Aerosol control on depth of warm rain in convective clouds

Aircraft measurements of cloud condensation nuclei (CCN) and microphysics of clouds at various altitudes were conducted over India during CAIPEEX (Cloud Aerosol Interaction and Precipitation Enhancement Experiment) phase I and II in 2009 and 2010 respectively. As expected, greater CCN concentrations gave rise to clouds with smaller drops with greater number concentrations (N-c). The cloud drop effective radius (r(e)) increased with distance above cloud base (D). Warm rain became detectable, i.e., rain water content >0.01 g/Kg, at the tops of growing convective clouds when re exceeded 12 mm. The re is determined by the number of activated CCN, N-ad, and D. The Nad can be approximated by the maximum measured values of Nc. Higher Nc resulted in greater D for reaching the re threshold for onset of warm rain, r(e)*, denoted as D*. In extreme cases of highly polluted and moist air that formed the monsoon clouds over the Indo-Gangetic plains, D* exceeded 6 km, well above the 0 degrees C isotherm level. The precipitation particles were initiated there as supercooled raindrops at a temperature of -8 degrees C. Giant CCN reduced r(e)* and D*, by initiating raindrops closer to cloud base. This effect was found mainly in dusty air masses over the Arabian Sea. Besides, the aerosol effect on D*, D* was found to decrease with increase in cloud water path.