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Communication incl. Poster: BibTeX citation key:  Miller
Miller, M. A. 2009. Controls on the Atmospheric Radiative Divergence Budget in West Africa. Work presented at Third International AMMA Conference, July 20—24, at Ouagadougou, Burkina Faso.
Added by: roussot 2009-11-09 18:31:12
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Categories: Monsoon system and its variability, Weather to Climatic modelling and forecasting
Keywords: Aerosol, Clouds - Convection, Precipitation, Radiation
Creators: Miller
Publisher: African Monsoon Multidisciplinary Analyses (Ouagadougou, Burkina Faso)
Collection: Third International AMMA Conference

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Abstract
The atmospheric radiative flux divergence is defined as the difference between the net incoming radiation entering an atmospheric column at the top of the atmosphere (TOA) and the radiation that exits the column at the surface. Positive values of the radiative flux divergence indicate net warming of the atmospheric column through the absorption of radiation and negative values indicate cooling through the emission of radiation. The factors that control the atmospheric radiative flux divergence in West Africa include clouds, dust, biomass burning, column water vapor, and surface vegetation. As part of the African Monsoon Multidisciplinary Analysis, the US Department of Energy’s Atmospheric Radiation Measurement (ARM) program deployed a Mobile Facility (AMF) in Niamey, Niger during 2006 to measure the surface radiative, latent, and sensible heat fluxes, and to characterize the atmospheric state using an array of surface based remote sensors including cloud radar (Miller and Slingo, 2007). The AMF continuously measured the fluxes at the surface whilst the Geostationary Earth Radiation Budget experiment aboard Meteosat 8 measured the broadband radiative fluxes at the TOA, whereupon the column radiative flux divergence could be computed every 15 minutes. This approach offered unique and unprecedented capabilities to analyze the factors that control this divergence, which operate at time scales that are compatible with these measurements. Instruments deployed in association with the AMF made possible the identification of specific infrared spectral signatures associated with radiatively significant dust, which is observed throughout the year in West Africa (Turner, 2008). Of the three minerals that can be detected using this approach, Kaolinite was observed in 94% of all cases and a trajectory dependent mixture of Gypsum and Kaolinite was found to be the most frequently observed mineral combination (66.6%). An analysis of cloud fraction observed during the wet season showed a bimodal distribution with one maximum near the freezing level (5-km) and a second in the layer between 10 and 15 km. A strong relationship between the Lifting Condensation Level (LCL) and the occurrence of clouds and precipitation was present in the AMF data, while no relationship between the Convective Available Potential Energy and these variables was found (Kollias et al., 2009). The net and component radiative fluxes were computed for all of 2006 using one-day mean values to circumvent sampling issues (Slingo et al., 2009). The longwave radiative flux divergence in the column was reasonably constant through 2006 at approximately -175 Wm-2. This constancy in the net longwave flux divergence despite strong impacts from controlling mechanisms at the surface and TOA is because, as column water vapor increases, the atmosphere loses longwave energy to the surface with about the same increasing efficiency that it traps outgoing longwave radiation. The shortwave radiative flux divergence was approximately 100 Wm-2 in 2006 with clouds found to affect the component fluxes more than the net flux, whereupon the impact of cloudiness is to redistribute heat within the column rather than to change the column absorption rate. The net radiative flux divergence indicates that the atmosphere loses energy to space during the course of the year at a steady rate of approximately -75 Wm-2.
Added by: roussot