Different predictions by two NWP models of the surface pressure field east of Iceland

In February and March 1996, the French NWP model Arpège was run to give operational forecasts
five days ahead. These forecasts were a part of preparations for FASTEX (Fronts and Atlantic StormTrack
Experiment), whose field phase took place in 1997. The extensive collection and availability of
model output during this period gave an excellent opportunity to study in real time the behaviour of
different NWP models during the period of the year when strong winds and rapid changes in weather
conditions are frequent. In this paper, we study a case where the ECMWF model predicted an
incorrect pressure gradient at Northeast Iceland, while Arpège correctly simulated the gradient. The
model errors are connected to anomalies in the mid-tropospheric vorticity field, which is in general quite
different in the two models. The differences in the vorticity fields are discussed in relation to orographic