83rd Annual

Monday, 10 February 2003
Seasonal radiation balance of Africa
Donald P. Garber, NASA/LRC, Hampton, VA; and G. L. Smith and T. Wong
Poster PDF (885.3 kB)
Africa has a broad range of climate classes, from desert and savanna to rain forest, or deep convection regions. The climate class is intimately related to the radiative climatology. The importance of radiation in the formation and maintenance of the deserts has been recognized since Hadley and more recently Charney. Radiation also plays a major role in sustaining tropical convection. For much of the continent, monsoons account for most of the rain. The monsoons are due to the seasonal movement of the intertropical convergence zone, as it "follows" the Sun, in response to the radiative forcing. In essence, the weather and climate system is a heat engine, driven by radiation.

Because of the importance of radiation in weather and climate, the Clouds and the Earth's Radiant Energy System (CERES) program flew radiation budget instruments on the Tropical Rainfall Measuring Mission (TRMM) and the TERRA and AQUA spacecraft. Also, the Surface Radiation Budget program has developed an 8-year set of surface radiation fluxes based on satellite data. This paper examines maps of outgoing longwave radiation fluxes and net radiation fluxes over Africa and their relations to the climate processes. Plots of radiative flux as a function of latitude and time for a given longitude show movements of the monsoons with season and also their intraseasonal variations. At 20 degrees east, the net radiative flux in the convection region is 90 W/m**2 and greater. This surplus of radiation provides the energy required to maintain the convection. In the dry region it is -30W/m**2 or more. This deficit of radiation comes from the conversion of potential energy as the air subsides in these regions. In January, the convection is centered at 15 degrees S (the Congo Basin), and moves just north of the Equator in July, where the lack of water vapor causes the convection to weaken. The convection increases to cover a broad region (20 degrees of latitude) with its return south.

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