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Communication incl. Poster: BibTeX citation key:  Guichard
Guichard, F., Kergoat, L., Gounou, A., Cappelaere, B., Galle, S., Mougin, E., Ramier, D., Timouk, F. & Lloyd, C. R. 2009. Couplings of surface thermodynamics and radiative fluxes across West Africa: an exploration in time and space from ground data. Work presented at Third International AMMA Conference, July 20—24, at Ouagadougou, Burkina Faso.
Added by: roussot 2009-09-08 10:48:58
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Categories: Ocean processes
Creators: Cappelaere, Galle, Gounou, Guichard, Kergoat, Lloyd, Mougin, Ramier, Timouk
Publisher: African Monsoon Multidisciplinary Analyses (Ouagadougou, Burkina Faso)
Collection: Third International AMMA Conference

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Abstract
The West African monsoon takes place over a region that is characterized by strong meridional gradients of surface and atmospheric properties. These in turn are thought to be closely connected to the monsoon meridional extend. The aim of this study is to characterize and to quantify basic contrasts in the functionning of this coupled system from the Soudanian to the northern Sahelian areas through an analysis of surface thermodynamics and surface radiative budgets along a seasonal cycle. Such an analyse provides ground truth allowing to assess the relevance of mechanisms inferred from more academical considerations and numerical simulations. Multi-year-site datasets acquired within the AMMA and CATCH projects are used for this purpose: these include several meterological stations, from Djougou (1.7E,9.7N), Banizoumbou and Wankama (around 2.6E,13.5N), Agoufou (1.5W,15.3N) and Bamba (1.2W,17N).
Large differences are found in the seasonal cycles of both surface equivalent potential temperature (thetae) and surface net radiation (Rnet) amont the measurement sites. A relative minimum of Rnet is taking place during the core of the monsoon season in Djougou, while at the same time it reaches a single yearly maximum further north, a broad one in Niamey and a sharper stronger one further north in Agoufou. This maximum then decreases further north in Bamba. Differences are stronger for the wettest years, when Rnet is lower in Djougou (a feature linked to a decrease of the incoming radiation SWin), while Rnet increases for the wettest years further north in the Sahel, even though SWin decreases there too; it occurs because this change is more than cancelled by a sharp decrease of the surface longwave upward flux and albedo. However, early in the season, the radiative impact of dusts and aerosols appear as particularly large in Agoufou and Bamba. The equivalent potential temperature displays a similar seasonal cycle, which involves distinct fluctuations of temperature and moisture. Overall, the interannual variability of these parameters is found to be the more pronounced in the Sahel.
Strong links are found between thetae and Rnet accross the measurement sites. However, surface radiative fluxes and thermodynamics, clouds and convection happen to be related in distinct ways among those sites. This notably involves major differences in the diurnal cycle of humidity, which are consistent with contrasted boundary-layer dynamics. This in turn leads to distinct diurnal cycles of low-level thetae. In Djougou, thetae displays a very well-defined daytime maximum whose amplitude does not vary much along the monsoon season, nor from one year to another, while in the Sahel, the diurnal cycle of thetae varies substantially at these times scales, in both phase and magnitude. Data also suggest a stronger control of low-level temperature fluctuations on the cloud cover in the Soudanian area. There, interactions between the surface and the atmosphere appear to involve a stronger coupling with cloud radiative properties than in the Sahel where the low levels seem to be more directly affected by surface features, such as their response to precipitating events, and by atmospheric circulations associated with these events, such as convective outflows. These relationships are summarized in the form of diagrams providing climatic signatures of this multi-year and multi-site dataset which can be used as model diagnosctic tools.
Our results provide new insights concerning the relationships of the low atmospheric levels to the West-African monsoon. They highlight large seasonal fluctuations of the north-south gradient, in terms of both magnitude and extend; these fluctuations involves distinct modes of interactions between the surface and the overlying low-level atmosphere from the Soudano-Soudanian site to the northern Sahelian site.
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