The direct assimilation of cloud-affected satellite infrared radiances in the ECMWF 4D-Var

A system has been developed to make use of infrared radiance data in cloudy conditions. The central strategy is to extend the 4D-Var analysis control vector to include parameters which describe the cloud conditions and simultaneously estimate these parameters together with temperature and humidity inside the main analysis. In the current prototype configuration of the system, the extra cloud variables are decoupled from the model physics and only cloudy data in completely overcast conditions are used. Early experiments with the new system have produced some encouraging results. Additional use of overcast radiance data improves the analysis fit to independent radiosonde observations and results in some small, but statistically significant, improvements in forecast quality. Copyright (C) 2009 Royal Meteorological Society