Determining Aerosol Indirect Effects from Satellite Data

The literature of observational studies of the indirect effects of aerosol upon clouds is characterized by a range of contrary (positive and negative) findings. We discuss why this is the case. We illustrate the general difficulty by way of an analysis of deep convective cloud structure, using DARDAR ice water content (IWC) profile data in the Tropics, and Moderate Resolution Imaging Spectroradiometer (MODIS) aerosol optical depths (AODs). DARDAR profiles are a joint retrieval of CloudSat and CALIPSO data. We average IWC profiles as a function of several AOD bin ranges, for individual regions and seasons, calculate derivatives (i.e. % change in IWC per 0.1 AOD), and then determine the means and 95% confidence limits of probability distribution functions (PDFs) of the IWC derivatives. The individual derivatives are either positive or negative, and the means of the PDFs are fairly small. A variety of microphysical and cloud dynamic processes of different signed influence are surely at play. Sampling specifics (e.g. number of profiles sampled, regional differences, the local time sampled, the sampling of a 3D cloud region by a random 2D slice, etc) all contribute to a range of findings that, on average, are fairly small.