Mesoscale Unsaturated Downdrafts Driven by Rainfall Evaporation: A Numerical Study

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  • 1 National Center for Atmospheric Research, CO 80307
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Abstract

It has been proposed that evaporation of precipitation failing from widespread thick masses of nimbostratus derived from cumulonimbus can account for the mesoscale unsaturated downdrafts inferred to occur within certain tropical mesosystems.

This paper discusses experiments made with a numerical model suitable for testing this idea. Dynamics of the explicitly described (large-scale) flow are governed by the hydrostatic unfiltered equations specialized to two dimensions. The value of f is for 10°N. Cumulus convection is parameterized through a one-dimensional plume model which allows for vertical transport of water substance in vapor and liquid form. The water budget for the large scale includes vapor and both cloud and precipitation and allows for transformations between these categories.

Computations are sensitive to the assigned value of β, the ratio of mass flux upward through the bases of convective clouds to the large-scale upward mass flux through 900 mb. For β=1.0, the initial wave disturbance (wavelength 103 km) weakens. Rapid deepening of the initial disturbance in runs with β=1.35 and 1.50 is a result of low-level warming by “compensating subsidence” between clouds.

That evap6rative cooling can induce a mesoscale downdraft of 10 cm s−1 is demonstrated by a pair of model computations, one including evaporation and the other not. In the former there develops a mesosystem similar to several recently reported in the literature. Furthermore, evaporation is sufficient to terminate deepening of the initial wave disturbance. Close to half the evaporation is the end result of mesoscale ascent within the large-scale (anvil) cloud. Comparison of computations with observation indicates that evaporation can account for much but possibly not all of the mesoscale subsidence underneath the anvil.

Abstract

It has been proposed that evaporation of precipitation failing from widespread thick masses of nimbostratus derived from cumulonimbus can account for the mesoscale unsaturated downdrafts inferred to occur within certain tropical mesosystems.

This paper discusses experiments made with a numerical model suitable for testing this idea. Dynamics of the explicitly described (large-scale) flow are governed by the hydrostatic unfiltered equations specialized to two dimensions. The value of f is for 10°N. Cumulus convection is parameterized through a one-dimensional plume model which allows for vertical transport of water substance in vapor and liquid form. The water budget for the large scale includes vapor and both cloud and precipitation and allows for transformations between these categories.

Computations are sensitive to the assigned value of β, the ratio of mass flux upward through the bases of convective clouds to the large-scale upward mass flux through 900 mb. For β=1.0, the initial wave disturbance (wavelength 103 km) weakens. Rapid deepening of the initial disturbance in runs with β=1.35 and 1.50 is a result of low-level warming by “compensating subsidence” between clouds.

That evap6rative cooling can induce a mesoscale downdraft of 10 cm s−1 is demonstrated by a pair of model computations, one including evaporation and the other not. In the former there develops a mesosystem similar to several recently reported in the literature. Furthermore, evaporation is sufficient to terminate deepening of the initial wave disturbance. Close to half the evaporation is the end result of mesoscale ascent within the large-scale (anvil) cloud. Comparison of computations with observation indicates that evaporation can account for much but possibly not all of the mesoscale subsidence underneath the anvil.

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