Quantitative aspect in circulation type classifications – An example based on evaluation of moisture flux anomalies

Abstract

This paper is motivated by an effort to add a quantitative aspect to the process of classifying circulation types and to enable the comparison of various regions from the viewpoint of the synoptic–dynamical causes of hydro-meteorologically extreme events. First, we stress and demonstrate the necessity of adding a quantitative aspect to the classification process when extreme weather events are being studied or even forecasted on the basis of the classification. We suggest an approach based on the statistical anomalies of a proper meteorological variable. Three-day grid-related means of the zonal and meridional components of moisture flux were used as the criteria for the suggested circulation type classification. The circulation type is determined independently in individual grid points. The direction of the most significant moisture flux anomaly determines the circulation type; the degree of its extremeness enables us to evaluate the conditions quantitatively. Eight circulation types are introduced, corresponding to the cardinal directions, as well as an indistinct type comprised of three-day periods without significant moisture flux anomalies in the vicinity of the grid point. This method was subsequently applied to heavy rain events that occurred in the warmer half-years during 1958–2002 within selected Central-European river basins. We detected four regions with different circulation types responsible for heavy rains and demonstrated a close relationship between the dominant circulation type and the seasonal distribution of heavy rain events in the regions. Events accompanied by anomalies of the ‘direct’ flux of moisture (from the west or south) typically occur in autumn; they dominate in the southeastern Alps (circulation types SW, S, SE). Precipitation events with ‘overturning’ moisture flux (mainly from the north and northeast) are concentrated in high summer. They dominate in northern Austria, the Czech Republic, western Slovakia, and Saxony (circulation types NE, N, NW). Both variants occur in northern Switzerland and southwestern Germany with prevailing circulation types W, SW (more frequent in autumn) and N, NE (more frequent in summer). In northwestern Germany, significant moisture flux anomalies are not typical features of heavy rains during warmer half-years. Central-European extreme rain events were accompanied with especially significant moisture flux anomalies.