Precipitation characteristics of trade wind clouds over the Atlantic Ocean near Barbuda are derived from radar and aircraft data and are compared with satellite-observed cloud fields collected during the Rain in Cumulus over the Ocean (RICO) field campaign. S-band reflectivity measurements Z were converted to rainfall rates R using a Z-R relationship derived from aircraft measurements. Daily rainfall rates varied from 0 to 22 mm day-1. The area-averaged rainfall rate for the 62-day period was 2.37 mm day-1. If corrected for evaporation below cloud base, this value is reduced to 2.23 mm day-1, which translates to a latent heat flux to the atmosphere of 63 W m-2. When compared with 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 predawn hours and a minimum in the midmorning 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 ∼ 1045 local time. These relationships revealed that between 5.5% and 10.5% of the cloudy area had rainfall rates that were > 0.1 mm h-1, and between 1.5% and 3.5% of the cloudy area had rainfall rates that were > 1 mm h-1. Cloud-top heights between ∼ 3 and 4 km and BRFs between 0.4 and 1.0 contributed ∼ 50% of the total rainfall. For cloudy pixels having detectable rain, average rainfall rates increased from ∼ 1 to 4 mm h-1 as cloud-top heights increased from ∼ 1 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.
ASJC Scopus subject areas
- Atmospheric Science