Trade wind cloud evolution observed by polarization radar—relationship to aerosol characteristics

One of the classical unsolved problems in cloud physics is the explanation of the observed short time between initial cloud formation and the onset of precipitation in warm clouds. Using data collected by the National Center for Atmospheric Research S-PolKa radar, which was operated continuously on Barbuda during the Rain In Cumulus over the Ocean (RICO) field campaign, the microphysical evolution of trade wind cumuli will be characterized focusing on the potential role of giant nuclei in influencing the ZDR signal. Specifically, a large, statistically significant number of trade wind cumuli have been tracked from early echo development through rainout using the computer program Soloii and analyzed individually with the time-height section approach which is designed to analyze the spatial and temporal evolution of the equivalent radar reflectivity factor (Ze), the differential reflectivity (ZDR), and the correlation between these two fields. Other measures used for analysis include ‘averaged' Ze and ZDR values which are calculated for entire constant-elevation angle sweeps through the cloud. Furthermore, this study will investigate precipitation initiation and attempt to define ‘time zero' in trade wind cumuli development as well as investigate the causes behind the microphysical pulsations observed in these clouds.