Use of cloud‐cleared radiances in three/four‐dimensional variational data assimilation

Abstract The direct use of TOVS (TIROS Operational Vertical Sounder) cloud‐cleared radiances in a three/four‐dimensional variational data assimilation scheme is described. This scheme uses a fast radiative transfer model and its adjoint. Radiances are used together with all the other observational data. Global spectral fields of mass, wind and humidity are analysed simultaneously under certain mass/wind balance constraints which control the degree to which gravity waves enter into the analysis. In this way the need for a subsequent initialization is avoided. The scheme thus combines retrieval, analysis and initialization in one step and makes it possible to achieve a more optimal combination of the information contained in the radiances, the conventional data and the background (a six‐hour forecast). At spectral truncation T63, a global three‐dimensional variational analysis (3D‐Var) of TOVS radiances and conventional data is compared with the ECMWF operational Optimum Interpolation scheme, which uses TOVS radiance information in the form of profiles of temperature and humidity, retrieved using a one‐dimensional variational method. The results of 3D‐Var are in good agreement with the operational analysis at the same resolution. In an application of the four‐dimensional scheme (4D‐Var), data covering a period of 24 hours were used simultaneously. In 4D‐Var consistency in time is ensured through the evolution of the forecast model and its adjoint. Using the adiabatic version of the ECMWF forecast model (spectral resolution T42) we show that 4D‐Var is able to extract additional information from the dynamics of the model. In particular we show an impact on the tropical wind field from the use of the humidity‐sensitive TOVS channels.