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Tropical Rainfall and Boundary Layer Moist Entropy

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  • 1 Department of Physics and Atmospheric Science, Dalhousie University, Halifax, Nova Scotia, Canada
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Abstract

In the Tropics, the variation of rainfall with sea surface temperature (SST) is highly nonlinear. Rainfall shows no dependence on SST for SST increases from 19° to 26°C, abruptly increases by a factor of 5 as SSTs increase from 26° to 29°C, and then rapidly declines. It is argued that this nonlinear dependence is a response to the nonlinear dependence of convective mass on SST. Convective mass is a measure of the mass in the convective boundary layer thermodynamically able to participate in deep convection by virtue of its positive convective available potential energy (CAPE). Monthly mean estimates of convective mass were obtained at various islands in the tropical Pacific and Caribbean from the NOAA/National Climatic Data Center high-resolution radiosonde database. In the inner Tropics, the tendency for temperatures above the boundary layer to be homogeneous plays an important role in the rapid increase in rainfall near the convective threshold SST. At SSTs below the convective threshold, near-surface winds are generally directed from cold to warmer SSTs, so that horizontal advection of equivalent potential temperature (θe) will tend to suppress moist entropy, and rainfall, in these regions. In areas of the ocean with SSTs larger than the convective threshold, the mean frequency distribution of θe in the boundary layer becomes independent of SST. This occurs both as a response to the homogeneity of temperatures in the inner Tropics, and to the tendency for wind speeds in the boundary layer to decrease with SST for SSTs larger than the convective threshold. In the subtropics, temperature fluctuations are much larger than in the inner Tropics, and can be expected to play a much greater role in determining precipitation patterns.

Abstract

In the Tropics, the variation of rainfall with sea surface temperature (SST) is highly nonlinear. Rainfall shows no dependence on SST for SST increases from 19° to 26°C, abruptly increases by a factor of 5 as SSTs increase from 26° to 29°C, and then rapidly declines. It is argued that this nonlinear dependence is a response to the nonlinear dependence of convective mass on SST. Convective mass is a measure of the mass in the convective boundary layer thermodynamically able to participate in deep convection by virtue of its positive convective available potential energy (CAPE). Monthly mean estimates of convective mass were obtained at various islands in the tropical Pacific and Caribbean from the NOAA/National Climatic Data Center high-resolution radiosonde database. In the inner Tropics, the tendency for temperatures above the boundary layer to be homogeneous plays an important role in the rapid increase in rainfall near the convective threshold SST. At SSTs below the convective threshold, near-surface winds are generally directed from cold to warmer SSTs, so that horizontal advection of equivalent potential temperature (θe) will tend to suppress moist entropy, and rainfall, in these regions. In areas of the ocean with SSTs larger than the convective threshold, the mean frequency distribution of θe in the boundary layer becomes independent of SST. This occurs both as a response to the homogeneity of temperatures in the inner Tropics, and to the tendency for wind speeds in the boundary layer to decrease with SST for SSTs larger than the convective threshold. In the subtropics, temperature fluctuations are much larger than in the inner Tropics, and can be expected to play a much greater role in determining precipitation patterns.

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