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.

中文翻译：

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环流条件对中欧夏季寒潮发生的影响

该研究的目的是评估中欧夏季寒冷事件的多年变化和趋势，并确定有利于其发生的环流条件。该研究是根据 1966 年至 2015 年期间从德国和波兰获得的数据进行的。感冒发作被理解为单一的寒冷天和寒冷期。冷天定义为最高气温低于观测确定的局部概率密度函数的第 5 个百分位的一天，寒潮定义为至少连续两个冷天。观察到的夏季气温升高转化为寒冷发作次数的减少。在研究区域的南部地区记录了它们数量的最大变化。一般，在中欧夏季寒冷事件发生期间，记录到低于平时的海平面压力，以及 300 hPa 位势水平的持续性较低，表明存在冷气团。基于最小方差法，即 Ward 法，指定了三种循环类型来导致寒冷事件的发生。指定的气压条件导致来自北区的冷气团平流。在每种类型中，最大的 T850 异常记录在中欧，甚至低于 -5°C。指定了三种循环类型导致寒冷事件的发生。指定的气压条件导致来自北区的冷气团平流。在每种类型中，最大的 T850 异常记录在中欧，甚至低于 -5°C。指定了三种循环类型导致寒冷事件的发生。指定的气压条件导致来自北区的冷气团平流。在每种类型中，最大的 T850 异常记录在中欧，甚至低于 -5°C。