Use of cloud cleared radiances in three/four dimensional variational assimilation
Utilisation de radiances "décontaminées" de l'influence des nuages dans l'assimilation variationnelle à trois ou quatre dimensions
Andersson, E. ; Pailleux, J. ; Thépaut, J.-N. ; Eyre, J. ; McNally, A. ; Kelly, G. ; Courtier, P.
The direct use of TOVS (TIROS Operational Vertical <br>Sounder) cloud-cleared radiances in a three/four-dimensional variational<br> data assimilation scheme is described. This scheme uses a fast <br>radiative transfer model and its adjoint. Radiances are used together <br>with all the other observational data. Global spectral fields of mass, <br>wind and humidity are analysed simultaneously under certain mass/wind <br>balance constraints which control the degree to which gravity waves <br>enter into the analysis. In this way the need for a subsequent <br>initialization is avoided. The scheme thus combines retrieval, analysis <br>and initialization in one step and makes it possible to achieve a more <br>optimal combination of the information contained in the radiances, the <br>conventional data and the background (a six-hour forecast).</p><p align="justify">At<br> spectral truncation T63, a global three-dimensional variational <br>analysis (3D-Var) of TOVS radiances and conventional data is compared <br>with the ECMWF operational Optimum Interpolation scheme, which uses TOVS<br> radiance information in the form of profiles of temperature and <br>humidity, retrieved using a one-dimensional variational method. The <br>results of 3D-Var are in good agreement with the operational analysis at<br> the same resolution.</p><p align="justify">In an application of<br> the four-dimensional scheme (4D-Var), data covering a period of 24 <br>hours were used simultaneously. In 4D-Var consistency in time is ensured<br> through the evolution of the forecast model and its adjoint. Using the <br>adiabatic version of the ECMWF forecast model (spectral resolution T42) <br>we show that 4D-Var is able to extract additional information from the <br>dynamics of the model. In particular we show an impact on the tropical <br>wind field from the use of the humidity-sensitive TOVS channels.</p>
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