Subcloud Layer Energetics of Precipitating Convection

Abstract

The thermodynamic modification of the subcloud layer in the GATE area is shown to be a function of precipitating convection. A critical rate of 2 mm h?1, based on the Z ? R relationship, in conjunction with 4 km ? 4 km scale 15 min mean radar maps, distinguishes between evaporation of precipitation in the subcloud layer (no change in moist static energy h) and vertical mass transport associated with penetrative downdrafts (decreases in h) into this layer from near and above cloud base. The spatial extent of the outflow of the active downdrafts is limited to a convective-mesoscale area directly under and as much as 15 km downwind of the precipitation causing the change. A more extensive wake region occurs on the upwind side of the precipitating region. The initial thermodynamic environment directly affects energy transport per unit mass by moist convection. Precipitating cells which operate upon an initially undisturbed atmosphere cause a net transfer of 60% more energy per unit mass than those convective clouds which operate upon regions previously modified by precipitation and downdrafts. Results suggest that large, linearly shaped, moving cloud lines are the centers of the most efficient energy transfer per unit mass.