Improving simulations of severe winter storms by initial modification of potential vorticity in sensitive regions

In this paper simulations of three selected severe winter storms over the North Atlantic are presented. Modifications of potential vorticity (PV) fields according to features in water vapour (WV) images are combined with information from singular vectors (SVs) in an attempt to improve the initial state over data‐sparse regions. The apparent mismatch between features in the WV images and the upper‐level PV anomalies in the numerical analyses is corrected, mainly at levels indicated as sensitive by the fastest‐growing SVs. The procedure is based on the fact that modification of the analysis in regions where SVs show a pronounced signal may have a large impact on the simulation. Model reruns, based on the inverted corrected PV fields, were then performed. Three cases are presented: the two French Christmas storms of 1999, and the storm affecting UK, the North Sea and southern Norway on 30 October 2000. In these cases a substantial improvement of the simulations of the storms was achieved. The PV modifications were carried out by a digital analysis system, implemented at the Norwegian Meteorological Institute. This system allows the PV modifications to be done interactively within an operational time limit. Copyright © 2006 Royal Meteorological Society