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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).

ALMIP2 Objectives

    In ALMIP Phase 2, LSMs will be evaluated using observa- tional data from three heavily instrumented supersites from the AMMA-Couplage de l'Atmosphère Tropicale et du Cycle Hydrologique (CATCH) observing system (Mali, Niger, and Bénin). The AMMA-CATCH window covers a north-south transect encompassing a large eco-climat- ic gradient. Two main experiments will be performed with the first one at the mesoscale for each of the three super sites, using a grid resolution of approxiamtely 5 km. A second set of experiments will be performed at the local scale for several selected sites within each of the mesoscale squares. The simulations will encompass the 2005­2007 Intensive Ob- serving Period with a special focus on the analysis during the Special Observing Period in 2006. In addition to evaluation us- ing field data, LSM simulations will also be compared to results from detailed vegetation process and hydrological models that have already been extensively validated over this region. The results will be used in conjunction with those from ALMIP-1 in an effort to evaluate the effect of scale change on the repre- sentation of the most important processes from the local to the regional scale.

    A coordinated set of model experiments over the three AMMA- CATCH sites will provide a good idea of the contrasting char- acteristics and processes in the Sahel and Soudano-Guinean regions. The typical endorheic nature of the surface hydrology of the two Sahelian sites (Mali and Niger) for which catch- ments are limited to scales on the order of a few tens of square kilometers, includes both high runoff-prone and infiltration- prone surfaces. This is in contrast to the large hydrological catchments over the Benin site. Each site observing system provides both forcing (i.e., micrometeorological description and soil and vegetation properties) and validation data (surface fluxes, soil moisture, water table, and runoff). In addition, remote sensing images have been processed in order to infer land surface properties at the mesoscale, such as land cover, leaf area index maps, land surface temperature, albedo, and superficial soil moisture.

The main scientific questions to be addressed in ALMIP Phase 2 are:

  1. Which processes are missing or not adequately modeled by the current generation of LSMs over this region (infiltration over crusted soils, endorheic hydrology...)?

  2. How do the various LSM respond to changing the spatial scale? The relation between meso and regional scales will be made using ALMIP Phase 1 results.

  3. Can relatively simple LSMs simulate the vegetation response to the atmospheric forcing on seasonal time scale (for several annual cycles) for the diverse climates/vegetation covers?

  4. How can LSM simulate mesoscale hydrology given their relatively simple representation of such processes?

  5. What are the impacts of uncertainties/differences in the precipitation on the surface fluxes and hydrological responses of the LSM models?