Laser radar (lidar) provides an excellent tool for characterizing the physical properties of atmospheric aerosols which play a very important role in modifying the radiative budget of the Earth's atmosphere. One of the important issues in lidar research is to derive accurate backscattering or extinction coefficient profiles required for understanding the basic mechanisms in the formation of aerosols and identifying their sources and sinks. Most of the inversion methods used for deriving the aerosol coefficients assume a range independent value for the extinction-to- backscattering ratio [lidar ratio, (LR)]. However, it is known that in a realistic atmosphere the value of LR is range dependent and varies with the physical and chemical properties of the aerosols. In this paper, we use a variant of widely applied Klett's method to obtain the range dependent LR values and derive the aerosol extinction profiles with good accuracy. We present the lidar derived aerosol extinction profiles in the upper troposphere and lower stratosphere corresponding to different seasons of the year of two distinctly different stations in the Indian subcontinent namely Trivandrum (8.33° N, 77° E), Kerala, India, a coastal station and Gadanki (13.5° N, 79.2° E), Tirupati, India an inland station. The range dependent LR is derived corresponding to different seasons of the year at the two stations. The lidar ratio, aerosol extinction coefficient (AEC), aerosol scattering ratio and aerosol optical depth show strong to medium seasonal variation at both the stations. The lidar ratio values at Trivandum vary in the range of 11-38 sr whereas the values range from 20-34 sr at Gadanki. AEC values at the Trivandum station vary from 7.9x10-6 to 6.9x10-5 m-1 and at Gadanki station the variation is from 1.27x10-5 to 6.9x10-5 m-1. It is proposed to use back-trajectory analysis to understand the sources of aerosol at the two stations.
Differential Absorption Lidar (DIAL) is a very effective technique for standoff detection of various toxic agents in the
atmosphere. The Lidar backscattered signal received usually has poor signal to noise (SNR) ratio. In order to improve the
SNR, statistical averaging over a number of laser pulses is employed. The aim of the present work is to select a particular
statistical averaging technique, which is most suitable in removing the noise in Lidar return signals. The DIAL system
considered here uses laser transmitters based on OPO based (2-5 μm) and TEA CO2 (9-11μm) lasers. Eight commonly
used chemical warfare agents including five nerve agents and three blister agents have been considered here as examples
of toxic agents. A Graphical User Interface (GUI) software has been developed in LabVIEW to simulate return signals
mixed with the expected noise levels. A toxic agent cloud with a given thickness and concentration has been assumed to
be detected in the ambient atmospheric conditions at various ranges up to 5 Km. Data for 200 pulses per agent was stored
in the computer memory. Various known statistical averaging techniques were used and number concentrations of
particular agent have been computed and compared with ideal Lidar return signal values. This exercise was repeated for
all the eight agents and based on the results obtained; the most suitable averaging technique has been selected.
Simulation studies have been carried out to analyze the performance of a Differential Absorption Lidar (DIAL) system for the remote detection of a large variety of toxic agents in the 2-5 μm and 9-11 μm spectral bands. Stand-alone Graphical User Interface (GUI) software has been developed in the MATLAB platform to perform the simulation operations. It takes various system inputs from the user and computes the required laser energy to be transmitted, backscattered signal strengths, signal-to-noise ratio and minimum detectable concentrations for various agents from different ranges for the given system parameters. It has the flexibility of varying any of the system parameters for computation in order to provide inputs for the required design of proposed DIAL system. This software has the advantage of optimizing system parameters in the design of Lidar system. As a case study, the DIAL system with specified pulse energy of OPO based laser transmitter (2-5 μm) and a TEA CO2 laser transmitter (9-11μm) has been considered. The proposed system further consists of a 500-mm diameter Newtonian telescope, 0.5-mm diameter detector and 10-MHz digitizer. A toxic agent cloud with given thickness and concentration has been assumed to be detected in the ambient atmospheric conditions at various ranges between 0.2 and 5 km. For a given set of system parameters, the required energy of laser transmitter, power levels of the return signals, signal-to-noise ratio and minimum detectable concentrations from different ranges have been calculated for each of these toxic agents.
