A Simple Model of Evaporatively Driven Dowadraft: Application to Microburst Downdraft

R. C. Srivastava National Center for Atmospheric Research, Boulder, CO 80307

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Abstract

A simple one-dimensional, time-dependent model of an evaporatively driven downdraft is presented. The model is described by equations for raindrop evaporation, raindrop concentration, water substance, thermodynamic energy and vertical air velocity. At the top of the downdraft, the pressure, temperature, relative humidity, vertical air velocity and the raindrop size distribution are specified; Marshall–Palmer and monodisperse drop size distributions are considered. The environment of the downdrafts is assumed to be still and steady and is specified by the height distributions of temperature and water vapor. The bottom of the downdraft is considered to be open. Calculations are presented for downdrafts developing in the subcloud layer of high-based cumulus. It is found that the intensity of the downdraft at the bottom of the column, increases with 1) increasing lapse rate of temperature in the environment, 2) increasing rainwater mixing ratio at the top of the downdraft 3) increasing relative humidity of the environment and 4) decreasing raindrop size; it decreases with increasing mixing of environmental air into the downdraft. At the bottom, the downdraft is usually very dry, and its temperature does not differ from that of the environment by more than a few degrees; a cooling is observed only at high rainwater mixing ratio and lapse rate of temperature close to the dry adiabatic. In relatively stable lapse rates, weak oscillatory motion and transient formation of “scud” cloud are found to occur. A comparison with microburst observations during the Joint Airport Weather Study (JAWS) Project suggests that a majority of those microbursts was evaporatively driven, and originated near the cloud base.

Abstract

A simple one-dimensional, time-dependent model of an evaporatively driven downdraft is presented. The model is described by equations for raindrop evaporation, raindrop concentration, water substance, thermodynamic energy and vertical air velocity. At the top of the downdraft, the pressure, temperature, relative humidity, vertical air velocity and the raindrop size distribution are specified; Marshall–Palmer and monodisperse drop size distributions are considered. The environment of the downdrafts is assumed to be still and steady and is specified by the height distributions of temperature and water vapor. The bottom of the downdraft is considered to be open. Calculations are presented for downdrafts developing in the subcloud layer of high-based cumulus. It is found that the intensity of the downdraft at the bottom of the column, increases with 1) increasing lapse rate of temperature in the environment, 2) increasing rainwater mixing ratio at the top of the downdraft 3) increasing relative humidity of the environment and 4) decreasing raindrop size; it decreases with increasing mixing of environmental air into the downdraft. At the bottom, the downdraft is usually very dry, and its temperature does not differ from that of the environment by more than a few degrees; a cooling is observed only at high rainwater mixing ratio and lapse rate of temperature close to the dry adiabatic. In relatively stable lapse rates, weak oscillatory motion and transient formation of “scud” cloud are found to occur. A comparison with microburst observations during the Joint Airport Weather Study (JAWS) Project suggests that a majority of those microbursts was evaporatively driven, and originated near the cloud base.

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