The effect of circulation conditions on the occurrence of cold episodes in summer in Central Europe

Abstract

The aims of the study were to assess the multiannual variability and trends of cold episodes in summer in Central Europe and to determine circulation conditions favouring their occurrence. The research was conducted based on data obtained for stations from Germany and Poland during the period 1966–2015. Cold episodes were understood as single cold days and cold spells. A cold day is defined as a day on which the maximum air temperature is lower than the 5th percentile of the local function of probability density determined from observation, and a cold spell was defined as at least two consecutive cold days. The observed increase in the air temperature in summer translated into a decrease in the number of cold episodes. The greatest changes in their number were recorded in the southern regions of the study area. On average, during the occurrence of cold episodes in summer in Central Europe, lower than usual sea level pressure was recorded, as well as lower persistence of the 300 hPa geopotential level, indicating the presence of cold air masses. Based on the minimum variance method, known as the Ward method, three circulation types were designated to cause the occurrence of cold episodes. The designated baric conditions caused advection of cold air masses from the northern sector. In each type, the largest T850 anomalies were recorded in Central Europe, reaching even less than −5°C.