Lidar techniques are based on the interaction of the laser beam with various constituents of the atmosphere like aerosols,
gas molecules etc. Various atmospheric conditions like temperature turbulence, refractive index variation, fog, rain etc.
really influence the transmission properties of the laser beam. An Imaging Lidar provides a 3-D Image of the targets like
clouds when used vertically up in the atmosphere or any terrestrial object on the ground when used horizontally. Various
image processing techniques are used to improve the image quality by using various mathematical models related to
atmospheric conditions. A portable IR Imaging lidar system has been designed and developed for imaging the terrestrial
targets during nighttime in complete dark conditions. The system is also being used for study of the structure of clouds in
the troposphere. The system mainly consists of a CW laser source operating in the IR region and a CCD array-imaging
device with zooming capability to cover the long range. The CCIR standard video output available from the CCD camera
is monitored by a high resolution monochrome monitor. The video output is digitized using a frame grabber board. The
digitized image is subjected to online and offline processing methods. The image signal depends on the integral response
of the laser source, reflection/scattering properties of the objects, atmospheric effects etc. Based on the image processing
methods needed to improve the quality of image under different atmospheric conditions, known a priori, an empirical
model is developed. This paper describes the imaging lidar system developed and the image processing.
Lidar observations had been conducted to study the long-range transport of aerosol and their effect at tropical station,
Trivandrum during the period of 2001-2003. The presence of aerosol layers was observed on many days below about 5
km during the above period. The monthly values of aerosol extinction coefficient profile showed the presence of aerosol
layer in the height region up to about 5 km during the summer monsoon periods. However, during the Asian winter
monsoon period the aerosol layers were observed in the altitude region between 0.6 and 3 km. The extinction values
were high in the winter season and were typically found to be 3.4×10-4 m-1. The aerosol optical depth was calculated by
integrating the extinction values in the aerosol layer region and it was found to be between 0.2 and 0.35. The plausible
reasons for the formation of these layers were explained using the wind circulation pattern and air back trajectories.
KEYWORDS: Clouds, LIDAR, Mass attenuation coefficient, Climatology, Geometrical optics, Time metrology, Aerosols, Signal attenuation, Temperature metrology, Physics
The cirrus clouds which are global in nature have been identified as one of the important constituents if the atmosphere.
They play a dual role in the earth radiation budget increasing the Earth's albedo while simultaneously decreasing the
emission of Infrared radiation to space. Tropical cirrus clouds come in a variety of forms ranging from optically thick
anvil cirrus closely associated with deep convection to optically thin cirrus layers frequently observed near the
tropopause. For better understanding of the formation, subsistence and dissipation of cirrus clouds extended studies are
necessary. From earlier investigations it is realized that the climatology of cirrus clouds is distinctly different at the low
latitude coastal station at the west coast of India. Some of the important characteristics of the cirrus clouds like time
history of formation and dissipation, geometrical and optical properties during the winter time have been investigated
using the ground based Mutiwavelength Lidar system designed and developed in house at the Space Physics Laboratory,
Vikram Sarabhai Space Centre, Trivandrum, India. The lidar provides a vertical resolution of 3.75m by making use of
the modified receiver electronics of the MWL system. The high resolution measurements have facilitated the study of the
fine internal structure, optical depth extinction coefficient and other parameters of importance of cirrus clouds. The
present paper describes lidar system and the results obtained over a period of one year covering all the seasons and the
peculiar characteristics of the cirrus during winter time at this coastal station.
Computer simulations have been carried out to optimize the IR Differential Absorption Lidar (DIAL) system in order to measure the gaseous pollutants released by the industries. The concentration of the gaseous pollutants due to elevated sources is computed using the Gaussian dispersion model. For given atmospheric conditions and stack physical parameters, the downwind distance (x) at which the SO2 reaches the safe limit of its toxicity has been computed at given other two coordinates (y, z) with respect to chimney. The gaseous pollutants released by the industries will be effectively monitored by the proposed DIAL system, which will be placed at New Delhi (28.35 degrees N, 77.12 degrees E), India. The performance of the Lidar has been optimized based on the various system parameters incorporating the atmospheric conditions and stack physical parameters. Further, the backscattered return powers at on- & -off line wavelengths, the required energy to be transmitted and the position at which the lidar system should be posted have been computed in order to monitor SO2.
One of the main functions of a Doppler Lidar system is to measure the atmospheric wind speed and direction. It is done by measuring the Doppler shift in frequency of the backscattered laser beam. A frequency stabilized Nd: YAG laser source operating at its fundamental wavelength of 1064 nm will be used in the proposed incoherent detection system. Edge technique is employed along with a high-resolution optical filter in this system to achieve high accuracy in the measurement of wind velocity. The performance of Doppler lidar system is estimated with realistic parameters. Analysis of the uncertainty in wind measurement is made by considering factors like lidar return signal levels, laser spectral width, etalon filter pass band (FWHM) etc. An accuracy of 0.25 m/s has been achieved in the wind speed up to an altitude of 5 km with 15-m range resolution in the proposed ground based Doppler Lidar system.
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