An improved general fast radiative transfer model for the assimilation of radiance observations

An improved version of the RTTOV (Radiative Transfer for the Television Infrared Observation Satellite (TIROS) Operational Vertical Sounder) fast radiative transfer model used operationally at the European Centre for Medium-Range Weather Forecasts (ECMWF) for the assimilation of Advanced TIROS Operational Vertical Sounder (ATOVS) radiances has been developed. This new model computes radiances for the Atmospheric Infrared Sounder and reproduces line-by-line radiances and Jacobians for the surface-sensing, water vapour and ozone channels of the ATOVS with significantly improved accuracy. The profile-dependent predictors used by the improved model to parametrize the atmospheric optical depths are based on the approach followed by the ECMWF fast radiative transfer model for the Infrared Atmospheric Sounding Interferometer (RTIASI). To improve the accuracy of the fast model in reproducing line-by-line radiances for the Infrared Atmospheric Sounder. modifications have been made to the predictors used in RTIASI by introducing a revised set of predictors for ozone and adding new predictors to model the water vapour continuum type absorption. To eliminate discontinuities in the water vapour Jacobians observed in RTIASI, data are now weighted prior to performing the regression.