmitter and receiver such
that, it is possible to orient the lidar system at any angles of azimuth and
elevation.
c. A diode pumped, high repetition rate Nd:YAG laser operating at micro joule
energies as laser transmitter. The laser beam is initially expanded and then
collimated to send in to the atmosphere.
d. A high gain, low dark count PMT as photon counting device for detecting the
laser-backscattered photon returns. Good dynamic range (in terms of range) is
achieved by employing high repetition rate low pulse energy concept with PMT
based photon counting technique.
e. A single card PC based multi-channel scaler, which provides high range
resolution height profiles. At present 30m (200 nsec is the dwell time) is range
resolution is employed in the prototype lidar system. It is also possible to
operate the system with 15 m range resolution (100 nsec is the dwell time).
f. A narrow receiver field-of-view achieved with fine focal aperture, which made
the possibility of operating the lidar system even during daytime on continuous
basis in an unattended way.

Statement of Invention
A boundary layer micro pulse lidar system according to the present invention comprises a laser head to generate a laser source, a beam expander to expand the laser beam output to achieve eye safety, the expanded laser beam being projected into the atmosphere through mirrors positioned at 45" angles, a telescope to receive the backscattered light, a Photo Multiplier Tube (PMT) as a detector system for single photon counting, a data acquisition unit and analyzer for acquiring and processing the signal from the detector to profile aerosols/particulates and atmosphere clouds, wherein the said laser head comprises a laser diode, a Nd doped Yttrium-Aluminium-Garnet crystal (Nd:YAG) which generates the laser, an acoustic opto-modulator (AOM), potassium tri-phosphate (KTP), which is a crystal that generates wavelength at the second harmonic of YAG resonating wavelength and other output optics.
BRIEF DESCRIPTION OF THE DRAWINGS
Figure 1 shows the schematic diagram of Micro Pulse Lidar (MPL) system of the prior
art.
Figure 2 shows the schematic diagram of the Micro Pulse Lidar (MPL) system of the
present invention.
Figure 3 laser output laser pulse energy variations with respect to repetition rate of laser
pulse.
Figure 4 shows the detailed schematic block diagram of the Boundary layer lidar system
Figure 5 shows a Schematic diagram of the boundary layer
Figure 6 shows the BLL observation of atmospheric backscatter profile recorded at
08:51 LT (during daytime) over Gadanki site.
Figure 7 shows the Diurnal variation of range-squared signal shows the typical ABL
evolution
Figure 8 shows the temporal evolution of Entrainment zone depth
Figure 9 shows the BLL data showing atmospheric oscillations at the top of the ABL.
Figure 10 shows the diagram of the formation of gravity waves
Figure 11 shows height profiles of lidar extinction, water vapor mixing ratio,
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atmospheric temperature, wind speed and wind direction
Figure 12 shows BLL observation of elevated multiple layers of aerosol on two
separate dates
Figure 13 shows Typical height profile showing (a) Basic photon count observed
from BLL system (b) Corresponding range squared corrected signals indicating
backscattered signals from (a) cloud and (b) aerosols.
Figure 14 shows Derived height profiles of aerosol backscattering coefficient from
BLL lidar backscatter returns. The height profiles provide total backscatter, molecular
and aerosol backscatter coefficients.
Figure 15 shows Typical height profile of aerosol observed from BLL system over
Gadanki. The shaded lines show the variability in the observed aerosol profiles
Figure 16a shows BLL observation of long range transported aerosol
Figure 16b shows A 7 day HYSPLIT back trajectories showing the air mass advection
from Far East
Figure 17a shows Lidar derived mean height profiles of aerosol extinction observed
over Gadanki during a certain period.
Figure 17b shows A comparison of aerosol extinction and water vapor mixing ratio
profiles showing the hygroscopic nature of boundary layer aerosol
Figure 18 shows MODIS satellite observation of dust storm over Indus-Valley
Figure 19 shows ARIES-NARL Lidar observation of dust storm above Manora peak,
Nainital. a) Temporal variation of range squared signal b) Lidar derived backscatter
ratio during the period 2200 - 2300 LT
Figure 20 shows BLL system observation of low-level cloud during a light rain
period.
Figure 21 shows BLL system observation of low-level cloud during a light rain time
over Gadanki.
Figure 22 shows BLL observation of middle level cloud over Gadanki site
Figure 23 shows BLL observation of high altitude clouds over Gadanki.
Figure 24 shows BLL system observations of high altitude clouds during winter period
over Gadanki
Figure 25 shows Wave generated K-H bellows in cirrus layers observed by BLL over
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Gadanki site.
Figure 26 shows BLL observation of multiple clouds occurrence in the atmosphere
Figure 27 shows Comparison of BLL cloud top and MODIS cloud top in terms of
pressure
DETAILED DESCRIPTION OF THE INVENTION
Figure 1 shows the conventional MPL system developed by Spinhirne. This system comprises a laser head (100), which is a high repetition rate, low pulse energy laser source operating at green wavelength. A laser head controller (10!) is provided to controls the output green laser wavelength repetition rate and energy. A transmitting optics arrangement (102) is provide to subject the laser source output beam to pass through several optical elements like beam expansion optics, half wave plate, beam steering optics, focusing lens, depolarizer (103) and finally achieves eye safety and delivers through telescope into the atmosphere. A telescope (104) is a 200mm diameter cassegrain telescope. The filter (105) is an optical narrow band filter and the detector (106) is an avalanche photo diode (APD) detector. A digitizing/MSC unit (107) is provided for digitizing the APD signal for range gating. A computer is provided for processing data archived and for further processing.
The above-mentioned prior art MPL system developed by Spinhirne has drawbacks, which were mentioned earlier. The present invention overcomes these drawbacks by providing a novel MPL system that will be described below with reference to Figures 2 to 4.
The MPL system according to this invention comprises a laser head (200) situated on one side of a telescope (201). The laser head (200) comprises a laser diode (LD), a Nd doped Yttrium-Aluminium-Garnet crystal (Nd:YAG) which generates the laser, an acoustic opto-modulator (AOM), Potassium tri-phosphate (KTP), which is a crystal that generates wavelength, the second harmonic of YAG resonating wavelength and other output optics. The laser is guided into the atmosphere using two mirrors (M) kept at 45° angles. A beam Expander (202) is provided to expand the laser beam output to achieve
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eye safety. The laser beam diameter is 0.4 mm and its divergence is less than 1.5mrad. The laser beam was expanded using the beam expander (202) to 3 mm in diameter and collimated to have the beam divergence of about 200 urad. The eye safety is achieved with this beam divergence for the heights above 150m from ground. A thenno electric cooling device (206) is provided adjacent to the laser diode (LD) to control the heat dissipated from the laser head (200).
