Global soil wetness project: forecast and assimilation experiments performed at Météo-France
Projet d'humidité globale du sol : expériences d'assimilation et de prévision réalisées à Météo-France
Douville, H. ; Bazile, E. ; Caille, P. ; Giard, D. ; Noilhan, J. ; Peirone, L. ; Taillefer, F.
Année de publication
1999
Global soil moisture data of high quality and resolution are not <br>available by direct observation, but are useful as boundary and initial <br>conditions in comprehensive climate models. In the framework of the <br>Global Soil Wetness Project (GSWP), the ISBA land-surface scheme of <br>Météo-France has been forced with meteorological observations and <br>analyses in order to study the feasibility of producing a global soil <br>wetness climatology at a 1°×1° horizontal resolution between January <br>1987 and December 1988. A control experiment and several sensitivity <br>tests have been performed, suggesting that soil moisture remains one of <br>the most difficult climatological parameters to model and that any <br>computed climatology must be considered with great caution. The <br>prescription of the soil depth is particularly critical, showing the <br>relevance of the absolute value of the soil water content and the <br>interest for land surface schemes to include a deep layer beyond the <br>rooting depth. Compared to a river flow climatology, the runoff <br>simulated over large river basins seems to be underestimated because of <br>deficiencies in both the ISBA scheme and the GSWP experiment design. In <br>order to obtain a more reliable climatology, a global reanalysis of soil<br> moisture has been attempted, using a sequential optimal interpolation <br>technique, in which soil moisture is corrected by iterative comparison <br>between simulated and observed near-surface air temperature and relative<br> humidity. Preliminary tests have been performed for July 1987, showing <br>the potential of this method in idealized conditions. In practice, many <br>uncertainties, either in the observations, the land surface properties <br>or the atmospheric forcing, are liable to jeopardize the quality of the <br>reanalysis, suggesting the need for more consistent data within the GSWP<br> framework. Some outlooks are presented for improving the robustness of <br>the assimilation technique, which lead to encouraging results.</div>
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