An objective classification method for Hess and Brezowsky Grosswetterlagen over Europe

An objective method for classifying synoptic weather regimes over Europe and the North East Atlantic, based closely on the widely used Grosswetterlagen (GWL) series of Hess and Brezowsky, has been constructed. Climate mean composite patterns of mean-sea-level pressure (MSLP) and 500 hPa geopotential height fields are calculated from ECMWF ERA40 re-analyses for each of the 29 GWLs, for winter and summer separately. A daily catalogue of objective-GWLs is then constructed using pattern correlations with these composite fields, employing a sequence of logical filtering steps to remove transient features, ensuring that each GWL event lasts at least 3 days, in accordance with Hess and Brezowsky's original concepts. Objective-GWL catalogues are produced with both ERA40 and NCEP re-analyses, covering the period 1948 to the present, and its robustness with respect to either re-analysis set is examined. Systematic differences between the objective and original GWL series are found, since the original series is strongly focussed on central Europe and has a certain level of subjectivity, whereas the objective series views regimes on a larger spatial scale. Empirical indices of the large-scale circulation over Europe are derived from the Objective-GWL series and used to examine the characteristics of seasonal circulation variations and of the variability, trends and extremes of the European circulation during the re-analysis period.