Radar vs. passive satellite determinations of Probability of Precipitation(POP)

Abstract:

Proper identification of precipitating clouds in marine low clouds can significantly advance our understanding on aerosol-cloud-precipitation interactions. Freud and Rosenfeld (2012) showed that coalescence rate of droplet increases sharply when cloud effective radius (Re) exceeds 14 μm, leading to rain formation. However, there is a large difference between the fraction of clouds with Re>14 μm and CloudSat-observed precipitation probability, as the former is significantly larger than the latter. Here we use A-Train satellite observations to show three possible reasons for the apparent discrepancy: 1) Re>14 μm means that the cloud will precipitate somewhere (some area under the cloud) and sometime during its life cycle, while CloudSat-observed precipitation probability represents the precipitating fraction of the cloud area. This difference explains why the fraction of clouds with Re>14 μm is much larger than CloudSat-observed precipitation probability; 2) MODIS-derived Re based on 2.1 μm channel has been overestimated compared to surface in situ observations; 3) CloudSat reaches its detection limit on shallow clouds when cloud liquid water path is smaller than 250 g·m^-2.