Abstract
Abstract. Over 3 weeks in May and June 2018, an exceptionally large number of thunderstorms hit vast parts of western and central Europe, causing precipitation accumulations of up to 80 mm within 1 h and several flash floods. This study examines the conditions and processes that made this particular thunderstorm episode exceptional, with a particular focus on the interaction of processes across scales. During the episode, a blocking situation persisted over northern Europe. Initially, the southwesterly flow on the western flank of the blocking anticyclone induced the advection of warm, moist, and unstably stratified air masses. Due to the low-pressure gradient associated with the blocking anticyclone, these air masses were trapped in western and central Europe, remained almost stationary, and prevented a significant air mass exchange. In addition, the weak geopotential height gradients led to predominantly weak flow conditions in the mid-troposphere and thus to low vertical wind shear that prevented thunderstorms from developing into severe organized systems. Due to a weak propagation speed in combination with high rain rates, several thunderstorms were able to accumulate enormous amounts of precipitation that affected local-scale areas and triggered several torrential flash floods. Atmospheric blocking also increased the upper-level cut-off low frequency on its upstream regions, which was up to 10 times higher than the climatological mean. Together with filaments of positive potential vorticity (PV), the cut-offs provided the mesoscale setting for the development of a large number of thunderstorms. During the 22 d study period, more than 50 % of lightning strikes can be linked to a nearby cut-off low or PV filament. The exceptionally persistent low stability over 3 weeks combined with a weak wind speed in the mid-troposphere has not been observed during the past 30 years.
Highlights
The period from May to mid-June 2018 was among the most active periods of severe convective storms associated with heavy rain, hail, convective wind gusts, and even tornadoes over large parts of western and central Europe (WetterOnline, 2018a, b, c; DWD, 2018a)
Having shown that a quasi-stationary air mass, which was conducive to convection, prevailed over vast parts of central Europe during the study period, we explore cut-off low activity that can prepare the atmosphere for convective initiation due to the associated large-scale lifting
We investigated an exceptionally large number of thunderstorms in western and central Europe over a 3-week period, from mid-May to mid-June 2018, using a combination of observational data and model data to gain a more holistic view of the prevailing dynamical and thermodynamical conditions and the decisive trigger mechanisms for this unusual thunderstorm episode
Summary
The period from May to mid-June 2018 was among the most active periods of severe convective storms associated with heavy rain, hail, convective wind gusts, and even tornadoes over large parts of western and central Europe (WetterOnline, 2018a, b, c; DWD, 2018a). From 26 May to 1 June 2018, thunderstorms caused insured losses of about USD 300 million and overall losses of about USD 430 million according to Munich Re’s NatCatSERVICE (Munich Re, 2019). It was the costliest convective storm event in western Europe that year
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