WINTER SYNOPTIC PROCESSES THAT CAUSE LARGE ANOMALIES OF EXTREME HIGH AIR TEMPERATURES OVER EASTERN EUROPE

Abstract

The paper describes typical synoptic processes that led to extreme-high temperatures for the last 30 years (1987-2017) in Eastern Europe. There are three main groups of processes that lead to significant air temperature anomalies for the period of 12 UTC, which are observed simultaneously over more than 25% of the selected domain. Those three groups of situations describe 67% of all episodes with large areas of extremely high air temperature above the 95th percentile of the distribution function for each grid points in the Eastern European sector. For the synoptic processes of the first group, the characteristic feature is the presence of a deep circumpolar low belt above the northern polar zone at the 500 hPa level. It causes a powerful zonal air flow. For the surface pressure field, there is a movement of a cyclone or a series of cyclones from the North Sea towards Scandinavia or northern Russia. In the case of the processes of the second group, an upper trough is observed over Western Europe for several days. The Eastern European sector is covered by the upper ridge or being under its western periphery. For the surface pressure field, a system of cyclones with typical locations over Great Britain, Scandinavia and Central Europe is observed. The typical configuration of the field at the 500 hPa level for the processes of the third group is characterized by an upper trough extending from Scandinavia to Central Europe or affecting the western part of the Eastern European sector. Nearly 67% of days with large areas with extremely high air temperatures included in the catalogue refer to the mentioned in the article three groups of synoptic processes. From the sample of 30 years 1987-2017, the first group of processes describes 10 episodes, the second –13 episodes and the third – 7 episodes. The remaining 35% of days with large anomalies are mostly short-lived and observed during several or even one day. Moreover, such anomalies are characterized by fewer areas of coverage and typically observed over the south or south-west part of the domain.