A 4D‐Var re‐analysis of FASTEX

A complete re‐analysis of the Fronts and Atlantic Storm‐Track EXperiment (5 January–27 February 1997) is performed. It provides a reference description of the life cycle of an ensemble of cyclogeneses over the north Atlantic. Many aspects make this re‐analysis noteworthy. In particular, the FASTEX period is characterized by an exceptional increase in the number of dropsoundings or radiosoundings concentrated on the storm track. Another striking point of the re‐analysis is the use of a four‐dimensional variational (4D‐Var) assimilation scheme on such a long period associated with the French Arpège (Action des Recherche Petite Echelle et Grand Echelle) global stretched model, that allows a locally high resolution in the area where dense networks of dropsondes were deployed. A twin re‐analysis with the same characteristics but without any FASTEX data is also performed. It shows that the impact of the special FASTEX data is quite noticeable in the resulting analyses and is lower, but still perceptible, in the associated short‐range forecasts (6 h–36 h). The impact of FASTEX observations on a particular FASTEX case is also shown.