The 23rd Conference on Hurricanes and Tropical Meteorology

4C.1
SUBCLOUD VERTICAL VELOCITY FIELDS AND THE GROWTH OF CUMULUS CLOUDS

Michael Garstang, University of Virginia, Charlottesville, VA; and K. Levey and J. Halverson



The sequence of vertical velocities in the subcloud layer before and after the formation of a cumulus cloud are traced from multiple, simultaneous tethered balloon observations in the cloud and mixed layers. Buoyant plumes originating at the surface serve to deepen the mixed layer with the top of the mixed layer growing at a rate of up to 10 cm s-1. Cloud growth is initiated by the mixed layer reaching the convective condensation level and not by discrete vertical velocity plumes extending from the surface to the condensation level. Downward and upward velocities are initiated at cloud base by the release of latent heat of condensation. Both upward and downward velocities grow downwards from cloud base to near the surface. The magnitude of the up- and downdrafts are the same (few meters per second) but the number of updrafts exceed the number of downdrafts in and below cloud base by 50%. Mass balance must be achieved by cloud-induced subsidence in the cloud-free air amounting to centimeter per second. The cloud-induced subsidence suppresses the mixed layer in the cloud-free regions inhibiting cloud growth and forcing cloud spacing. Cloud core transports exceed 2000 W m-2 but occupy less than 1% of the total area. Less than 15% of the air originating in the mixed layer below cloud base reaches cloud top (12 km).

The 23rd Conference on Hurricanes and Tropical Meteorology