Techniques for retrieving cloud microphysical properties remotely from reflected radiances in the visible and near-infrared have an established history. Simultaneous retrievals of cloud optical depth and effective radius are possible because cloud droplets composed of pure water do not absorb at visible wavelengths but are moderately absorbing at some near-infrared wavelengths in the water vapor windows of that spectral region. Global retrievals have, in the past, relied extensively on measurements made by the Advanced Very High Resolution Radiometer (AVHRR) on operational polar orbiting satellites. The wavelengths used are centered at 0.63 and 3.7 microns, respectively, and the thermal window channel is utilized to remove the emission component of the 3.7 micron radiance during the day, when these reflectance measurements are made.

Retrieval algorithms are based on plane parallel radiative transfer, therefore the heterogeneity typically present in cloud fields introduces biases in these retrievals. These could be severe when Global Area Coverage (GAC) data are used to extract cloud properties because the horizontal resolution of the pixel is 1 x 4 km at nadir and the scene could be partly cloudy. We have examined histograms of optical depth and effective radius retrieved from low level clouds in the North Atlantic using two screening criteria - a simple threshold and the spatial coherence method. Not surprisingly, many partially filled pixels yield effectively low cloud optical depths. What is more intriguing is that there is no preferred bias in the retrieved effective radius.

In addition to the optical depth and effective radius, an attempt has also been made to estimate the liquid water path, geometrical thickness and droplet concentration of the clouds using ancillary data from a meteorological analysis. Since AVHRR GAC and the NCEP/NCAR reanalysis are available globally for several years, it is proposed that it may be possible to identify anthropogenic signatures in cloud microphysical properties on a global scale.