Performance measurement with advanced diagnostic tools of all‐sky microwave imager radiances in 4D‐Var

In this paper, a comprehensive assessment of the impact of all‐sky radiance observations in the operational European Centre for Medium‐Range Weather Forecasts' assimilation and forecast system is presented using advanced diagnostic tools. In particular, the observation influence in the assimilation process and the related contribution to the short‐range forecast error of radiance observations from microwave sensors is evaluated with recently developed diagnostic tools based on the adjoint version of the model. Recent operational changes to the assimilation of these observations result in a more beneficial impact on the initial condition and the short‐range forecast. The largest information content is obtained for observations that are located in clear‐sky regions as indicated by the model short‐range forecast and the observations themselves. However, the largest decrease in the forecast error is provided by observations detected in cloudy regions. It is shown that the use of Special Sensor Microwave/Imager data helps to reduce model systematic errors in the central eastern equatorial Pacific, where the intertropical convergence zone is located, and the Arabian Sea, where the monsoon season occurs.