Estimate of solar radiative forcing by polluted clouds using OMI and SCIAMACHY satellite data

Absorbing aerosols, like smoke, can potentially have a large heating effect if they are situated above bright clouds. However, the detection of aerosols in the presence of clouds is not possible for satellite imagers operating in the visible. We use the Absorbing Aerosol Index (AAI) from OMI and SCIAMACHY ultraviolet radiances to detect aerosols in cloudy scenes. We first determine the global occurrence of events in 2006 with high effective cloud fraction (> 0.5) and appreciable amount of absorbing aerosols (AAI > 1) from OMI and SCIAMACHY data. These events we call polluted clouds. Next we zoom in on SW-Africa, where polluted clouds occur most frequently, especially in the months JJAS. Using SCIAMACHY reflectance measurements, polluted clouds appear to have a shortwave spectral reflectance that is lower than that of clean clouds from the UV until about 1050 nm. We determine the difference in reflectance between clean clouds and polluted clouds, and integrate the spectral difference from 280 to 1050 nm to obtain the solar radiative forcing of polluted clouds as compared to clean clouds. We find that in case of polluted cloud events in SW-Africa, a shortwave heating of the atmosphere in the range of 0-80 W/m2 occurs, with a mean of 35 W/m2.