Electrical, microphysical and dynamical observations in summer monsoon clouds

Electrical, microphysical and dynamical conditions in ground-based clouds were studied using the observations carried out at Mahabaleshwar (17 degrees 56'N, 73 degrees 40'E; 1382 m ASL), a hill station, during the summer monsoon season of 1977. There is a significant correlation between the rain intensity and the corresponding atmospheric electric potential gradient. The diurnal curves of rainfall and negative electric potential gradient exhibited two peaks. The peak in rainfall during the morning hours and the afternoon peak in electric potential gradient were statistically significant. The early morning peak in rainfall was attributed to the enhanced convergence caused by the radiational imbalance in the cloud and cloud-free regions during active monsoon conditions. The afternoon peak in the electric potential gradient was attributed to the cloud formation due to convection. The atmospheric electric potential gradient showed sign reversal from its normal fair weather positive to negative at the time of the onset of rain. The reversal of the electric potential gradient and the incidence of positively charged raindrops at the surface were almost simultaneous. Positive raindrop charges were recorded 1-2 minutes prior to the occurrence of negative electric potential gradient showing a steep increase during heavy rain spells. The raindrop charges were predominantly positive. The possible physical mechanisms for electrification of monsoon clouds have been discussed. The cloud condensation nuclei and surface temperature also showed peaks during the morning hours. The microphysical observations suggest that the rain formation takes place in monsoon clouds both by the collision-coalescence and ice crystal processes. A simple 1-D model was used to compute different cloud physical parameters. For a cloud with its top at 9 km the precipitation efficiency is in the range 60-70% of the adiabatic liquid water content. It has an average rainfall rate of 15 mm hr(-1), vertical velocity of 3-5 m s(-1) and average life time of 30 minutes.