Climatology of Vb cyclones, physical mechanisms and their impact on extreme precipitation over Central Europe

Abstract

Abstract. Cyclones, which develop over the western Mediterranean and move northeastward are a major source of extreme weather and known to be responsible for heavy precipitation over the northern side of the Alpine range and Central Europe. As the relevant processes triggering these so-called Vb events and their impact on extreme precipitation are not yet fully understood, this study focuses on gaining insight into the dynamics of past events. For this, a cyclone detection and tracking tool is applied to the ERA-Interim reanalysis to identify prominent Vb situations. Precipitation in the ERA-Interim and the E-OBS data sets is used to evaluate case-to-case precipitation amounts and to assess consistency between the two data sets. Both data sets exhibit high variability in precipitation amounts among different Vb events. While only 23 % of all Vb events are associated with extreme precipitation, around 15 % of all extreme precipitation days (99 percentile) over the northern Alpine region and Central Europe are induced by Vb events, although Vb cyclones are rare events (2.3 per year). To obtain a better understanding of the variability within Vb events, the analysis of the 10 heaviest and lowest precipitation Vb events reveals noticeable differences in the state of the atmosphere. These differences are most pronounced in the geopotential height and potential vorticity field, indicating a much stronger cyclone for heavy precipitation events. The related differences in wind direction are responsible for the moisture transport around the Alps and the orographical lifting along the northern slopes of the Alps. These effects are the main reasons for a disastrous outcome of Vb events, and consequently are absent in the Vb events associated with low precipitation. Hence, our results point out that heavy precipitation related to Vb events is mainly related to large-scale dynamics rather than to thermodynamic processes.

Sensitivity of extreme precipitation to temperature: the variability of scaling factors from a regional to local perspective

“…The geographic location transitions from high Alpine terrain in the north-west of the region to lower-elevation forelands in the south-east, where a strong warming trend with a substantial increase in heat days has been observed over the last decades (Kabas et al 2011). Moisture advection from the Mediterranean Sea and orographic lifting provide essential ingredients for extreme precipitation events on the southern Alpine slopes (Cassola et al 2016;Panziera et al 2015;Schicker et al 2010;Sodemann and Zubler 2010) and lows over the Mediterranean moving northeastward ('Vb'-cyclone tracks) regularly result in excessive rain and large-scale flooding in the southern and south-eastern Alpine region (Volosciuk et al 2016;Messmer et al 2015;Hofstaetter and Chimani 2012).…”

Comparison of landslide forecasting services in Piedmont (Italy) and Norway, illustrated by events in late spring 2013

“…Central Europe and the northern Alpine region are exposed to high-impact events associated with the Vb cyclones (Messmer et al, 2015). This type of cyclone was mentioned by Köppen (1881) and later, defined by Van Bebber (1882, 1891) who proposed a cyclone classification system based on the main storm circulation trajectories in Europe (Messmer et al, 2015;Roald, 2008), describing one of them as Vb.…”

“…Central Europe and the northern Alpine region are exposed to high-impact events associated with the Vb cyclones (Messmer et al, 2015). This type of cyclone was mentioned by Köppen (1881) and later, defined by Van Bebber (1882, 1891) who proposed a cyclone classification system based on the main storm circulation trajectories in Europe (Messmer et al, 2015;Roald, 2008), describing one of them as Vb. In later classifications like GWL/SVG proposed by James (2007), this synoptic weather regime is known as 11 TM "Tief Mitteleuropa = Low (Cut-Off) over Central Europe", while in the GWT classification it is known as "TME Central European low".…”

“…A description of the basic climatology of this weather type is provided in Messmer et al (2015), given insight into the Vb cyclones' variability and investigating their physical mechanisms.…”

“…These cyclones transport large amounts of atmospheric moisture to central Europe and northern side of the Alps, thus triggering extreme precipitation events (Messmer et al, 2015). The potential of transporting extreme precipitation to central Europe is especially high if these cut-off low systems are positioned in the northern or eastern parts of the Alps (Awan and Formayer, 2016).…”