Global Evaluation of the Fidelity of Clouds in the ECMWF Integrated Forecast System

Weather forecasting centers mainly assimilate infrared sounder data in clear-conditions or in channels with their main sensitivity to the atmosphere above the cloud tops. Sometimes channels with stronger cloud sensitivity are used in overcast conditions, but currently no cloud information is used from infrared sounders, and all-sky assimilation approaches are still under development. However, cloudy radiances could already be used for validating the quality of clouds in forecasts in a passive assimilation. We illustrate this by comparing the radiances observed (obs) with the AIRS (Atmospheric Infrared Sounder) to calculated radiances (cal) based on the clouds specified in the ECMWF (European Centre for Medium Range Weather Forecasting) Integrated Forecast System (IFS). Our analysis is based on a 12 hour ingest of AIRS data into the ECMWF assimilation system. We show that the standard deviation of (obs-cal) using the 1231 cm-1 atmospheric window channel is a metric of the fidelity of the clouds in the IFS. The global standard deviation of 5 K appears to be dominated by clouds in the IFS which are not seen in the AIRS data, and vice versa. Our metric capitalizes on the unique sensitivity of infrared sounders to clouds for the routine monitoring of the fidelity of clouds in weather forecasts.