Disentangling the role of microphysical and dynamical effects in determining cloud properties over the Atlantic

Abstract

MODIS satellite data reveal that over the Atlantic Ocean (20°S–30°N) in June–August 2002 indirect aerosol effects cause a decrease in the cloud top effective radius of stratiform clouds of 2.9 μm and an increase in cloud fraction of 21%, when increasing the aerosol optical thickness (AOT) from the cleanest 5 percentile to an AOT of 0.2. Thus, indirect aerosol effects are responsible for 72% (−8.8 W m−2) of the −12.2 W m−2 decrease in the shortwave radiation at the top‐of‐the atmosphere (TOA). Global climate model simulations with and without indirect aerosol effects confirm a decrease in TOA shortwave cloud forcing of −9 W m−2 over the Atlantic from the cleanest to the highest AOT due to indirect aerosol effects. While MODIS shows an increase in cloud fraction due to aerosols, in the model aerosols cause primarily an increase in cloud water. Thus, unlike the analysis from MODIS, the increase in cloud fraction with increasing AOT is dominated by changes in dynamical regimes, not by aerosol indirect effects.