Seminal role of clouds on solar dimming over the Indian monsoon region

Abstract

In contrast to most of the world where solar dimming has changed over to solar brightening since late eighties, dimming continues unabated over the Indian region. This study investigates new insight into the origin of dimming over India. As the insolation at the surface is controlled by aerosols and clouds, we tried to separate out the two controlling factors by examining clear and cloudy sky days. From 1981-2006, the rate of dimming is found to be twice as large during cloudy conditions (~12 W/m²/decade) compared to that during clear sky conditions (~6 W/m²/decade). The clear sky dimming is attributed to increasing aerosols. While the rate of dimming by clouds is similar during summer and winter monsoon seasons, the increased contribution to dimming by clouds during summer seems to come from increasingly deeper clouds covering increasingly larger area. During winter, dimming in cloudy conditions appears to be due to indirect effect of aerosols.