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.
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.
LIDAR operates by transmitting light pulses of few nanoseconds width into the atmosphere and receiving signals
backscattered from different layers of aerosols and clouds from the atmosphere to derive vertical profiles of the physical
and optical properties with good spatial resolution. The Data Acquisition System (DAS) of the LIDAR has to handle
signals of wide dynamic range (of the order of 5 to 6 decades), and the data have to be sampled at high speeds (more
than 10 MSPS) to get spatial resolution of few metre. This results in large amount of data to be collected in a short
duration. The ground based Multiwavelength LIDAR built in Space Physics Laboratory, Vikram Sarabhai Space Centre,
Trivandrum is capable of operating at four wavelengths namely 1064, 532, 355 and 266 nm with a PRF of 1 to 20 Hz.
The LIDAR has been equipped with a computer controlled DAS. An Avalanche Photo Diode (APD) detector is used for
the detection of return signal from different layers of atmosphere in 1064 nm channel. The signal is continuous in nature
and is sampled and digitized at the required spatial resolution in the data acquisition window corresponding to the height
region of 0 to 45 km. The return signal which is having wide dynamic range is handled by two fast, 12 bit A/D
converters set to different full scale voltage ranges, and sampling upto 40 MSPS (corresponding to the range resolution
of few metre). The other channels, namely 532, 355 and 266 nm are detected by Photo Multiplier Tubes (PMT), which
have higher quantum efficiency at these wavelengths. The PMT output can be either continuous or discrete pulses
depending upon the region of probing. Thick layers like clouds and dust generate continuous signal whereas molecular
scattering from the higher altitude regions result in discrete signal pulses. The return signals are digitized using fast A/D
converters (upto 40 MSPS) as well as counted using fast photon counters. The photon counting channels are capable of
counting upto 200 MHz with a spatial resolution of few metres. The LIDAR data generated comes in burst mode and
gets transferred to computer system. Pulse to pulse averaging is done rangebinwise for SNR improvement. The range
normalized signal power is computed and the vertical profiles of backscatter and extinction coefficients are derived. This
paper describes the intricacies in the design of the high resolution DAS developed in-house to obtain the scientific data.
The optimization methodology used for handling the data is also described.
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.
Ground based lidars are widely used all over for the study of physical and optical properties of aerosols and clouds in the
atmosphere. The observed parameters on aerosols and clouds and their dependence on various meteorological parameters
are being studied using the ground based lidars at different laboratories. But the results obtained are mostly applicable to
local / regional particular to the lidar observation site. Space borne lidar is a unique system for observing the global
distribution of aerosols and clouds. It provides vertical profiles of the physical properties of the clouds and aerosols with
global coverage. Such data is useful for the validation of climate models and for process studies related to the climate
change and also for studies on transport of aerosols and pollutants. Retrieval of optical properties of clouds and aerosols
from the data obtained by the space borne lidar is very complex. Currently we are developing algorithms to produce
calibrated data products for space borne and ground based lidars. A software to produce simulated lidar backscatter
profiles applicable to space borne and ground based lidars has been developed, which generates data that matches the
expected performance of the lidars under varying conditions. Output simulated data includes 1064 nm total backscatter
profiles and 532 nm profiles for both the parallel and perpendicular polarization states. This paper describes the
methodology used for inverting the ground based lidar data and the strategy for validating the data which will be
obtained from the proposed space borne lidar to be launched by ISRO.