Precipitation Characteristics of Trade Wind Clouds during RICO Derived from Radar, Satellite and Aircraft Measurements

Precipitation characteristics of trade-wind clouds over the Atlantic Ocean near Barbuda are derived from radar and aircraft data and compared with satellite-observed cloud fields collected during the Rain In Cumulus over the Ocean (RICO) field campaign. S-Band reflectivity measurements were converted to rainfall rates using a Z-R relationship derived from aircraft measurements. Daily rainfall rates varied from 0 to 22 mm day. The area-averaged rainfall rate for the 62 day period was 2.37 mm day. If corrected for evaporation below cloud base, this value reduces to 2.23 mm day, which translates to a latent heat flux to the atmosphere of 63 W m. When compared to the wintertime ocean-surface latent heat flux from this region, the average return of water to the ocean through precipitation processes within the trade wind layer during RICO was 31-39%. A weak diurnal cycle was observed in the area averaged rainfall rate. The magnitude of the rainfall and the frequency of its occurrence had a maximum in the pre-dawn hours and minimum in the mid-morning to early afternoon on 64% of the days. Radar data were collocated with data from the Multiangle Imaging SpectroRadiometer (MISR) to develop relationships between cloud-top height, cloud fraction, 866 nm bidirectional reflectance factor (BRF) and radar-derived precipitation. The collocation took place at the overpass time of ~10:45 AM local time. These relationships revealed that between 5.5% 10.5% of the cloudy area had rainfall rates > 0.1 mm hr, and between 1.5% 3.5% of the cloudy area had rainfall rates > 1 mm hr. Cloud-top heights between ~ 3 4 km and BRFs between 0.4 1.0 contributed ~ 50% of the total rainfall. For cloudy pixels having detectable rain, average rainfall rates increased from ~ 1 mm hr to 4 mm hr as cloud-top heights increased from ~ 1 km to 4 km. Rainfall rates were closely tied to the type of mesoscale organization, with much of the rainfall originating from shallow (< 5 km) cumulus clusters shaped as arcs associated with cold pool outflows.