Assimilation of IASI temperature and humidity retrievals with scene‐dependent observation operators in clear‐sky scenes

The potential of assimilating Infrared Atmospheric Sounding Interferometer (IASI) temperature and humidity retrievals with their scene‐dependent observation operators, a product developed by the European Organisation for the Exploitation of Meteorological Satellites (EUMETSAT), has been investigated in the European Centre for Medium‐Range Weather Forecasts (ECMWF) system. The results are compared with the corresponding radiance assimilation impact. The experiments are done in a depleted observing‐system framework to emphasise the impact originating from the new data. The focus in the first assimilation experiments has been on retrievals over sea and in clear‐sky scenes, as they have high and homogeneous quality. The results demonstrate a clear statistically significant positive impact on temperature, humidity, and wind forecasts. However, the impact is somewhat smaller in magnitude for the retrieval assimilation compared with the radiance assimilation above 700 hPa.