Cloud characteristics and channel selection for IASI radiances in meteorologically sensitive areas

Introduction Advanced infrared sounders, such as the Atmospheric InfraRed Sounder (AIRS, launched on board the Aqua satellite in May 2002) and the Infrared Atmospheric Sounding Interferometer (IASI) scheduled to be on board the first operational European polar-orbiting satellite Metop, measure or will measure radiation in many thousands of channels. They have the potential to provide atmospheric temperature and composition information at a much higher vertical resolution and accuracy that can be achieved with the previous generation of satellite instruments such as the High resolution Infrared Radiation Sounder. As it is neither feasible nor efficient to assimilate all the channels in a Numerical Weather Prediction (NWP) system, channel selection policies have been designed in the NWP context. Previous studies have proposed methods in order to reduce the number of channels to keep only the ones, which bring the most useful information. In particular Rabier et al. (2002) and Fourrié and Thépaut (2003) have used a method based on the information content following Rodgers (1996) for clear sky conditions.