Impact of a simplified physical package in 4D‐var analyses of fastex situations

In the context of the FASTEX (Fronts and Atlantic Storm‐Track EXperiment), this paper presents a number of tests and validations of the four‐dimensional variational (4D‐Var) data assimilation system under development at Météo‐France, over the Intensive Observation Periods IOP3 and IOP17, in particular the impact of adding a simplified physical package within the incremental approach. the impact of using a 6‐hour 4D‐Var system (instead of a standard optimum interpolation or a 3D‐Var scheme) is positive for the description and the forecast of Atlantic cyclones. Moreover, despite the lack of tuning and remaining deficiencies in the parametrization schemes. the analysis and prediction of Atlantic cyclones seem better handled with a 4D‐Var system including physical processes in the assimilating model.