The use of potential vorticity as an operational forecast tool

Abstract

Abstract The results from a project to evaluate the use of potential vorticity in operational forecasting are presented. An analysis of the temporal and spatial coherence of analysed and forecast potential vorticity fields showed that features with space scales downn to a few hundred kilometres could be tracked for between one and two days. Of features in these fields, 90% had a corresponding signal in the vertical motion fields, showing that they were real features having an important influence on weather‐bearing systems. A comparison between potential vorticity fields and water‐vapour imagery shows a close relationship in the region of the polar front. Examples are given showing how this can be used to detect errors in model analyses and forecasts. The use of potential vorticity as a diagnostic of model behaviour is discussed. Many authors have stressed the role of potential vorticity anomalies in major cyclogenesis. In these cases the forcing is usually clear from other diagnostics. It is suggested here that it is in highlighting areas of weaker forcing or relatively small‐scale model errors where potential vorticity is most useful to the operational forecaster.