Impacts of absorbing aerosols on South Asian rainfall

Anthropogenic aerosols in the lower troposphere increase the absorption and scattering of solar radiation by air and clouds, causing a warmer atmosphere and a cooler surface. It is suspected that these effects contribute to slow down the hydrological cycle. We conducted a series of numerical experiments using a limited area atmospheric model to understand the impacts of aerosol radiative forcing on the rainfall process. Experiments with different radiative conditions under an idealized setting revealed that increasing atmospheric forcing and decreasing surface forcing of radiation causes reductions in rainfall. There was no relationship of top of the atmosphere forcing to the rainfall yield. The model was then used to simulate a domain covering southern part of Sri Lanka, over for the period from November 2002 to July 2003. For a given radiative forcing, instances with lower rainfall yields showed larger fractional reductions in rainfall. The trends in seasonal rainfall observed over the site in past 30 years in a different study confirms this finding. We conclude that the negative impact of increase of anthropogenic aerosols on rainfall would be more severe on regions and seasons with lower rainfall yields. The consequences of this problem on the industries that critically depend on well-distributed rainfall like non-irrigated agriculture and on the general livelihood of societies in low-rain areas can be serious.