Rayleigh / Mie Lidar

he lidars are used to get the following studies : (a) Density and temperature profiles of the part of the middle atmosphere that is devoid of aerosols (z>30 km). (b) Vertical profiles of backscattering and extinction coefficients of the aerosols in the troposphere and in the lower stratosphere. (c) Properties of clouds. System Description A pulsed monostatic lidar system was setup at the National Atmospheric Research Laboratory at Gadanki (13.5°N, 79.2°E), Tirupati, India in collaboration with Communication Research Laboratory, Japan, for the study of atmospheric aerosols and thermal structure of stratosphere and mesosphere . The lidar uses an Nd:YAG laser operating at its second harmonic of 532 nm with energy of 600 mJ, pulse width of 7 ns and PRF of 50 Hz. The laser beam is expanded and sent into the atmosphere. The receiver system employs two channels one to cover middle and upper atmosphere altitudes christened as Rayleigh channel and another to cover troposphere and lower stratosphere altitudes christened as Mie channel. The Rayleigh channel uses 750mm telescope in Newtonian configuration.

In order to avoid saturation effects return signal from Rayleigh channel is divided into two to cover upper stratosphere (U) and another above those heights (R). The optical path contains IF filter (1nm) and a PMT is used for detection. A 350-mm-diameter Schmidt-Cassegrain-type telescope with a FOV of 1 mrad is used for photon collection. PMT with a narrow band interference filter (1.13nm) centered at 532 nm is used after the polarization beam splitter which separates the beam into Cross and Co polarized components. For both Rayleigh and Mie channels, an MCS-Plus (EG and G ORTEC) multi channel photon counter is used for recording the photon counting signals as a function of time (altitude). The dwell time of the counting system is 2 µs, which corresponds to an altitude resolution of 300 m. The backscattered returns summed for 250 seconds and are stored in a computer hard disk for off-line analysis. The lidar system installed at the Gadanki station has been in regular operation since 1998. Presently the lidar system is operated for about two hours in the night regularly, subject to sky condition. In addition to this, the system is operated in Common Mode Operation (CMO) twice in a week for five hours. Temperatures are derived using downward integration with the highest altitude taken as 90 km. Mie lidar products are obtained using inversion methods.