Synoptic patterns associated with thunderstorms in Poland
The purpose of this study is an analysis of the impact of synoptic patterns over the landmass of Europe with specific reference to the frequency of the occurrence of thunderstorms in Poland during the period 1971-2008. The subjective analysis of synoptic maps has led to the identification of seven types of synoptic situations for thunderstormy days over the area of Poland. There are four types with characteristic frontal storms: Type 1, a front moving from the western sector connected with a low pressure area situated to the north of Poland; Type 2, similar to Type 1 but with a warm and humid mass of tropical air moving ahead of the front itself; Type 3, a front moving from the North initiated by a high pressure area occurring over Scandinavia; Type 4, with a low pressure centre and accompanying combination of atmospheric fronts over Poland. Two of the identified types of synoptic situations are characterised by days with internal air mass thunderstorms: Type 5, without an atmospheric front and without a high pressure system over Poland; Type 6, with an atmospheric high over the area of Poland. There is also an intermediate type, Type 7, with storms occurring behind the warm atmospheric front moving towards Poland from the South. For the above identified types of synoptic situations an averaged image of the atmospheric pressure field over the area of Europe for both sea level and the height of the 500 hPa geopotential, and any anomalies, was presented by using data concerning the pressure field originating from reanalyses. Applying the correlation research, probability and regression the influence of the above identified seven types on the occurrence of stormy days was defined. Types 2, 4, and 7 bring the most serious likelihood of the occurrence of stormy days. More than 60 % of analysed days are related to the activity of cyclonal systems these being Types 1, 2, 3 and 4. Type 2 synoptic situation is identified as the most conducive to the occurrence of days with thunderstorms over Poland.