Meteorological bias in satellite estimates of aerosol-cloud relationships

Abstract

Several recent studies have reported a substantial correlation between satellite retrievals of aerosol optical depth (AOD) and cloud fraction, which is ascribed to an aerosol microphysical mechanism. Another possible explanation, however, is that the history of meteorological forcing controls both AOD and cloud fraction. The present study examines the latter hypothesis by comparing meteorological conditions along parcel back-trajectories for cases of large and small AOD and cloud fraction. Cloud and aerosol observations are obtained from the MODIS instrument aboard Terra, and meteorological information is obtained from ECMWT analyses. For continuity with previous investigations, the analysis focuses on the stratocumulus cloud region of the Northeast Atlantic during June through August 2002, the season of maximum cloud cover. Results show that scenes with large AOD and large cloud fraction had origins closer to Europe and experienced greater lower tropospheric static stability (LTS) during the past 2-3 days than did scenes with small AOD and small cloud fraction. Controlling for variations in LTS reduces the dependence of cloud fraction on AOD by at least 54%. We conclude that meteorological forcing must be accounted for in assessing aerosol impacts on cloud forcing, and that doing so requires a Lagrangian analysis of parcel histories. Copyright 2007 by the American Geophysical Union.