The laser backscattered due to contact with molecules, water droplets, ice crystals, dust and haze aerosol in the atmosphere is received by a telescope, preferably, a cassegrain telescope (203) as shown in Figure 2 and 4. The diameter of the telescope is 150mm and its F-value is 9. The geometrical form factor for a coaxial lidar having no apertures (other than the objective lens or mirror of the telescope) or obstructions is unity, provided the divergence angle of the laser beam is less than the opening angle of the telescope (Measures, 1984). Therefore, an iris (204) having a diameter (pin-hole) of 0.5mm is used to obtain a field of view of about 400// rad. A narrow band interference filter (205), whose center-wavelength and bandwidth were 532mm and 0.5nm respectively, was used to reduce background light. The filter used in this system is not temperature controlled, hence as specified it needs to be maintained at a temperature of about 22°C. For this reason, the lidar system is kept in a temperature controlled cubical.
In the system according to the present invention, a Photon Multiplier Tube (PMT) is used as a detector system. The PMT is positioned behind the Interference filter (205). In the prior art systems, Avalanche Photodiode (APD) were used as detector operating in photon counting mode. Since prolonged operation of APD in the breakdown region can potentially damage the diode, hence it requires avalanche quenching circuitry. However, the PMT used in the present invention are simple systems for use in the single photon counting applications. Because of their inherently high gain and fast response it makes them ideal for single photon counting. Moreover, their noise performance is much better compared to APDs. Also, the PMTs do not require any pulse effect corrections.
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The data acquisition unit used in the lidar system of the present invention is a PC based multichannel analyzer (EG & G Ortec model MCS pci). A Multi Channel Scalar (MCS) records the counting rate of events as a function of time. When a scan is started, the MCS begins counting input events in the first channel of its digital memory. At the end of the pre-selected dwell time, the MCS advances to the next channel of memory. At the end of the pre-selected dwell time, the MCS advances to the next channel of memory to count the events. This dwell and advance process is repeated until the MCS has scanned through all the channels in its memory. A display of the contents of the memory shows the counting rate of the input events versus time. In repetitive measurements, where the start of the scan can be synchronized with the start of the events, multiple scans can be summed to diminish the statistical scatter in the recorded pattern. The MCS-pci can profile counting rates versus time, or it can function as a multiple-stop time digitizer for measuring flight times of multiple photons or particles. Typical applications are in time-resolved single-photon counting. It can count rates up to 150 MHz or 65,536 time bins (channels). Dwell times selectable from 100 ns to 1300 s/channel. It can do single scan once, or average repeated scans. There is no dead time between channels and no double counting. There is no end-of-scan dead time. In the lidar the instrumental bin width was normally set at 200 ns, which corresponding to an height resolution of 30 m. Usually 300000 shot integrated photon count profile constitutes a raw data profile that corresponds to a time resolution of 120 sec.
Application to atmospheric boundary layer studies
The boundary layer is that part of the troposphere that is directly influenced by the presence of the earth's surface, and responds to surface forcings with a timescale of about an hour or less (Stull, 1988). These forcings include frictional drag, evaporation, heat transfer, pollutant emission, and terrain induced flow. The processes occurring in this region greatly affect the lives of mankind. In this region, man spends most of his life in. Figure 2.1 shows the schematic diagram of the boundary layer (Piiromen, 1994). The boundary layer formed from sunrise to sunset is characterized by the formation of thermal plumes from solar heating. These plumes carry with them moisture that
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contributes to the formation of convective clouds. Solar heating is also responsible for the transport of heat and aerosols and thus mixing occur in this time frame. From sunset to sunrise, the boundary layer is characterized by a stable layer formed when the solar heating ends and radiative cooling starts to occur in the lowest part of the atmospheric boundary layer. Above the stable layer is the residual layer, which is the remnant of the daytime boundary layer.
The Atmospheric Boundary Layer (ABL) thickness is quite variable in space and time, ranging from hundreds of meters to a few thousand meters. It is practice in air pollution meteorology to use the term mixed layer (ML) since pollutants that are emitted into the ABL become gradually dispersed and mixed through the action of turbulence. The mixed layer depth (MLD) is the height of the top of the ML and is an important parameter to characterize the ABL and its structure. In some papers, the MLD is also called the mixing height (Seibert et al., 1998; 2000). Measurements, parameterizations and predictions of the MLD have many theoretical and practical applications such as the prediction of pollutant concentrations, in numerical weather prediction and climate modeling (Seibert et al., 1998; 2000) and the study of turbulence in ABL. Different mathematical methods were reported to determine or estimate the MLD (Endlich et al., 1979; Melfi et al., 1985; Boers et al., 1988; Hooper and Eloranta, 1986; Senff et al., 1996; Hayden et al., 1997; Flamant et al., 1997; Menut et al., 1999; Cohn and Agevine, (2000); Brooks, 2003). During daytime, radio soundings are the most common data source to retrieve the MLD based on wind, temperature and pressure profiles but in most stations regular launches are made only twice a day at predetermined synoptic times (0000 and 1200 UTC). Consequently, radio soundings can often be used as a reference for comparison with modeled ABL heights only around midnight and noon. However, nighttime measurements of ABL are still a gap region to modelers, which can be compensated from lidar soundings. Active remote sensing systems such as lidars use aerosols as tracers of the ABL dynamics (Bhavani Kumar, 2006). The optical power measured by a lidar device is proportional to the aerosol content of the atmosphere. The lidar signal shows a strong backscattering within the ML, which decreases through a transition zone and becomes weak in the free troposphere (FT).
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These contrasts are the basis for the lidar estimations of the MLD. Under convective conditions, the ABL can be divided into three different layers: the surface layer (SL), the mixed layer (ML) and the entrainment zone (EZ). The latter represents a transition zone between the ML and the stable FT above. Usually, the EZ is defined in a horizontal average sense (Deardorff et al, 1980). It is important to distinguish between the instantaneous (or local) MLD that varies between the EZ top and the middle of the ML and the average MLD which is the middle of the EZ (Stull, 1988). Ground-based aerosol lidars give a high-resolution picture of the instantaneous ML top that is marked by a large contrast between the backscatter signal from aerosol-rich structures below and cleaner air above.
Method of Interpretation of Lidar data
The Lidar System according to the invention emits a short laser-pulse at given wavelength and given duration into the atmosphere. The laser pulse is absorbed and scattered by atmospheric molecules through Rayleigh-scattering and by particles through Mie-scattering. The backscattered fraction of light within the field of view of the coaxial telescope is collected and transmitted to the detector. The use of the lidar technique for boundary layer ranging relies on the altitude resolved measurement of atmospheric backscatter intensity from outgoing laser radiation. The backscatter intensity measured by lidar is proportional to the signal backscattering by particles and air molecules present in the atmosphere. The expression that relates laser energy output (Eo ) and the backscattered signal P(r), in the case of a backscatter single wavelength lidar is given by (Bhavani Kumar et al., 2007)

