USE OF CLOUD-CLEARED RADIANCES IN 3-DIMENSIONAL 4-DIMENSIONAL VARIATIONAL DATA ASSIMILATION

The direct use of TOVS (TIROS Operational Vertical Sounder) cloud-clea red radiances in a three/four-dimensional variational data assimilatio n scheme is described. This scheme uses a fast radiative transfer mode l and its adjoint. Radiances are used together with all the other obse rvational data. Global spectral fields of mass, wind and humidity are analysed simultaneously under certain mass/wind balance constraints wh ich 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 ste p and makes it possible to achieve a more optimal combination of the i nformation contained in the radiances, the conventional data and the b ackground (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 Inter polation scheme, which uses TOVS radiance information in the form of p rofiles 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 hou rs 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 T4 2) 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 trop ical wind field from the use of the humidity-sensitive TOVS channels.