Large‐eddy simulations of the semidirect aerosol effect in shallow cumulus regimes

Abstract

This study investigates the influence of absorbing aerosols on marine boundary layer clouds using large‐eddy simulations. The study focuses on two different shallow cumulus cloud regimes: an intermediate trade wind cumulus regime with moderate cloud fraction (0.1–0.5) and a pure trade wind cumulus regime with low cloud fraction (0.05–0.2). An absorbing aerosol mixture was used that reflected the polluted conditions observed during the Indian Ocean Experiment. The aerosol layer had a midvisible optical depth of 0.4 and single scattering albedo of 0.88. Simulations with these aerosol conditions are then compared with aerosol‐free simulations to calculate direct and semidirect forcings associated with changes in cloud fraction and liquid water path. In the intermediate trade cumulus regime the aerosol heating reduced the daytime cloud fraction by about 10%. This led to a semidirect radiative forcing of 8 W m−2, which almost canceled the negative direct radiative forcing of the aerosol (−10 W m−2). However, in the pure trade cumulus regime the daytime cloud fraction was reduced by less than 2%, and the semidirect radiative forcing was just 2 W m−2. Hence this study demonstrates that the semidirect aerosol effect is highly dependent on cloud regimes.