Contribution de S. GUEDJ, F. KARBOU & F. RABIER:


Towards the assimilation of SEVIRI observations over land.



This work aims to improve the assimilation of low-level SEVIRI (Spinning Enhanced Visible and Infra Red Imager) IR (Infra-red) observations over land to better constrain atmospheric analyses in meso scale models operated by Météo-France. To this date, over land surfaces, only high-peaking Water Vapour channels are operationally assimilated whereas IR channels are totally rejected. The assimilation of IR observations over land is possible only if several limitations are accounted for: a reliable description of the surface emissivity, a more accurate estimation of the surface temperature and an effective bias correction scheme. Some feasibility studies have been undertaken in order to assimilate more SEVIRI observations in the ALADIN French system. The land surface emissivity was described using climatologies from the EUMETSAT Land-SAF (Satellite Application Facilities). Using these climatologies helped to improve the RTTOV performances when simulating SEVIRI brightness temperatures (Tb) over Europe. The land surface emissivity and SEVIRI Tb were also used as input parameter in the radiative transfer model to retrieve the surface temperature (Ts) over Europe. The retrieved Ts was compared with independent Ts estimates (MODIS, Land-SAF products, \u2026) and was then used within the assimilation process to constrain the analysis of surface temperature. A description of the methods for emissivity/temperature retrievals will be presented. An evaluation of the retrieved Ts against independent measurements will be also presented. Finally, we will give an overview of assimilation and forecast experiment results once SEVIRI IR observations are assimilated over Europe.

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