Interpreting the cloud cover – aerosol optical depth relationship found in satellite data using a general circulation model J. Quaas1, B. Stevens2, P. Stier2, and U. Lohmann3 1Max Planck Institute for Meteorology, Hamburg, Germany 2Atmospheric, Oceanic and Planetary Physics, University of Oxford, UK 3Institute for Atmospheric and Climate Science/ETH Zurich, Switzerland
Abstract. Statistical analysis of satellite data shows a positive correlation between
aerosol optical depth (AOD) and total cloud cover (TCC). Reasons for this
relationship have been disputed in recent literature. The aim of this study
is to explore how different processes contribute to one model's analog of the
positive correlation between aerosol optical depth and total cloud cover
seen in the satellite retrievals. We compare the slope of the linear
regression between the logarithm of TCC and the logarithm of AOD, or the
strength of the relationship, as derived from three satellite data sets to
the ones simulated by a global aerosol-climate model. We analyse model
results from two different simulations with and without a parameterisation
of aerosol indirect effects, and using dry compared to humidified AOD.
Perhaps not surprisingly we find that no single one of the hypotheses
discussed in the literature is able to uniquely explain the positive
relationship. However the dominant contribution to the model's AOD-TCC
relationship can be attributed to aerosol swelling in regions where humidity
is high and clouds are coincidentally found. This finding leads us to
hypothesise that much of the AOD-TCC relationship seen in the satellite data
is also carried by such a process, rather than the direct effects of the
aerosols on the cloud fields themselves.
Citation: Quaas, J., Stevens, B., Stier, P., and Lohmann, U.: Interpreting the cloud cover – aerosol optical depth relationship found in satellite data using a general circulation model, Atmos. Chem. Phys., 10, 6129-6135, doi:10.5194/acp-10-6129-2010, 2010.