Retrieving microphysical properties near the tops of potential rain clouds by multispectral analysis of AVHRR data

Properties of potentially precipitating cloud tops are retrieved from the radiances emitted and reflected from them, as measured by the Advanced Very High Resolution Radiometer (AVHRR) onboard the NOAA-11 satellite. Only clouds that are optically thick in the visible wave band and filling the field of view are considered as candidates for precipitation. Therefore, effects of emitted and reflected radiation from below the clouds, which is important in semi-transparent or broken clouds, are avoided altogether. The retrieval is done by comparing the measured radiance to the theoretically calculated radiance from clouds having various microphysical properties. The likelihood for precipitation formation processes is then estimated and verified against actual observations of precipitation, using a weather radar. It was shown that optically thick clouds with retrieved particle effective radius greater than about 14 βm correspond well to areas with radar echoes, indicating the existence of precipitation size particles. This results is consistent with the fact that existence of drops having a radius of at least 12 μm is required for efficient precipitation formation in clouds with relatively warm tops, by the mechanisms of warm rain processes as well as ice multiplication processes.