P(r) =

he,

[j3{r)\^T:{r)\nYNHRAt (1)

where
P(r) = Expected number of photons detected in the range interval (r-Ar/2, r+Ar/2)
E0 - Laser energy (Joules)
h = Plank's constant (6.63 x 10 34 J s-1)
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c = Velocity of light (3 x 108 m s"1)
X = Lidar operating wavelength (532 rnn)
p(r) = backscatter cross-section (m 1 s r 1)
AR = Receive telescope area
R = laser pulse rate (s"1)
Ta(r) = One-way transmission of atmosphere
rj = Efficiency of lidar system
N/j = Expected photon count per range bin per pulse due to background noise
and dark counts from detector A / = Integration time (s)
The first term in Eq (1) gives the number of transmitted laser photon within the time bin length of At. The second term provides the probability that a transmitted photon backscattered by scatterers in to a unit solid angle at angle of it (180°). The term refers to the volume backscatter coefficient of the atmosphere, which relates to the amount of photons that would be backscattered when an atmospheric thickness is crossed by the laser pulse length of r and is given in units of'nf'sr1'. This is equal to the product of the effective backscatter cross-section of scatterer, their number density and the scattering layer thickness. The third term indicates the probability that a scatter photon collected
by the receiving telescope i.e., by the receive aperture to the scatter. The term —f- is the
r'
solid angle of the receiving telescope aperture relative to the scattering source. This term gives the geometric fraction of the surface of a sphere with radius r for which the backscattered photons can be colleted by the telescope with the aperture area Q?AR units. The fourth term informs the amount of atmospheric transmission during the light transmission from laser source to distance r and from distance r to receiver. The term Ta(r) is the one-way transmittance of the atmosphere. The fifth term provides the optical efficiency of overall lidar components e.g., steering mirror, collimating lens, optical IF filters etc. The sixth term is the expected photon count per range bin per unit due to background and detector noise. It is the number of photons detected from the background including any light sources other than the emitted laser light such as
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airgiow emission, star light, and photo detector dark counts. The dark count is caused by spontaneous emission of photoelectrons from the cathode in the PMT. The expression (/) can be simplified as:
P(r) = Ps (r)+PN (2)
where the measured signal intensity P(r), given in terms of photon counts, corresponds to signal counts Ps(r) due to the atmospheric backscatter (comes from air molecules and aerosols) and counts P^fr) due to sky background signal. Usually the lidar signal is usually background signal corrected and transformed into a variable that removes the
range square —- dependence X(r) [Fernald, 1984] or its logarithm, S(r) [Klett, 1985]. r~
X{r) = [p{r)-PN]xr2 (3)
S(r) = In X(r) - (4)
Identification Of ABL Height Front Lidar Data
A general reliable method than that of the existing art involves the direct comparison of the backscatter signal with a fitted model Rayleigh backscattering profile. The boundary layer height can then be defined as the first altitude point for which the measured backscatter profile exceeds the Rayleigh model profile by some fixed amount, ij>. Following Melfi et al. [1985], $ is chosen to be 25%, although it is noted that the boundary layer height retrieved by this method is not particularly sensitive to reasonable values of >. The lidar backscatter profiles recorded are shown in Figure 6a and 6b respectively. The Rayleigh model is fitted between the indicated altitudes.
APPLICATION TO ABL STUDIES
To demonstrate the performance of the lidar system for ABL evolution, a continuous 15-day observation was conducted at Gadanki site (dry season) during clear sky period.
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The variation of range corrected signal S(r) over a diurnal period (24-hour) is shown in Fig. 7. Fig 7 provides the time evolution of lidar range squared signal S(r).
Over land surfaces, the boundary layer has a well-defined structure that evolves with the diurnal cycle [Stull, 1988]. From Fig. 7, one can observe the mixed layer (ML) development from the lidar signal enhancement during 06:00 to 15:00 LT. The mixed layer (ML) or Convective Boundary Layer (CBL) is formed by the convection that arises from solar heating of the earth's surface and is associated with organized thermal transport due to highly developed vertical motion. Soon after the sunset buoyant (eddy) production ceases and the atmosphere changes to a near neutral condition. A weak residual layer containing the remnants of the daytime mixed layer is observed in the background after 18:00 LT. In the late night hours, as the surface gets cooled the elevated eddies are directed downwards. This causes formation of a stable boundary layer (SBL) near to the surface. This is seen at late evening hours over land surfaces. The intensified signal returns in late evening hours clearly show the formation of SBL. The SBL has been observed to form in the height range of 200 to 600 m during dry season due to low night temperatures. Thus, the portable lidar system has been successfully used to study the structure, dynamics and the evolution of Atmospheric Boundary Layer using aerosols as atmospheric tracers. Using the portable lidar system, the internal sub-layers of ABL such as Mixed Layer (ML), Residual Layer (RL) and Stable Boundary Layer (SBL) or Nocturnal Boundary Layer (NBL) have been identified clearly over the tropical rural site Gadanki. This result shows the feasibility of the low-pulse energy lidar system application in atmospheric boundary layer measurements.
ENTRAINMENT ZONE (EZ) DEPTH
The top of the BL is generally associated with a transition region from rather polluted air in the ABL to mostly clean air in the free troposphere, hence a strong negative gradient in lidar signal is observed at the transition region. The vertical gradient D(r) of the lidar signal [Endlich et al., 1979] is expressed as
DW=*±iL*M] (5)
dr dr
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Figure 8 indicates the temporal variation of lidar signal S(r) and its first derivative D(r) from 06:00 to 18:00 Hrs LT on 6 Jan 2005. Figure 8 demonstrates that this transition region is clearly detected from lidar signals, with high temporal and spatial resolutions. The method of using D(r) provides a precise description of the boundaries between air masses identified by difference in the aerosol content. Positive gradients of X(r) correspond to increasing backscatter with altitude, and vice versa. A strong decrease with altitude in the aerosol concentration is expected at the entrainment zone [Endlich et al., 1979]. The largest negative vertical gradient in the lidar signal, associated with a sharp transition from "polluted" to "clean" air, can be used to identify this layer. The Entrainment Zone is the region of statically stable air at the top of the ML, where there is entrainment of free-troposphere (FT) air downward and the overshooting thermals upward. From the Figure 2.4 (b), one can observe that the entrainment is clear during early hours of morning and also late afternoon hours. But during strong convective period, from 09:00 to 15:00 hrs LT, the Entrainment zone appeared to be patchy. This is clear that during this time the upward thermals are strong and thus preventing air entering from FT in to BL.
OSCILLATION IN BOUNDARY LAYER
Figure 9 shows an example of an oscillating aerosol layer observed by the BLL system. The oscillation that produced the buoyancy waves was observed to start from 17:00 to 21:00 local time. In this time range, the period of oscillation is found to be equal to 12 min.
THEORY BEHIND THE ATMOSPHERIC OSCILLATIONS
A stable environment supports buoyancy waves. Atmospheric oscillations or sometimes called gravity waves are a common occurrence in the atmosphere and are formed due to shear instability, as shown in Fig 10 frontal acceleration, air flow over orographic regions, or geostrophic adjustments (Hertzog et al., 2001). For example, when clouds are present in the atmosphere, they act as obstructions to the flow of air. The waves are generated when air flows over the top of these obstacles, which is, in this case, a mountain. Clouds too are obstacles and can trigger gravity waves.. In the presence of fronts, waves can occur in pre- and postfrontal zones (Stull, 1988). Waves
22

