Diagnosis of forecast-analysis'' differences of a weather prediction system

An alternative approach is set out for the study of the difference between the forecast and contemporaneous analysis fields of a weather prediction system. Illustrations and interpretations are proffered of this difference when viewed in terms of potential vorticity (PV) for both a case study analysis and a single winter's climatology. The results indicate that the approach provides a compact and insightful description of the difference field's dynamics.