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Regional and Diurnal Variability of the Vertical Structure of Precipitation Systems in Africa Based on Spaceborne Radar Data

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  • 1 Department of Atmospheric Science, University of Wyoming, Laramie, Wyoming
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Abstract

The Tropical Rainfall Measuring Mission (TRMM) 2A25 radar reflectivity profiles and derived surface rain rates are used to describe the vertical structure of precipitation systems in Africa. Five years of data are used in both the boreal and austral summer rainy seasons. A number of climate regions are isolated and compared. To place the composite reflectivity profiles in context, they are contrasted against TRMM 2A25 observations over the Amazon.

In all of tropical Africa, precipitation systems tend to be deeper and more intense than in the Amazon, and shallow warm-rain events are less common. In all African regions, but especially in the Sahel and northern Savanna, storms are characterized by high echo tops, high hydrometeor loading aloft, little indication of a radar brightband maximum at the freezing level, and evidence for low-level evaporation.

Storms in Africa are generally most common, and deepest, in the late afternoon, and weaker shallow systems are relatively more common around noon. The diurnal modulation is regionally variable. The amplitude of the diurnal cycle of the mean echo top height decreases from the arid margins of the zenithal rain region toward the equatorial region, and is smallest in the Amazon. A secondary predawn (0000–0600 LT) maximum occurs in the Congo, in terms of rainfall frequency, rainfall intensity, and echo tops. The storm intensity indicators generally peak a few hours later in the Sahel and northern Savanna than in other regions in Africa.

The difference between all African regions and the Amazon, and the relatively smaller differences between regions in Africa, can be understood in terms of the climatological humidity, CAPE, and low-level shear values.

Corresponding author address: Dr. Bart Geerts, Department of Atmospheric Science, University of Wyoming, Laramie, WY 82071. Email: geerts@uwyo.edu

Abstract

The Tropical Rainfall Measuring Mission (TRMM) 2A25 radar reflectivity profiles and derived surface rain rates are used to describe the vertical structure of precipitation systems in Africa. Five years of data are used in both the boreal and austral summer rainy seasons. A number of climate regions are isolated and compared. To place the composite reflectivity profiles in context, they are contrasted against TRMM 2A25 observations over the Amazon.

In all of tropical Africa, precipitation systems tend to be deeper and more intense than in the Amazon, and shallow warm-rain events are less common. In all African regions, but especially in the Sahel and northern Savanna, storms are characterized by high echo tops, high hydrometeor loading aloft, little indication of a radar brightband maximum at the freezing level, and evidence for low-level evaporation.

Storms in Africa are generally most common, and deepest, in the late afternoon, and weaker shallow systems are relatively more common around noon. The diurnal modulation is regionally variable. The amplitude of the diurnal cycle of the mean echo top height decreases from the arid margins of the zenithal rain region toward the equatorial region, and is smallest in the Amazon. A secondary predawn (0000–0600 LT) maximum occurs in the Congo, in terms of rainfall frequency, rainfall intensity, and echo tops. The storm intensity indicators generally peak a few hours later in the Sahel and northern Savanna than in other regions in Africa.

The difference between all African regions and the Amazon, and the relatively smaller differences between regions in Africa, can be understood in terms of the climatological humidity, CAPE, and low-level shear values.

Corresponding author address: Dr. Bart Geerts, Department of Atmospheric Science, University of Wyoming, Laramie, WY 82071. Email: geerts@uwyo.edu

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