Attributing marine stratocumulus cloud break-up timing to anthropogenic aerosols

Marine stratocumulus clouds (Sc) cover extensive areas of the oceans and are often influenced by anthropogenic pollution from adjacent continents. When sufficient drizzle ensues, the fully cloudy Sc typically breaks up rapidly, leading to a lower cloud radiative effect due to the lower cloud cover. Here we use realistic Lagrangian large eddy simulations, coupled with satellite observations, to investigate Sc break-up timing in response to aerosol conditions. We show with a case-study how a fully-overcast Sc deck over the North East Atlantic Ocean would have broken up sooner had it not been influenced by anthropogenic aerosol from Western Europe. The timing of the Sc break-up exhibits self-consistency with the initial aerosol: the more polluted the overcast conditions, the later the break-up. The timing is strongly modulated by the diurnal cycle and large scale meteorology. Our results support the notion that the effect of the aerosol on Sc cloud radiative forcing via cloud cover adjustments is not saturated, in contrast to the effect of aerosol on cloud albedo at fixed cloudiness, which weakens as the background aerosol levels increase.