Use of potential vorticity in monitoring and improving numerical analyses and simulations of severe winter storms in Western Europe.

Abstract

Despite the large improvements of numerical weather prediction models (NWP) during the last decades, including more advanced data assimilation methods that allow extensive use of satellite data, such models still occasionally produce forecast failures. Though these forecast errors are rare, they tend to involve cases of rapid cyclogenesis that cause dangerous weather. This investigation deals with a method that allows the human forecaster to improve the numerical analyses and forecasts in cases of imminent strong cyclogenesis. The combined use of potential vorticity (PV) fields in the numerical analyses and information retrieved from Meteosat water vapor (WV) images reveals analysis errors early in the stage of strong cyclogenesis. It is demonstrated that manual correction of the PV fields in the analysis according to information retrieved from the WV images can improve the short range forecasts of rapid cyclogenesis substantially. This is achieved by performing inversion of the corrected PV fields. By this method a new, dynamically consistent analysis is produced, from which a numerical resimulation is carried out. It is further demonstrated that singular vectors constitute an additional important tool in this process, by allowing PV corrections to be performed in sensitive regions. This procedure makes it possible to determine the likely 3-dimensional structure of the required PV modifications. The beneficial effects by this method may have some limitations in cases of downstream developments.