The catastrophic floods in 2008, 2010 and 2020 in western Ukraine: Hydrometeorological processes and the role of upper-level dynamics

Abstract. Western Ukraine has encountered significant challenges due to three extensive summer rainfall events and major floods in July 2008, July 2010, and June 2020, resulting in numerous fatalities and substantial economic damage. This study investigates the hydrometeorological factors, as well as the atmospheric processes, that led to these three devastating flood events in the basins of the Tisza, Prut, Siret, and Dniester rivers in western Ukraine. The 2008 flood was the most severe, with river levels surpassing historical records. The flood in 2020 was notable for its hydrological complexity and was evolving more rapidly than the 2008 flood. The 2010 flood was more localized. 

A series of intense precipitation events extending over about 5 days were one of the key factors resulting in floods in all three cases. The prolonged heavy precipitation that caused these floods mainly occurred during the transition of the large-scale flow from a Scandinavian blocking pattern to a western Russian blocking regime and typically formed beneath an upper-level trough located over southeastern Europe. An essential synoptic feature for initializing the heavy rain events was a quasi-stationary upper-level cutoff low  that existed for about 5 days.  This persistence of the synoptic flow pattern allowed for the advection of warm, moist air from the Black Sea at low to mid-tropospheric levels toward the eastern slopes of the Carpathians, leading to orographic lifting that strongly contributed to precipitation in the region. While each flood event shared common mechanisms, such as Rossby wave breaking with subsequent formation of cutoff lows, atmospheric blocking and orographic lifting, the arrangement, interaction, and intensity of these processes varied. The 2010 event was marked by a combination of two consecutive Rossby wave breaking events and an intense atmospheric block in western Russia. In contrast, the 2008 and 2020 floods were characterized by a merging of the Scandinavian blocking regime with a blocking system over western Russia, resulting in the formation of a cutoff cyclone over Romania.  Thus, through the characterization of hydrometeorological conditions during western Ukraine floods, we aim to provide knowledge for better preparedness for future floods both in the region and throughout Eastern Europe.