A study of thermals in cumulus clouds

Air motions in the thermals contained within shallow Florida cumulus clouds were observed to be similar to the circulation observed in laboratory thermals. There was outward flow in the updraughts of individual thermals at most levels and there were usually downdraughts observed at the edges of the updraught or of the cloud. Widespread inward flow towards the centre of the cloud and a narrow, but strong, updraught was occasionally observed, reminiscent of the tail found at the rear of the laboratory thermals. A region of reduced liquid water content was frequently observed in the centre of several thermals where the updraught and horizontal 1D divergence were strongest, and complete holes were observed on two occasions. Although horizontal wind shear was generally weak, it was significant in a few cases, causing the flow pattern in the cloud to be asymmetric. Ascending regions of cloud with high values of liquid water content (cloud cores) were commonly observed at all altitudes, but generally the percentage of clouds measured with high liquid water content decreased with altitude. The observations of airflow and liquid water content structure in warm cumulus clouds described in this paper are consistent with the schematic model of a thermal where a core of high liquid water content survives for several kilometres above cloud base, but erodes as it ascends. Copyright © 2005 Royal Meteorological Society

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