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Large-Scale Atmospheric Dynamics and Sahelian Precipitation

Large-Scale Atmospheric Dynamics and Sahelian Precipitation Observations show a broad band of precipitation across northern Africa, with maxima evident in some analyses on either side of the continent. A low-resolution GCM with simple boundary conditions produces such a band and, by producing a double-maximum structure, suggests the operation of distinct mechanisms for generating rainfall in the east and west. The precipitation, moisture convergence, and low-level wind convergence anomalies are very similar, indicating that an understanding of the low-level dynamics is essential for understanding the precipitation perturbation over the land surface. A linear model analysis shows that the anomalous low-level convergence is primarily forced by condensational heating in the middle and upper troposphere over East Africa. Low-level condensation and dry convection are also important for driving convergence in the west. Understanding the response of the low-level flow is key for understanding how inhomogeneity at the surface is communicated into the precipitation field. Midtropospheric condensational heating stretches vortex columns and induces a positive vorticity tendency in the lower troposphere. To establish a climatology, the low-level dynamics must adjust to balance this tendency in a way that maintains moisture convergence. The balance is accomplished by the meridional advection of low absolute vorticity air from the south and frictional effects. http://www.deepdyve.com/assets/images/DeepDyve-Logo-lg.png Journal of Climate American Meteorological Society

Large-Scale Atmospheric Dynamics and Sahelian Precipitation

Journal of Climate , Volume 10 (6) – Jun 14, 1995

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Publisher
American Meteorological Society
Copyright
Copyright © 1995 American Meteorological Society
ISSN
1520-0442
DOI
10.1175/1520-0442(1997)010<1137:LSADAS>2.0.CO;2
Publisher site
See Article on Publisher Site

Abstract

Observations show a broad band of precipitation across northern Africa, with maxima evident in some analyses on either side of the continent. A low-resolution GCM with simple boundary conditions produces such a band and, by producing a double-maximum structure, suggests the operation of distinct mechanisms for generating rainfall in the east and west. The precipitation, moisture convergence, and low-level wind convergence anomalies are very similar, indicating that an understanding of the low-level dynamics is essential for understanding the precipitation perturbation over the land surface. A linear model analysis shows that the anomalous low-level convergence is primarily forced by condensational heating in the middle and upper troposphere over East Africa. Low-level condensation and dry convection are also important for driving convergence in the west. Understanding the response of the low-level flow is key for understanding how inhomogeneity at the surface is communicated into the precipitation field. Midtropospheric condensational heating stretches vortex columns and induces a positive vorticity tendency in the lower troposphere. To establish a climatology, the low-level dynamics must adjust to balance this tendency in a way that maintains moisture convergence. The balance is accomplished by the meridional advection of low absolute vorticity air from the south and frictional effects.

Journal

Journal of ClimateAmerican Meteorological Society

Published: Jun 14, 1995

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