On the Dynamics and Radiation of Cumulus Topped Marine Boundary Layers

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Description

Marine boundary layer (BL) cumuli are routinely observed over the tropical and subtropical oceans and play a key role in transporting enthalpy and momentum upwards from the surface. Although the hourly cloud coverage of these clouds is ~25%, they have a significant impact on the Earth’s radiation budget. The cumulus cloud cover is primarily determined by the boundary layer dynamics, which is heavily influenced by the surface turbulent fluxes, radiation and BL thermodynamics. To study the linkages between cumulus dynamics and radiation, we used data collected during the deployment of the first ARM Mobile Facility (AMF-1) at the island of Graciosa (GRW) in the Eastern North Atlantic and data collected at the Manus islands in tropical Western Pacific. We analyzed about 115 hours of data from GRW and about 200 hours of data from Manus Island during shallow cumuli conditions. Preliminary analysis suggests the tropical cumuli to have larger radar reflectivity near cloud top than the subtropical cumuli. The increase in hourly averaged updraft velocity from cloud base to cloud top was higher in GRW cumuli than in the cumuli observed over Manus. The mean profiles of hourly cloud fraction and updraft mass-flux are similar at the two sites. The radiation associated with these cumuli will be determined using the Rapid Radiative Transfer Model (RRTM), which will be constrained using a variety of measurements made at the sites. The co-variability of in-cloud dynamics with the boundary layer radiative cooling and the surface fluxes will be studied. Results from these comparisons together with the contrast in the dynamic and radiative structure of tropical and subtropical marine cumulus topped boundary layer will be presented.