Role of Topography and Aerosols in the Rainfall over the Western Ghats and Rain Shadow Regions as Inferred from Aircraft Measurements

Abstract

The topography and aerosol roles in the heavy rainfall zone at the Western Ghats and the scarce rainfall farther east at the rain shadow were studied during Cloud Aerosol Interaction and Precipitation Enhancement EXperiment (CAIPEEX-2009) conducted over the Indian sub-continent using an instrumented research aircraft. Analysis of cloud microphysics (cloud droplet number concentration (CDNC) and effective radii (Re)) and aerosol number concentrations showed that the warm rain initiation starts at early stages (lower cloud depths) over the coastal areas and becomes even lower at the crest of the western slopes, probably due to intense washout of the aerosols by the rain. However, the clouds lose their microphysically maritime character farther east over the rain shadow regions, where the clouds have to grow to higher depths to initiate the rain process. The aerosol and Cloud Condensation Nuclei (CCN) concentrations support the hypothesis that the shallow convective clouds (i.e., with tops < 6 km) lose their ability to rain over the rain shadow due to increased aerosol concentrations, which are probably attributed to air pollution.