are also formed within the entrainment zone. Hooke and Jones (1986) observed these undulations by acoustic echo sounders. These waves have wavelengths that are equal to the diameter of the thermals and are quickly absorbed in the mixing layer.
OBSERVATION OF AN ELEVATED BOUNDARY LAYER
At nighttimes the PBL tends to be lower in thickness while during the day it tends to have a higher thickness. The two reasons for this are the wind speed and thickness of the air as a function of temperature. Usually, the formation of PBL is dominated more by advection and thermal energy budgets than levels above the PBL. The earth gains most of its energy and losses most of its energy from the surface. It is warmed through solar heating and cooled through long-wave radiation emissions. Strong wind speeds allow for more convective mixing. This convective mixing will cause the PBL to expand. It can warm up significantly during the day and cool at night while the rest of the atmosphere stays at a fairly uniform temperature. At nighttime, the PBL contracts due to a reduction of rising thermals from the surface. Cold air is denser than warm air, therefore the PBL will tend to be shallower in nighttimes. An interesting observation of elevated boundary layer during nighttime has been observed over Gadanki on the night of 23 July 2006. This observation was supported by lidar and GPS data. Usually the determination of the depth of the PBL is done using the radiosonde data. The top of the PBL is often marked with a temperature inversion, a humidity gradient, and change in wind speed and/or a change in wind direction. However, there will be an abrupt change in air mass at the top of the boundary layer. This information will be obtained from the lidar data.
The Fig 11 shows the lidar extinction profile, GPS sonde derived temperature, water vapor, wind speed and wind direction. It can be seen from Figure that the structure of aerosol distribution is closely related to the structure of atmospheric water vapor. It shows clearly that the distribution of aerosol is hygroscopic in nature.
MULTIPLE LAYERING PHENOMENA
Multiple layers are easily identified in lidar scans by the horizontal stratification that is always present. The figure shows a thin stratified aerosol layer above NBL. The Above
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the thin aerosol layer a constant haze layer is also seen. Fig. 12 shows the layers of aerosol layers formed at height between land 3 km. Each aerosol layer represents a different aerosol concentration. The existence of these layers shows primarily that the atmosphere is stable. It can also be seen that the layers are oscillating in time.
APPLICATION TO AEROSOL STUDIES
INTRODUCTION
Our atmosphere consists of two different types of microscopic scatterers, namely the molecular (Rayleigh scatterers) and the particulates or aerosols (Mie scatterers), based on the optical effects observed in the atmosphere. Rayleigh scatterers have dimensions that are small compared to the interactive wavelength of light, while Mie scatterers have dimensions that are comparable or even greater than the wavelength of the interactive light. On the average, Rayleigh scatterers have dimensions of the order In the field of laser remote sensing of atmosphere, the most common type of lidar system is the elastic Mie scattering lidar. These lidars rely on the fact that Mie
scattering have large scattering cross section of about —-s 10~8cm2sr~x. This means
that even low concentrations or changes in the concentrations of dusts or aerosols can easily be detected (Measures, 1984). This makes this system ideal for continuous atmospheric monitoring. In general, Mie scatter lidars are utilized to monitor aerosols and to detect cloud base height and diurnal variations of the atmosphere. With a fine temporal resolution, a continuously operated lidar can easily detect real-time aerosol dynamics in the atmosphere.
It is well known that vertical layering of aerosols is vital in the atmospheric remote sensing [Franke et al., 2003]. The present emphasis is on the observation of physical
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properties of the atmospheric boundary layer aerosols and elevated aerosol layers which are most relevant to radiative forcing applications. In the lower troposphere, aerosols come from a number of sources and they cause visibility degradation when they are located in the boundary layer. Widespread surface sources include biogenic sources, volcanoes, deserts, oceans and fresh water, crustal and cryospheric aerosols, and biomass burning. This is commonly experienced during dust events (Husar et al., 2001; Sugimoto et al., 2003). Biofuels such as wood fuel, dung, and crop waste are the primary contributors of aerosols in rural areas of India (Habib 2004). In addition to ubiquitous soil and sea salt aerosols (coarse particles), urban aerosols have anthropogenic origins such as dust from paved and unpaved road (coarse particles), primary or secondary particles from exhaust of automobile combustions and industrial processes (fine particles). Aerosol emissions in urban regions result from fossil fuel sources such as diesel and kerosene mixed-fuel vehicles, industrial processes, and coal-fired power generation (Dickerson 2002, Prasad 2006). Industrialization and population expansion with agricultural and fuel-use practices induce high aerosol loading over the NIS (Lelieveld 2001, Prasad 2006). However, the anthropogenic influence is not entirely responsible for persistent aerosols. Meteorological phenomena induced by local geography (i.e., Himalayan Mountains and the Indian Ocean) exacerbate smog conditions by prolonging the lifetime of aerosols and clouds to further impact climate. Since the beginning of the age of industrialization, the increase of human and industrial activities has raised the production of aerosols released to the atmosphere. In the context of urban air pollution, the terminology of suspended particulate matter (SPM) is used to specify aerosols causing human health problems. These are solid or liquid particles with varying sizes (less than 10 mn) and shapes whose origins are often ascribed to industrial activities. Exposures to these fine particles, especially particulate matter smaller than 2.5 um (PM 2.5), can result to serious respiratory disorders (Abbey et al. (1999), Samet et al. (2000), Pope et al. (2002)). Atmospheric aerosols can also impact the local and regional radiation heat budget. Black carbon aerosols absorb incident solar radiation and heat the atmosphere more effectively than dust, sulfates, and organic carbons, which reflect more radiation and cool the atmosphere. However, the scientific understanding of the aerosol radiative forcing magnitude remains very
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low (Albritton and Filho 2001). Although the lifespan of aerosols is about one week, persistent meteorological phenomena and anthropogenic activities can provide an environment of greater climate impact (Rasch 2001). As part of understanding the effects of aerosols on human lives and the environment, it is also equally important to study the optical properties and dynamics of aerosols in the boundary layer so that pollution transport and local atmospheric variation can be clearly revealed.
Because aerosols play an important role in radiation budget of the atmosphere, quite a number of studies were conducted on this area (Holben et al. (1998), Dammann et al. (2002), Rajeev and Ramanathan (2002), Birmili et al. (1999) and Langmann et al. (1998)). By knowing the optical properties of the aerosols, we will be able to understand whether aerosols aid in the cooling or heating process (Kandel, 1999) of the atmosphere. To achieve this, altitude profiling of aerosol are required on long-term basis. It is possible with the elastic backscatter lidars. However, to obtain aerosol height distribution on long-term basis in an un-attended way needs Micro pulse lidars (Spinhirne, 1993; Bhavani Kumar, 2006), because big pulse lidars cannot be operated for longer periods. The Figure 13 shows a typical backscatter and its range squared signal from micro pulse based lidar system such as BLL, which is capable of providing the altitude distribution of aerosol and cloud information through the troposphere from ground up to tropopause within a time sampling of 10 minute.
LIDAR SIGNAL INVERSION - HEIGHT PROFILES OF AEROSOL
The lidar equation for return signals due to elastic backscatter by air molecules and aerosol particles can be, in its simplest form, written as (Fernald et al., 1972)

