Precipitation Characteristics of Trade Wind Clouds during RICO Derived from...

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Snodgrass, E. R., L. Di Girolamo, and R. M. Rauber (2009), Precipitation Characteristics of Trade Wind Clouds during RICO Derived from Radar, Satellite, and Aircraft Measurements, J. Appl. Meteor. Climat., 48, 464-483, doi:10.1175/2008JAMC1946.1.

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 day21. The area-averaged rainfall rate for the 62-day period was 2.37 mm day21. If corrected for evaporation below cloud base, this value is reduced to 2.23 mm day21, which translates to a latent heat flux to the atmosphere of 63 W m22. 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 h21, and between 1.5% and 3.5% of the cloudy area had rainfall rates that were .1 mm h21. 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 h21 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.

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