A Climatonomic Description of the Surface Energy Balance in the Central Sahel. Part I: Shortwave Radiation

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  • 1 Department of Meteorology, Florida State University, Tallahassee, Florida
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Abstract

The climate of the West African Sahel is characterized by unusually long (multiyear) persistence of anomalously wet or dry conditions. An increasing body of evidence suggests that land surface processes contribute to this persistence and to the severity of drought. In this study, we quantify land surface characteristics and fluxes in the Sahel in order to determine the degree to which they vary in response to rainfall fluctuations and anthropogenic effects on the surface. The Lettau climatonomy model is used to assess surface energy balance near Niamey, Niger. This article presents the first of three parts of the model, shortwave radiation climatonomy. Using irradiance at the top of the atmosphere as a forcing function, the model calculates global radiation, atmospheric heating, ground-absorbed solar radiation and planetary or top albedo. Sensitivity studies show that submedium absorption of solar radiation is primarily affected by surface albedo and cloudiness; the aerosol content of the heavily dust-laden Sahel atmosphere has little effect. A comparison with time series approximating global radiation on clear and partly cloudy days shows that the model accurately assesses the clear sky case but that better cloud parameterization would improve model results.

Abstract

The climate of the West African Sahel is characterized by unusually long (multiyear) persistence of anomalously wet or dry conditions. An increasing body of evidence suggests that land surface processes contribute to this persistence and to the severity of drought. In this study, we quantify land surface characteristics and fluxes in the Sahel in order to determine the degree to which they vary in response to rainfall fluctuations and anthropogenic effects on the surface. The Lettau climatonomy model is used to assess surface energy balance near Niamey, Niger. This article presents the first of three parts of the model, shortwave radiation climatonomy. Using irradiance at the top of the atmosphere as a forcing function, the model calculates global radiation, atmospheric heating, ground-absorbed solar radiation and planetary or top albedo. Sensitivity studies show that submedium absorption of solar radiation is primarily affected by surface albedo and cloudiness; the aerosol content of the heavily dust-laden Sahel atmosphere has little effect. A comparison with time series approximating global radiation on clear and partly cloudy days shows that the model accurately assesses the clear sky case but that better cloud parameterization would improve model results.

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