*(*)
PkhPtA^faMzV^e
z2 ■ — ' 2

l\a(z)ck
(6)

where P(z), is the lidar signal received from a range z at a wavelength A, PL is the emitted laser power , A0 is the telescope receiving area, ^(z)is the receivers spectral transmission factor, /3(z) is the atmospheric volume backscattering coefficient, £" (z) is the overlap factor between the field of view of the telescope and the laser beam, a (z)
26

is the extinction coefficient from atmospheric molecules and particles, c is the speed of light and r, is the laser pulse length. The lidar data have been processed with the algorithm described by Fernald [1984] and Klett [1985], and assuming to know the molecular (or Rayleigh) contribution to the signal from the CIRA 1986 standard Atmosphere. According to the Fernald - Klett formulation, the solution of the lidar equation is given by the equation (7)
M = exp{-fe)-S(z)]} (7)
-p[7)+l ]*exPB"*.M(z)]}
where, the total backscattering coefficient is the sum of Rayleigh and particulate contribution given by /?(z )=/?om, (z ) + /?„,„, (z), the /?(z0)is the boundary condition set
on /?(z)at the reference far-end range. The detail of the numerator is given in equation (8). The typical height profile of aerosol backscatter derived is shown in Figure 14
S(z0)-S{z)= ln[z()2 * P(z0)]-ln[z2 * p(z)]-± £?*Mt (8)
To derive the above equation, several assumptions were made
(a) The backscatter-to-extinction ratio is known,
(b) The reference range (z-ref) is taken in a region where the lidar profile followed the molecular atmosphere (generally between 6 and 8 km).

