Effects of Radiation and Turbulence on the Diabatic Heating and Water Budget of the Stratiform Region of a Tropical Cloud Cluster

Abstract

A two-dimensional, kinematic model, incorporating ice- and water-cloud microphysics, visible and infrared radiation, and convective adjustment, is used to diagnose the thermodynamic, water vapor, and hydrometeor fields of the stratiform clouds associated with a mesoscale tropical cloud cluster. The goal is to determine the relative contributions of radiation, microphysics, and turbulence to diabatic heating, and the effects that radiation has on the water budget of the cluster in the absence of dynamical interactions. The model was initialized with observed steady-state vertical and horizontal wind speeds, and thermodynamic fields corresponding to the stratiform region of a GATE tropical squall line. The wind field is held constant while thermodynamic and hydrometeor fields adjust to a steady state consistent with the prescribed winds. Advection of hydrometeors into the stratiform region from the convective line located just ahead of the stratiform region was estimated with the aid of a one-dimensional convective model, which indicated the vertical distribution of hydrometeors produced by the convective cells. These hydrometeor distributions were assumed to apply throughout the calculations on the upwind lateral boundary of the stratiform region, which lay just behind the line of convection. Particles produced by convection were thus advected into the stratiform region from this boundary by the prescribed wind field. During nighttime, energy loss due to infrared flux divergence was balanced by horizontal temperature advection and by vertical eddy flux convergence of moist static energy. The latter occurred when radiative destabilization led to convective overturning in the upper troposphere. Absorption of shortwave radiation by day warmed the cloud over a layer several kilometers deep, but infrared cooling at cloud top was still able to cause destabilization in the top 1 km of cloud. The solar and infrared radiation did not substantially change the hydrometeor fields, and thus had no effect on the water budget of the precipitating stratiform region. The depth of convective overturning in the upper troposphere was too small and water vapor mixing ratios were too low for radiatively driven convective overturning to affect the water budget of the stratiform precipitation region of the squall line. It was concluded that radiation does not directly affect the water budget of the stratiform region, and any radiative effect on hydrometeors must involve interaction with dynamics. An incipient anvil (cirrostratus cumulonimbogenitus) was represented by allowing hydrometeors from the convective line to blow off into a region devoid of other cloud or mean vertical air motion. Radiative destabilization maintained saturation in this cloud, and apparently contributed to the longevity of this cirrus, but no precipitation was formed.