Analyzing signatures of aerosol-cloud interactions from satellite retrievals and the GISS GCM to constrain the aerosol indirect effect

Evidence of aerosol-cloud interactions is evaluated using satellite data from MODIS, CERES, and AMSR-E; reanalysis data from NCEP; and data from the NASA Goddard Institute for Space Studies climate model. We evaluate a series of model simulations: (1) Exp N, aerosol direct radiative effects; (2) Exp C, like Exp N but with aerosol effects on liquid-phase cumulus and stratus clouds; and (3) Exp CN, like Exp C but with model wind fields nudged to reanalysis data. Comparison between satellite-retrieved data and model simulations for June to August 2002 over the Atlantic Ocean indicate the following: a negative correlation between aerosol optical thickness (AOT) and cloud droplet effective radius (R-eff) for all cases and satellite data, except for Exp N, a weak but negative correlation between liquid water path (LWP) and AOT for MODIS and CERES, and a robust increase in cloud cover with AOT for both MODIS and CERES. In all simulations, there is a positive correlation between AOT and both cloud cover and LWP (except in the case of LWP-AOT for Exp CN). The largest slopes are obtained for Exp N, implying that meteorological variability may be an important factor. On the basis of NCEP data, warmer temperatures and increased subsidence were found for less clean cases (AOT > 0.06) that were not well captured by the model. Simulated cloud fields compared with an enhanced data product from MODIS and AMSR-E indicate that model cloud thickness is overpredicted and cloud droplet number is within retrieval uncertainties. Since LWP fields are comparable, this implies an underprediction of R-eff and thus an overprediction of the indirect effect.