Optical properties of cirrus clouds at a tropical Indian station Gadanki, Tirupati (13.5°N, 79.2°E)

The optical properties of the cirrus clouds over a tropical inland station Gadanki, Tirupati were studied using a dual polarization lidar. The extinction coefficient, backscatter coefficient, optical depth and linear depolarization of the cirrus clouds are derived using the range dependent lidar ratio. This work reports the results obtained during the period of December 2006 to July 2007 which covers the three prominent seasons of the year in the Indian subcontinent. A variety of ice crystals like hexagonal thin plate, thick plate, columns, dendrites and aggregates were observed within the cloud. The geometrical and optical thicknesses of the clouds show strong seasonal variations. The occurrence frequency of thin cirrus clouds was found to be relatively high as compared to sub-visible and dense clouds. In almost all the cases, the cloud contains smaller ice crystals in the top part, larger crystals in the middle portion and mixed phase in the bottom portion. Compared to the winter and summer seasons the horizontally oriented ice crystals were observed more in monsoon period. The lidar ratio and linear depolarization ratio of the cirrus clouds were in the range of 3-40 sr and 0.1-1.5 respectively. The maximum linear depolarization ratio was observed for the clouds containing randomly oriented ice crystal with temperature below -80 degrees C. The lidar ratio was found to be maximum for the thin plate crystals and minimum for thick clouds with horizontally oriented ice crystals. The extinction and backscattering coefficients of the clouds were in the range of 0.3x10(-4) to 6 x10(-4) m(-1) and 0.12x10(-4) to 3x10(-4) m(-1) sr(-1) respectively during the observation period.