Winter synoptic processes that cause large anomalies of extreme-low air temperatures in Eastern Europe

Abstract

Formulation of the problem. Extreme-low temperatures are an adverse phenomenon for the country’s economy and human living conditions. It is important to have a climatological estimation of typical locations and probability of extreme-low temperatures caused by different synoptic processes, which ideally should be predicted in advance.

The purpose of the article. The paper describes the typical synoptic processes that lead to extreme-low temperatures over the past 20 years (1997-2017) in middle latitudes in Eastern Europe. The probability of significant areas with extreme-low temperature and their typical localization during various synoptic processes was investigated.

Methods. Definition of “extreme-low temperatures” refers to the temperatures below the 5th percentile of the surface actual temperature distribution in each grid point. To create the sample, anomalies of extreme-low air temperatures over significant areas (more than 20 percent of the sector) were chosen. To divide the synoptic processes into groups, a synoptic analysis was applied.

Results. From the sample of 1997-2017, 75 episodes with large areas of extreme-low temperature were identified. The sample contains 13 episodes that cover more than 30% of the Eastern Europe and last from 2 to 11 days, and 12 episodes that have one day duration. Cases with anomalies occupy from 20 to 30% of the territory rarely lasts more than 2-3 days (11 episodes). There are five main groups of processes that lead to significant air temperature anomalies that cover a large area of the chosen domain. These five groups of situations include 75% of all days with large areas of extreme-low air temperature. The first group of processes: there is the movement of the cyclone from the Black Sea towards the Volga river basin region, with the anticyclone located in the northwest of Russia displacing towards the Baltic region or Scandinavia. In this case, temperature anomalies cover almost the entire eastern part of the selected sector. The second group of processes: a vast anticyclone, expanding from Western Siberia in the direction of Belarus (extreme-low temperatures most of time observed in the central part of the sector). The third group of processes: the movement of the “southern” cyclone from the Black Sea to the territory of Ukraine with subsequent displacement to the east (extreme-low temperatures most of time observed in the north-eastern part of the sector). The fourth group of processes: motionless anticyclone over the northeast of the Eastern European sector (high probability of extreme-low temperatures in the south-eastern part of the sector). The fifth group of processes: the northward trajectory of the “southern” cyclone to the territory of Ukraine while strengthening of the anticyclone over the Scandinavia. This one is similar to the first group of processes but differ by more western trajectories of cyclones, and hence different locations of extreme temperature anomalies (north-western part of the sector and eastern part of Ukraine). The remaining 25% of the days with large anomalies are mostly short-lived and observed during 1-2 days.

Scientific novelty and practical significance. The created catalogue of processes will allow recognizing the potential extreme situations that might be useful for middle and long-range forecasting.