Towards and improved understanding and representation of key land surface and hydrological processes over West Africa : The AMMA Land surface Model Inter-comparison Project, Aaron Boone CNRM-GAME, Météo-France

West Africa has been subjected to extreme climatic variability over the last half century, with predominantly relatively wet years during the 50s and 60s being followed by a much drier period during the 70s-90s. This had significant socioeconomic consequences for the people of this region. Seasonal to inter-annual prediction of the west-African monsoon, which is the main precipitation driving mechanism, is research topic of utmost importance, however, a thorough understanding of this complex system has proved illusive. The deficiencies with respect to modeling the African monsoon arise from both the paucity of observations at sufficient space-time resolutions, and because of the complex interactions of the relevant processes at various temporal and spatial scales between the biosphere, atmosphere and hydrosphere over this region.

The AMMA (African Monsoon Multidisciplinary Analysis) project was organized in recent years with the main goal of obtaining a better understanding of the intra-seasonal and interannual variability of the west-African monsoon (WAM). In particular, land-atmosphere coupling has been shown to be significant in this region. For example, the magnitude of the north-south gradient of surface fluxes (related to soil moisture and vegetation) exerts a strong influence on the position of the tropical front and the African Easterly Jet (AEJ). Therefore a high priority goal of AMMA is therefore to better understand and model the influence of the spatio-temporal variability of surface processes on the atmospheric circulation patterns and the regional water cycle. In order to attain this goal, the AMMA Land surface Model Inter-comparison Project (ALMIP) was conceived. The idea is to force multiple state-of-the-art land surface models with the best quality and highest (space and time) resolution data available in order to better understand the key processes and their corresponding scales.  ALMIP1 focused on regional scale processes and surface states over the period 2002-2007, using a spatial resolution akin to that of a regional climate or high resolution global climate model. In ALMIP Phase 2, LSMs have been forced and evaluated using observational data from three heavily instrumented “supersites” from the AMMA-Couplage de l'Atmosphère Tropicale et du Cycle Hydrologique (CATCH) observing system which covers a north-south transect encompassing a large eco-climatic gradient. In this talk, an overview of key results from ALMIP phases 1 and 2 will be presented. Certain applications will be discussed (the evaluation of NWP forecasts, the ability of GCMs to simulate the surface fluxes and state variables, regional scale water budget estimates), key processes which are scale dependent in the models, the ability of the models to simulate mesoscale hydrological processes (budgets and lateral flow), and an evaluation of models using local scale field data in order to identify key processes which are not well modeled over this region. Finally, perspectives on the next steps towards developing improved land surface processes in coupled models will be given.