Rainfall mechanism over the rain-shadow region of north peninsular India

Abstract

Rainfall mechanism over the rain-shadow region of north peninsular India during the summer monsoon season has been investigated using dynamic, thermodynamic, cloud microphysics and cloud dynamic (CMCD) forcings. Daily rainfall data has been used to understand rainfall variability. Daily ECMWF wind data for the period 2009–2011 have been used to study the wind divergence, shear and vertical velocity profiles. Daily thermodynamic parameters from upper air soundings of Hyderabad (17.448°N, 78.381°E), have been examined. Aircraft data have been used to study CMCD parameters. The divergence is found between surfaces to 850 hPa level whereas the convergence is at 850 hPa, which comes down to surface level during presence of low pressure systems. The divergence is observed at low (700 hPa), mid (600–300 hPa) and upper (250–150 hPa) tropospheric levels. In consequence of these divergence structures, three types of cloud systems viz. shallow, congestus and deep are developed with bases just above 850 hPa and tops at corresponding three divergent levels. The vertical profiles of relative humidity observed by radiosonde data have been analyzed to get the frequency distribution of shallow, congestus and deep clouds. The highest frequency observed is that of congestus clouds. The thermodynamic structure shows dry surface level and warm and moist middle troposphere with tongues of dry air and multilevel inversions which have been attributed to advection of aerosol-rich dry air. The aircraft observations showed high aerosol concentrations from surface to 5 km, polluted clouds with cloud droplet effective radius smaller than that required for collision–coalescence process. The factors which are responsible for causing low rainfall over the rain-shadow area have been identified.