Assessment of water budgets computed from NWP models and observational datasets during AMMA-EOP

Understanding the water cycle in the West African Monsoon (WAM) system is a major objective of AMMA. The water cycle is the result of the interplay of various coupled atmospheric – ocean – continental surface processes. The identification of the mechanisms involved and the scales at which they operate is approached here through the investigation of water budgets terms (precipitable water vapour PWV, water vapour fluxes WVF, precipitation P, and evapo-transpiration ETP). The present study focuses on the intercomparison of datasets that are currently used for the computation of such water budgets are large scale. Numerical Weather Prediction (NWP) models provide their own complete description of the water cycle but with limitations (especially in P and ETP terms). During the AMMA EOP, many additional observations went into the NWP operational analyses which might thus be of unprecedented quality. We describe here the seasonal cycles (monthly-mean and short-term variability) of PWV, WVF, P and ETP obtained from ECMWF operational analyses, as well as NCEP reanalysis 1 and 2, for years 2005 to 2007. Significant differences are seen between the different analyses, with NCEP reanalysis 1 being of much poorer quality (in term of overall bias, seasonal cycle and spatial structure). Water vapour convergence and budget closure from the different NWP datasets are also examined in several boxes over continental West Africa. There also, significant differences are found for the different analyses. The water budget estimates are also assessed comparing some terms with observational datasets (PWV from GPS, radiosondes, and satellites; P from EPSAT-SG and GPCP; ETP from land and ocean surface models). A combined water budget is computed, using NWP estimates for WVF and observation-based datasets for P and ETP. This is used to investigate the origine of the sources of water (e.g., oceanic versus continental), the significance of water recycling and atmospheric advection of water, and the link between the intra-seasonal variability in the water budget terms and the coupled atmospheric – ocean – continental surface processes.