(c) The value of /?(z) at the reference range was accordingly chosen as been equal to pm(z) (no aerosol contribution).
(d) The value of C was chosen as been equal to 0.035, which is an average value among those suggested for rural, urban and maritime aerosols (Kovelev, 1993).
OBSERVATION OF BOUNDARY LAYER AEROSOL OVER GADANKI
Using the BLL system, the height profiles of lower troposphere aerosol, at 532 nin wavelength, have been derived up to an altitude of 7 km using the standard Klett
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technique from BL lidar nighttime data. These are the first observations of the lower troposphere aerosols taken from the Gadanki site. The following observations are bought out from the preliminary analysis of the data. (1) Most of the time, a thick aerosol layer will always be present in the lowermost troposphere which top 1.5-2.0 km above ground level, as shown in Figure 15. We consider that the layer correspond to the local mixing layer. Parameswaran et al. [1997] from an observational study of night time aerosol concentrations in the lower atmosphere at a tropical coastal station reported that accretion of aerosol occurs in a stable atmosphere sandwiched between two turbulent regions which are normally observed within the boundary layer.
OBSERVATION OF LONG-RANGE AEROSOL
It is well recognized that the lidar measurements are valuable in aerosol studies in that they potentially supply vertical information on aerosol optical properties. Lidar profiles sampling the air mass advected from the Indian sub-continent show multiple layers present over the Indian ocean as a result of convection and long range transport of aerosol originating from arid and semi arid regions of the world [Muller et al., 2001; Ramanathan et al., 2001]. At times the air mass shows 3 km deep pollution layer above the boundary layer [Ansmann et al., 2000].
Such elevated aerosol plumes were also observed over the northern India and also over southern India, particularly during winter season due to dry convective lifting of pollutants at distant sources and subsequent horizontal upper air long range transport [Ratnana et al., 2004; Niranjan et al., 2007]. Figure 16(a) shows the aerosol backscatter coefficient as a function of altitude for. It may be seen that a thin aerosol layer was observed from an altitude of 2 to 3 km indicating high altitude aerosol layers. The aerosol layers found above the boundary layer could be transported several thousands of km without significant removal and can contribute significantly to the column aerosol optical depth, at times more than the boundary layer [Franke et al., 2003]. The observation shown above indicate that the air mass advecting over the eastern coast of India into Bay of Bengal not only contains the anthropogenic contribution from the
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Indian sub-continent but also at times contains the plumes advecting from far east regions with significant contribution of biomass aerosol. Figure 16(b) indicates a 7 day HYSPL1T back trajectories showing the air mass pathway from far east region. It is considered that these layers were on the way of long-range transport. Such type layers are ejected in to the free troposphere by distant sources and get transported globally.
HYGROSCOPIC NATURE OF AEROSOL
The lidar system detects the laser-backscattered radiation from air molecules and aerosols up to about 3 km during daytime and up to about 17 km during night. Using the system, temporal variation in the vertical profiles of aerosols over Gadanki is being studied.
Fig. 17(a) shows the average monthly aerosol extinction profiles measured over Gadanki. During winter months the aerosol height profiles are limited to 2 km altitude region where as one can observe an increase in aerosol amount during summer months, pre-monsoon period, in the 2 to 4 km altitude region. This shows a rise in local boundary layer during summer months. This increase could be probably due to increased hygroscopic growth of aerosol particles in the 2 to 4 km altitude region. This was confirmed by a study of comparison with the GPS sonde derived water vapor mixing ratio with that of the lidar derived aerosol extinction profiles. Figure 17(b) shows the comparison profile, which clearly indicate that the aerosols in the lower atmosphere are hygroscopic in nature.
OBSERVATION OF SCAVENGING OF ATMOSPHERIC AEROSOL BY ARIES-NARL LIDAR AT A HIGH-ALTITUDE STATION, NAINITAL
Recently a portable lidar system was set up at Manora Peak, Nainital (29° 22' N, 79°27' E, 1960 m MSL) by National Atmospheric Research Laboratory (NARL), Gadanki under a joint scientific collaborative programme between ARIES (Aryabhatta
29

Institute for Observational Sciences), Nainital and NARL, Gadanki. The lidar system was operated on two days on 16 and 17 May 2006. During the observations the lidar system has collected backscatter returns from the lower atmospheric aerosol and also high altitude cloud such as cirrus. These are the first observations taken from a high altitude location in the northern part of India. The height distribution of aerosol was seen up to the height of 3 km above the ground level.
The high altitude clouds were observed in the height ranging between 8 and 11 km. During this period, an interesting feature was noticed that there was a drastic reduction in the height distribution of aerosol particles due to scavenging process caused by heavy rain on 17 May 2006, consequently a significant reduction in AOD was also observed. The AOD obtained on 16 May 2006 was about 0.208 at ^.=0.532 urn, which showed a significant reduction to a value of 0.08 on 17 May 2006 due to the scavenging effect in the height range of 0.2 to 3.0 km.
OBSERVATION OF DUST STORM IN THE INDUS VALLEY
A large dust storm blew through the Indus Valley, along the border between Pakistan and India, on June 12, 2006. The Moderate Resolution Imaging Spectroradiometer (MODIS) flying onboard NASA's Aqua satellite captured this image the same day. In this picture (Figure 18), the dust heads toward the Himalaya Mountains in the top right corner of the image. In the lower-left corner of the image, sprays of clouds appear to blow in the same direction as the dust, away from the Arabian Sea and toward the northeast. Once the dust reaches the mountains, it changes direction and blows along their southern edge. Aerosol emissions originating from the Northern Indian Subcontinent (NIS) region have a significant effect on local and regional climate. Using ARIES-NARL lidar an aerosol analysis was conducted over the Manora peak , Nainital during June 2006 (Figure 19). The lidar analysis shows the dust layer appears above the local boundary layer and peaks at a height of about 1 km AGL. The height distribution extends up to 4 km.
30

