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An infrared Transmittance database for fast radiative Transfert model To assimilate the satellite information, meteorological forecasting models need, at each iteration, an accurate radiative transfer model. This model must be very fast and line by line models are inadequate. So much faster models adapted to predefined filters, have been developed: Meteo-France has chosen the fast RTTOV model which uses, for working pressure levels, 3 independent polynomial modelisations with non-linear terms functions of the temperature, the specific humidity and the ozone of the input atmospheric profile. The coefficients of the polynomial equations are off- line calculated by linear regression on atmospheric transmittances, computed with a line-by-line model for specific spectral filters and for a small number of diverse atmospheric profiles. At each new satellite, the regressed coefficients have to be recomputed. The purpose of this study is to develop the tools for quickly calculating these coefficients: To avoid the problem of large computing time inherent in line-by-line calculations, we have generated a database of elementary infrared transmittances for a set of 32 profiles representative of the earth. The transmittances have been computed with the line by line FASCOD3 model for separately uniformly-mixed gases, water vapour and ozone for 40 pressure levels and 5 scan angles. The results are stored at a spectral resolution of 0.1 cm-1. This large computing time step has to be made only one time and for each new radiometer, the elementary transmissions are convolved with the new filters and then used to compute the set of filter adapted coefficients. This second step takes less than 2 hours on working station. In a first section, we will present the elaboration of the transmission database. Then, we will evaluate the quality (through RTE calculations) of the elementary transmissions and of the regressed coefficients by comparison to the direct line-by-line results, for the NOAA7 and NOAA11 filters. In a last part, we will compare the radiances computed with this RTTOV version and another fast radiative model ITPP, for a set of more than 4700 collocated satellite and radiosonde situations, for NOAA11 and NOAA12 filters.
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