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MMM SEMINAR NCAR

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Entrainment in Trade Wind Cumuli

Hermann Gerber

Gerber Scientific

Reston, Virginia

We have had for a long time a reasonable conceptual idea how cumulus (Cu) clouds evolve: latent heat from condensation drives the positive buoyancy, entrainment of dry air from outside the Cu dilutes the cloud and changes the droplet size spectrum, and coalescence and collection of the drops leads to precipitation. It thus is surprising that we are still unable to quantify sufficiently with observations or modeling these basic physical processes. The recent RICO (Rain in Cumulus over the Ocean) study on Caribbean trade wind cumuli was designed to meet again this challenge. We report here on RICO data analysis dealing with numerous aircraft passes through the Cu, passes which are used to estimate the clouds’ vertical evolution and to quantify the entrainment and mixing processes. We take advantage of unique incloud microphysical data collected with a resolution of 10cm, as well as data from other probes on the NCAR C-130 aircraft. We determine for the first time which of the oft-described entrainment/mixing processes, “homogeneous or inhomogeneous”, dominates these Cu, the length scale of the entrained parcels, the percentage of cloud-free voids within the Cu caused by entrainment, and the role played by the newly activated cloud drops in the entrained air. Recommendations are given for additional observational and modeling research on Cu.

Thursday, 1 June 2006, 3:30 PM

Refreshments 3:15 PM

NCAR-Foothills Laboratory

3450 Mitchell Lane

Bldg 2 Auditorium (Rm.1022)