Kaustav Chakravarty

Microphysics of clouds and rain over the Western Ghat

In an attempt to unravel the interactions between cloud microphysics and dynamics that make shallow clouds precipitate heavily in this region, some unique observations of rain and cloud microphysical parameters are presented here from two stations, Pune and Mahabaleshwar, one each on the lee and windward sides, respectively, of the Western Ghat (WG) mountains in peninsular India. To elucidate rain microphysics, we used the raindrop size distribution (DSD) by fitting three parameter Gamma functions to the observed raindrop spectra. Over Pune, during stratiform rain with bright band (BB) at 0°C isotherm; concave upward DSD shapes are observed below the BB which becomes concave downward at lower altitudes. It is due to breakup process of large raindrops which increases drop concentration at midsizes suggesting coalescence, collision, and breakup processes. Both slope and intercept parameters of Gamma DSD decrease during no BB condition as altitudes decrease, signifying collision and coalescence processes. Over Mahabaleshwar, bimodal and monomodal DSD are observed during light and heavy rainfall, respectively. With shallow storm heights, small raindrops mainly contribute to both types of rainfall. The DSDs are parameterized, and their radar reflectivity factor-rainfall intensity relationships are evaluated suggesting the dominance of collision-coalescence processes. Aircraft measurements of orographic clouds over the WG suggest interaction of cloud mass with the ambient updraft speed. The orographically forced updrafts foster rapid condensational growth of cloud droplets triggering coalescence process within few hundred meters of cloud depth. Hence, these clouds are dynamically forced to produce precipitation over the WG.

Rain Depolarization Measurements on Low Margin Ku-Band Satellite Signal at a Tropical Location

The phenomenon of depolarization of a Ku-band satellite signal caused by rain has been studied with a simple experimental system in which the attenuation of the copolar component and the enhancement of the cross-polar component signal have been measured at a tropical location. The degradation of cross-polarization discrimination (XPD) due to rain over the Earth space path has been obtained from these measurements and compared to that estimated from the ITU-R model, indicating the characteristic feature of a tropical location.