This analysis found that 500nm daily average aerosol optical depth (AOD) values as high as 1.6 AOD consisted primarily of fine mode particles during the dry monsoon phase, while the wet monsoon phase produced much lower 500nm daily average values near 0.1 AOD with coarse mode particles. Satellite imagery and retrieved data, back trajectory analyses, and model output provided supporting information on aerosol source regions, transport, and chemistry.
PROFILING ATMOSPHERIC CLOUDS
INTRODUCTION
Research in the last few years has shown that clouds are a major variable in Earth's climate system. The climate is very sensitive to small (10-30 percent) changes in clouds. This sensitivity is very important in trying to figure out what man-made changes, such as increasing carbon dioxide, will do to Earth's climate. Depending how these changes affect clouds, their influence on climate could be either enhanced or damped out. We all know that the energy to the Earth comes from the sun. An equal amount of energy must go back into space or Earth's temperature will change. The picture shows that clouds make the biggest contribution to reflected energy (think about how bright clouds can appear ~ this is reflected energy), and also to emitted heat (clouds act like radiators in the atmosphere ~ though much colder than a radiator in a building). You can think of clouds as one thermostat that sets Earth's temperature. If. for example, you increase the average thickness of low clouds a little (i.e., make them more reflective), the Earth's temperature will decrease a little. The effect is as if you turned down the thermostat a little. We have learned that clouds can act to either warm or cool the Earth. High clouds are often thin and not very reflective. They let lots of the sun's warmth in. They also sit high in the sky, where the air temperature is quite cold; so they do not emit a lot of heat. On balance, high clouds tend to warm the Earth. Low clouds are often quite thick and reflect lots of sunlight back to space. They are also lower in the atmosphere where the air is warmer so they emit more heat. On balance, low clouds tend to cool the Earth.
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Hence, it essential to build a regional or global cloud climatology to properly access the affect of clouds on the Earth's radiation budget, in order to do it so, we need instruments on different platforms.
One of the potential applications of lidar is in the studies of characterization of clouds. Usually strong backscattering arises from clouds due to relatively large scattering cross-sections of cloud particles and huge number density of scatterers. Lidar has the capability to delineate the position and spatial structure of clouds in the atmosphere. Because of its high spatial resolution, it can be used to locate cloud base and its top with a good precision that is not possible with any other remote sensing technique. This feature of lidar has attracted more researchers recently in investigations of clouds at various height regions as clouds play a critical role in the radiation budget of the earth atmosphere (Liou 1986). The recent reports on backscatter lidar employed in studies of upper tropospheric clouds (Nee et al. 1998; Winker and Trepte 1998; Piatt et al. 1998; Boehm and Verlinde 2000; Bhavani Kumar et al. 2001; Roumeau 2000) indicate the potential application of the instrument in cloud research..
OBSERVATION OF RAIN CLOUD USING BLL
The formation of clouds comes from rising air parcels. Air rises because of orographic lift, convective lift, and convergent lift. Orographic lifts are formed when air is forced to move upward as it encounters a cooler, denser body of air or when it meets high landforms such as a mountain. Convective lift occurs when air encounters a warm surface, heats up and becomes less dense compared to the surrounding air. This usually occurs during daytime when the sun drives the convective circulation on the ground. Convergent lift occurs when air is whirled toward the center of a cyclone colliding with itself and is forced to move upward. This is the mechanism that drives air upward during storms or tornados. As air parcels rise, they cool down and condense. Rain clouds occur when the clouds start to produce moisture. Examples of these types of clouds are the nimbostratus and the cumulonimbus. Nimbostratus clouds are layered, uniform, rain clouds. These clouds are generally dark, and associated with large areas of continuous rain. In the case of cumulonimbus clouds, they are formed by the upward movement of warm air currents. These types of clouds are accompanied by
32

compensating downdrafts of cold air and are common in warm and humid weather (Wallace and Hobbes, 1977). For raindrops to occur there must be particles in the air, such as dust or aerosols, at temperatures above freezing.
When particles are cooled below the freezing temperature, water condenses on them. As this happens, the particles become heavy and start to fall. Rain clouds can also be formed locally when air rising over a moist area causes the formation of cumulus clouds. As the moisture condenses, the clouds begin to grow darker. The movement of these rain clouds can be tracked using radar (Crane, 1996). Figure 20 and 21 show the streaks formed from falling raindrops. By measuring the vertical displacement of the streak and the time for the streak to cover the vertical displacement, the average speed of the streaks can be estimated. The value of the average speed can then be used to approximate the raindrop size.
OBSERVATION OF MIDDLE LEVEL CLOUD
The occurrence of clouds above 6 km and below 10 km is very rare. Here is a typical observation of cloud appearing in height range between 6 and 10 km, depicted in Figure 22. These clouds generally formed from the remnants of rainy cloud tops, which appear at heights between 3 and 5 km.
DETECTION OF HIGH LEVEL CLOUDS
The appearance of deep cumulus clouds during convective periods is a common phenomenon at tropical latitudes. Usually the upper portion of convective cumulous clouds constitutes ice that extends in the fibrous anvil, which at a later stage takes the form of cirrus. The formation of high altitude clouds such as cirrus in the tropics plays a particularly important role in the Earth-atmosphere radiation budget. Tropical cirrus develops in a variety of forms, ranging from optically thick anvil cirrus that is closely associated with deep convection (Houze 1993) to optically thin cirrus layers frequently observed near the tropopause (Boehm et al. 1999). Much of the cirrus in the atmosphere, especially in the tropics, arises from mesoscale convective activity (Houze 1993). Moreover, the tropical tropopause being a very cold region with temperatures often reaching as low as 190 K, forms a favorable situation for nucleation and
33

