A Model of Intense Downdrafts Driven by the Melting and Evaporation of Precipitation

R. C. Srivastava Laboratory for Atmospheric Probing, The University of Chicago, Chicago, IL 60637

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Abstract

A ono-dimensioral time-dependent model of a downdraft driven by the melting and evaporation of precipitation and precipitation loading is formulated. Equations for particle melting, particle evaporation, particle concentration, precipitation content, thermodynamic energy, and vertical air velocity are formulated and solved numerically. In the environment, the lapse rate of temperature and the relative humidity are prescribed. At the top of the downdraft column, the temperature, relative humidity, vertical air velocity, and the phase and size distribution of the precipitation particles are prescribed. Two types of particle size distribution are considered: a distribution consisting of a single exponential function of the negative of the particle diameter, and a distribution consisting of two exponential functions of different slopes joining continuously at a melted diameter of 3 mm. The effects of lapse rate of environmental temperature, precipitation content, its phase and size distribution on the intensity of the downdraft are examined. With lapse rate of temperature approaching the dry-adiabatic value, even very light precipitation can drive intense downdraft. As the stability of the thermal stratification is increased, progressively higher precipitation contents and eventually high precipitation contents in the form of ice are needed to drive intense downdraft. A size distribution of precipitation particles having a relative abundance of smaller particles also favors intense downdrafts. The downdraft can be driven solely below cloud base provided the cloud delivers the necessary precipitation at its base. A comparison with available meager observations of wet downbursts lends support to these results. Observations of the lapse rate of temperature and precipitation content, including its phase and size distribution, in situations of wet downbursts are recommended.

Abstract

A ono-dimensioral time-dependent model of a downdraft driven by the melting and evaporation of precipitation and precipitation loading is formulated. Equations for particle melting, particle evaporation, particle concentration, precipitation content, thermodynamic energy, and vertical air velocity are formulated and solved numerically. In the environment, the lapse rate of temperature and the relative humidity are prescribed. At the top of the downdraft column, the temperature, relative humidity, vertical air velocity, and the phase and size distribution of the precipitation particles are prescribed. Two types of particle size distribution are considered: a distribution consisting of a single exponential function of the negative of the particle diameter, and a distribution consisting of two exponential functions of different slopes joining continuously at a melted diameter of 3 mm. The effects of lapse rate of environmental temperature, precipitation content, its phase and size distribution on the intensity of the downdraft are examined. With lapse rate of temperature approaching the dry-adiabatic value, even very light precipitation can drive intense downdraft. As the stability of the thermal stratification is increased, progressively higher precipitation contents and eventually high precipitation contents in the form of ice are needed to drive intense downdraft. A size distribution of precipitation particles having a relative abundance of smaller particles also favors intense downdrafts. The downdraft can be driven solely below cloud base provided the cloud delivers the necessary precipitation at its base. A comparison with available meager observations of wet downbursts lends support to these results. Observations of the lapse rate of temperature and precipitation content, including its phase and size distribution, in situations of wet downbursts are recommended.

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