COMPARISON OF DOWNWELLING SURFACE LONGWAVE RADIATION FLUXES DERIVED FROM METEOSAT-8 WITH IN SITU MEASUREMENTS

Downwelling Surface Longwave Radiation Flux (DSLF) over land is one of the operational products delivered by the Satellite Application Facility for Land Surface Analysis (LSA SAF), which has started its initial operational phase on February 2005. The DSLF product is currently being produced every 30 minutes over the European Region of the Meteosat-8 disk. DSLF is the result of atmospheric absorption, emission and scattering within the entire atmospheric column and may be defined as the thermal irradiance reaching the surface in the thermal infrared spectrum (4-10µm). It is a particularly difficult parameter to retrieve since satellites cannot directly measure it. However hybrid methods that make a combined use of satellite and NWP data are one of the most suitable techniques for operational purposes. The adopted algorithm to compute DSLF consists of an hybrid method based on two different bulk parameterisation for clear and cloudy sky conditions using as input ECMWF forecasts of 2m-temperature, 2m-dew point temperature and total column water vapour (TCWV) as well as the cloud products from Satellite Application Facility on support to Nowcasting and Very Short Range Forecasting (NWC SAF) (Cloud Mask and Effective Cloudiness). The validation of DSLF product includes the comparison with in situ measurements and we present the first results aiming to assess the quality of the product against the user requirements (errors of the order of 5%). The study is presented for two ground truth sites over Europe (Carpentras and Roissy).