condensation processes, leading to the formation of optical thin cirrus in the vicinity of tropopause (Jenson et al. 1996).
It is well established that lidar has the capability to delineate the position and spatial structure of clouds in the atmosphere. Because of its high spatial resolution, it can be used to locate cloud base and its top with a good precision that is not possible with any other remote sensing technique.
This feature of lidar has attracted more researchers recently in investigations of clouds at various height regions as clouds play a critical role in the radiation budget of the earth atmosphere. The appearance of deep cumulus clouds during convective periods is a common phenomenon at tropical latitudes. Usually the upper portion of convective cumulous clouds constitutes ice that extends in the fibrous anvil, which at a later stage takes the form of cirrus. The formation of high altitude clouds such as cirrus in the tropics plays a particularly important role in the Earth-atmosphere radiation budget. Tropical cirrus develops in a variety of forms, ranging from optically thick anvil cirrus that is closely associated with deep convection to optically thin cirrus layers frequently observed near the tropopause. A typical lidar observation of cirrus system in the night of 11-12 September 2005 is shown in Figure 23. Figure 23 represents the temporal variation of backscatter ratio as a function of altitude for the night observation made on 11-12 September 2005.
Similar type clouds appear during winter period over Gadanki is also shown in Figure 24. The generation mechanism of this type clouds are different from the monsoon generated cirrus clouds.
OBSERVATION OF K-H BELLOWS IN HIGH LEVEL CLOUDS
Another "specialty" cloud is one that can develop due to Kelvin-Helmholtz (K-H) instability waves and sub-harmonic resonance with other waves in the atmosphere. A typical observation of K-H bellows in cirrus clouds observed by BLL system is shown in Figures 25. Atmospheric gravity waves are responsible for formation of such type clouds in the upper troposphere. This can result in an intertwined or spiral cloud pattern as shown. K-H instability is the result of strong wind shear. K-H clouds that form in
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early stages can resemble well-organized waves that appear to be breaking like ocean waves. The high spatial and temporal resolution measurements made the possibility of observing such type of instability structures in the upper troposphere clouds.
Large-scale shear in the upper troposphere could be the probable reason for development of ambient gravity-wave activity.
OBSERVATION OF MULTI-LAYER CLOUDS
Atmospheric clouds appear at different altitudes. Appearance all three different heights of clouds are shown in figure 26.
A high level cirrus is seen around 11 km , a middle level cloud is appeared around 6 km height and a low level cloud occurred on the top of the boundary layer is shown clearly in Figure 26. These clouds occur at multiple altitudes and generally appears above Southern Indian Ocean.
MODIS -BLL CLOUD HEIGHT COMPARISON
A study was carried out jointly by Indian Institute of Technology, Kharagpur and National Atmospheric Research Laboratory, Gadanki for cloud height comparison using BLL system at NARL, Gadanki and MODIS satellite pass data over Gadanki. The MODIS satellite passes over Gadanki at 16:00-17:00 time and also 04:00-05:00 LT. Using the satellite data and the BLL detected cloud height is compared. It is found that the comparison was highly correlated and matched with the MODIS detected cloud heights with BLL detected cloud heights.
From the study it is found that for high-level clouds the difference in cloud top pressures is less than 100 hPa in 80% of the cases and for low-level clouds the difference is less than 100 hPa in 86% of the cases. Linear Regression analysis shows that the slope of the linear fit is very close to unity (0.9975), which means that overall there is only a slight underestimation of cloud top pressure by MODIS.

We Claim
1. A boundary layer micro pulse lidar system comprising a laser head to generate a laser source, a beam expander to expand the laser beam output to achieve eye safety, the expanded laser beam being projected into the atmosphere through mirrors positioned at 45° angles, a telescope to receive the backscattered light, a Photo Multiplier Tube (PMT) as a detector system for single photon counting, a data acquisition unit and analyzer for acquiring and processing the signal from the detector to profile aerosols/particulates and atmosphere clouds, wherein the said laser head comprises a laser diode, a Nd doped Yttrium-Aluminium-Garnet crystal (Nd:YAG) which generates the laser, an acoustic opto-modulator (AOM), potassium tri¬phosphate (KTP), which is a crystal that generates wavelength at the second harmonic of YAG resonating wavelength and other output optics.
2. The system as claimed in claim 1 wherein the said telescope is a classical cassegrain telescope with a diameter of 150 mm and a F-value of 9.
3. The system as claimed in claim 1, wherein an interference filter is provided in front of the photo Multiplier tube to reduce the background light.
4. The system as claimed in claim 1, wherein an iris with diameter of at least 0.5mm is provided in between the telescope and the interference filter to receive a field of view of 400 urad.
5. The system as claimed in claim 1, wherein a thermo electric cooling unit is provided adjacent the laser diode to control the heat dissipated from the laser head.
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(>. The system as claimed in claim 1, wherein the data acquisition unit is a Multi
Channel Scalar analyzer connected to a personal computer.
7. The system as claimed in claim 3, wherein the said interference filter is a narrow band interference filter, whose center-wavelength and bandwidth were 532mm and 0.5nm respectively.

Documents:

http://ipindiaonline.gov.in/patentsearch/GrantedSearch/viewdoc.aspx?id=W9fJ3T/DGSYBT9REmko3Cw==&loc=egcICQiyoj82NGgGrC5ChA==

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Patent Number

271947

Indian Patent Application Number

597/CHE/2009

PG Journal Number

11/2016

Publication Date

11-Mar-2016

Grant Date

11-Mar-2016

Date of Filing

17-Mar-2009

Name of Patentee

INDIAN SPACE RESEARCH ORGANISATION

Applicant Address

ISRO HEADQUARTERS, DEPARTMENT OF SPACE, ANTARIKSH BHAVAN, NEW BEL ROAD, BANGALORE - 560 094.

Inventors:

#	Inventor's Name	Inventor's Address
1	Y. BHAVANI KUMAR	NATIONAL ATMOSPHERIC RESEARCH LABORATORY (NARL), PAKALA MANDAL, GADANKI 517 112.
2	A. JAYARAMAN	NATIONAL ATMOSPHERIC RESEARCH LABORATORY (NARL), PAKALA MANDAL, GADANKI 517 112.

PCT International Classification Number

G01C13/00

PCT International Application Number

N/A

PCT International Filing date

PCT Conventions:

#	PCT Application Number	Date of Convention	Priority Country
1			NA