Variants of synoptic-scale patterns inducing heavy rains in the Czech Republic

Abstract

A classification method is suggested and applied to distinguish the variants of synoptic-scale patterns inducing heavy, widespread, and steady rains in the Czech Republic. The method is based on divisive hierarchical clustering of the 78 heaviest rainfall events that affected various parts of the Czech territory in the warmer half-years from 1958 to 2002. The degree of moisture flux close to the rainfall area is selected as a similarity criterion for clustering. Subsequently, a fuzzy approach is employed to assess the membership degree of the individual events to four clusters obtained by the second level of divisive clustering. Weighted averaging of synoptic-scale thermodynamic patterns is used to determine four main variants of causal synoptic-scale conditions typical of these fuzzy clusters. Two frontal and two cyclonic variants are distinguished. The frontal variants are characterized by the recurring passage of cold frontal waves, possibly followed by the rapid passage of a frontal wave cyclone. Heavy rains occur mostly on the warm side of the quasi-stationary frontal boundary, where the intense southern component of moisture flux occurs. In the case of the passage of the frontal wave cyclone, heavy rains appear in the cold rearward sector of the cyclone, where the intense northern component of moisture flux develops. The cyclonic variants are characterized by the slow passage of a cyclone across Central Europe along various trajectories. Heavy rains are always located in the cold sector of the cyclone, which is surrounded by anomalous moisture flux that is continuously supplying the rainfall area with a large amount of moisture. The results of the present study open the door to a quantitative evaluation of thermodynamic conditions typical of heavy, widespread, and steady rains in the Czech Republic.