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Executive Summary:

    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. Seasonal to interannual prediction of the west-African monsoon, which is the main precipitation driving mechanism, has therefore become a 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 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), which is to be accomplished through an extended period of intensive observations and field campaigns together with model developments and improvements. In particular, land-atmosphere coupling is theorized to be significant in this region. 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 possibly the strength of the monsoon and the African Easterly Jet (AEJ). 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.

    The strategy proposed in AMMA to develop a better understanding of fully coupled system is to break the various components into more manageable portions which will then provide insight into the various important processes. The first step is to begin with the land surface in off-line or uncoupled (without atmospheric feedbacks) mode. The idea is to force 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. The AMMA project therefore affords the possibility to improve the understanding of critical land surface processes over west Africa within the context of an Land Surface Model (LSM) intercomparison project. The critical aspect of such a project is the LSM forcing database, which consists in two components, the land parameter data and the atmospheric forcing. In addition, the database consists in forcing at three distinct scales (regional, meso and local scale).

    In order to address the known limited ability of LSMs to simulate the surface processes over western Africa, ALMIP has the following main objectives:

  1. inter-compare results from an ensemble of state-of-the-art models
  2. determine which processes are missing or not adequately modeled by the current generation of LSMs over this region
  3. examine how the various LSM respond to changing the spatial scale (three scales will be analysed: the local, meso and regional scales)
  4. develop a multi-model climatology of ``realistic'' high resolution (multi-scale) soil moisture, surface fluxes, water and energy budget diagnostics at the surface
  5. evaluate how relatively simple LSMs simulate the vegetation response to the atmospheric forcing on seasonal and inter-annual time scales.