Differences in the mean atmospheric conditions during summer (June-August) extreme and non-extreme precipitation events in Sweden are analysed for cyclonic, anticyclonic and directional weather types based on the Lamb classification. Extreme and non-extreme events are defined as daily totals ≥40 mm and totals <40 mm and more than 1 mm respectively. The analyses are based on daily precipitation data from a data set consisting of 366 stations in Sweden, divided into 11 regions. The atmospheric conditions are described by daily National Centers for Environmental Prediction reanalysis data of various variables at nine levels (1000-200 hPa). The proportion of precipitation events that occurs during the cyclonic weather type is ∼45% for non-extreme events but increases to ∼70% for the extreme events. This can be related to higher mean vertical velocities for the cyclonic types compared with the directional and anticyclonic types, and higher mean specific humidity compared with the directional type.
The atmospheric circulations of extreme and non-extreme events are compared through composites of wind and geopotential heights at 500 and 850 hPa. The significance of the differences in mean values of the atmospheric variables during the two types of event is examined using a two-tailed t-test. The composites of extreme events occurring during cyclonic weather types are characterized by a more pronounced ridge over western Russia at 500 hPa compared with the non-extreme events. For the directional weather type the flow at 500 hPa is, on average, basically zonal for the non-extreme events, whereas for the extreme events a ridge is positioned east of Sweden over Finland. Most of the variables analysed show significant differences in mean during extreme and non-extreme events, and, not surprisingly, specific humidity and vertical velocity can be regarded as the key variables when describing climatological differences between extremes and non-extremes. The extreme events are favoured by lower westerly wind flow and wind speed and stronger southerly winds compared with the non-extreme events.