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Surface soil moisture from SSM/I Imagery

Research project T4/DD/44 (Research action T4)

Persons :

Description :

In order to enhance the understanding of the impact of global warming at a regional scale, the Institut d'Astronomie et de Géophysique Georges Lemaître of the UCL performs regional climate simulations over western Europe with the Modèle Atmosphérique Régional (MAR). This is a numeric atmospherical model that contains a detailed land surface scheme, including explicit computations of surface soil moisture. The correct representation of soil moisture in numeric climate models is of paramount importance since it is a key variable of the hydrological cycle, and with global warming an increased frequency of extreme hydrologic events such as flooding and extended droughts periods are expected to occur. Unfortunately, suitable data sets to validate simulated soil moisture over western Europe are lacking.

The most efficient way to obtain information about surface soil moisture with sufficient spatial and temporal sampling for large enough regions resides in satellite measured brightness temperatures in the microwave range, and the SSM/I is currently the only spaceborne microwave imager that can do so.

Recently, a physically based methodology developed at VITO [Passive Microwave Soil Moisture Retrieval (PSR)] has been demonstrated to be capable to explain observed variations in surface soil moisture. PSR is based on the difference between the vertically and horizontally polarised SSM/I brightness temperatures at 19.35 GHz. Perturbation effects related to atmospheric transmissivity and surface roughness are eliminated by using time series of SSNYI brightness temperatures over relatively short periods (weeks) and by considering variations of polarisation difference temperature rather than absolute values, assuming that the variations are caused by soil wetness changes only. A validation of the methodology was performed by means of ground data from the FIFE experimental campaign, yielding errors comparable to groundbased experimental error. Although the results obtained with this method are very encouraging, they are valid for a limited area and for a short period only. More work is required to test the feasibility of the method over larger areas and for longer periods.

The subject of this proposal is to extend the PSR methodology, from the site scale to the continental scale. In particular, VITO will:

- assess the performance of the PSR methodology at the scale of western Europe through consistency checks, in particular by comparing SSM/I output with theoretical (PSR-predicted) threshold values, and by comparing PSR output with climatology data referring to manifest wet and dry periods;
- produce a temporal series of maps of surface moisture for the domain and the period of interest to the user partner (western Europe, May-October 1990), together with clear specifications of the final product, the emphasis being on its accuracy.

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