MST radar and polarization lindar observations of tropical cirrus

Significant gaps in our understanding of global cirrus effects on the clima te system involve the role of frequently occurring tropical cirrus. Much of the cirrus in the atmosphere is largely due to frequent cumulus and convec tive activity in the tropics. In the Indian sub-tropical region, the deep c onvective activity is very prominent from April to December, which is a fav orable period for the formation of deep cumulus clouds. The fibrous anvils of these clouds, laden with ice crystals, are one of the source mechanisms for much of the cirrus in the atmosphere. In the present study, several pas sages of tropical cirrus were investigated by simultaneously operating MST radar and a co-located polarization lidar at the National MST Radar Facilit y (NMRF), Gadanki (13.45 degrees N, 79. 18 degrees E), India to understand its structure, the background wind field and the microphysics at the cloud boundaries. The lidar system used is capable of measuring the degree of depolarization in the laser backscatter. It has identified several different cirrus struct ures with a peak linear depolarization ratio (LDR) in the range of 0.1 to 0 .32. Simultaneous observations of tropical cirrus by the VHF Doppler radar indicated a clear enhancement of reflectivity detected in the vicinity of t he cloud boundaries, as revealed by the lidar and are strongly dependent on observed cloud LDR. An inter-comparison of radar reflectivity observed for vertical and oblique beams reveals that the radar-enhanced reflectivity at the cloud boundaries is also accompanied by significant aspect sensitivity . These observations indicate the presence of anisotropic turbulence at the cloud boundaries. Radar velocity measurements show that boundaries of cirr us are associated with enhanced horizontal winds, significant vertical shea r in the horizontal winds and reduced vertical velocity. Therefore, these m easurements indicate that a circulation at the cloud boundaries suggest an entrainment taking place close to these levels. The analysis of simultaneou s lidar and MST Radar observations can thus yield valuable information on t he structure and dynamics of the cirrus, specifically near the boundaries o f such clouds.