The role of large-scale dynamics in an exceptional sequence of severe thunderstorms in Europe May–June 2018

Mohr, Susanna; Wilhelm, Jannik; Wandel, Jan; Kunz, Michael; Portmann, Raphael; Punge, Heinz Jürgen; Schmidberger, Manuel; Quinting, Julian; Grams, Christian M.

doi:10.5194/wcd-1-325-2020

Cited by 39 publications

(48 citation statements)

References 92 publications

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“…; Mohr et al . ). Note, however, that ERA‐Interim (and reanalyses in general) may have a tendency to underestimate vertical wind shear (Taszarek et al .…”

Section: Environmental Conditions During Blockingmentioning

confidence: 97%

“…In May 2018 a blocking anticyclone over northern Europe, associated with repeated cut‐off lows forming along its southern flank, led to heavy precipitation and flash floods in western and central Europe (Mohr et al . ). In this case, the flow pattern provided key ingredients of stationary convection: moist air mass being trapped in the block and local‐scale ascent via a cut‐off low.…”

Section: Introductionmentioning

confidence: 97%

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Relationship between atmospheric blocking and warm‐season thunderstorms over western and central Europe

Mohr

Wandel

Lenggenhager

et al. 2019

Quart J Royal Meteoro Soc

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A statistically significant link is presented between atmospheric blocking located over the eastern North Atlantic and northern Europe and warm‐season thunderstorm activity over western and central Europe. Lightning data from 2001 to 2014 were used to identify thunderstorm days and blocking events were extracted from the ERA‐Interim reanalysis using an objective identification algorithm. The statistical link between the two phenomena is established through odds ratio analysis. Two areas – one over the eastern part of the North Atlantic and one over the Baltic Sea – were identified as locations where blocking influences the occurrence of deep moist convection in parts of western and central Europe. Based on the mean ambient conditions on days with blocking in these two areas, well‐known dynamic and thermodynamic mechanisms supporting or suppressing the development of thunderstorms were confirmed. The anticyclonic circulation of a block over the eastern part of the North Atlantic leads to a northerly to northwesterly advection of dry and stable air masses into Europe on the eastern flank of the block. In addition, these environmental conditions are on average associated with large‐scale subsidence of air masses (convection‐inhibiting conditions). In contrast, the southerly to southwesterly advection of warm, moist and unstable air masses on the western flank of a block over the Baltic Sea results in convection‐favouring conditions over western and central Europe. Both blocking situations are on average associated with weak wind speeds at mid‐tropospheric levels and with weak wind shear. As a consequence, thunderstorms related to atmospheric blocking over the Baltic Sea tend to be on average less organised.

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“…; Mohr et al . ). Note, however, that ERA‐Interim (and reanalyses in general) may have a tendency to underestimate vertical wind shear (Taszarek et al .…”

Section: Environmental Conditions During Blockingmentioning

confidence: 97%

Section: Introductionmentioning

confidence: 97%

Relationship between atmospheric blocking and warm‐season thunderstorms over western and central Europe

Mohr

Wandel

Lenggenhager

et al. 2019

Quart J Royal Meteoro Soc

Self Cite

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“…The flow is more amplified meridionally, and Rossby waves break downstream of Switzerland, resulting in weak upper-level zonal winds over central and eastern Europe and weaker surface zonal winds. In addition, blocking anticyclones over Scandinavia can contribute to more persistent flow over central Europe (Mohr et al, 2020). There are fewer fronts over western Europe on clustered hail days than on isolated hail days, and the fronts are located further from northern Switzerland (Fig.…”

Section: Atmospheric Conditions Prior To and During Hail Events North Of The Alpsmentioning

confidence: 99%

“…Large-scale flow patterns have been linked to the spatial and annual variability of thunderstorms in Europe (Piper et al, 2019;Mohr et al, 2019), and two case studies highlight two particularly long-lasting sequences of consecutive thunderstorm days in central Europe (Piper et al, 2016;Mohr et al, 2020). Piper et al (2016) compare a 15-day episode of thunderstorms in Germany in May-June 2016 with the period 1960-2014 and find that this event was exceptional for its number of days with prevailing extreme precipitation or convection-favoring conditions.…”

Section: Introductionmentioning

confidence: 99%

“…Piper et al (2016) compare a 15-day episode of thunderstorms in Germany in May-June 2016 with the period 1960-2014 and find that this event was exceptional for its number of days with prevailing extreme precipitation or convection-favoring conditions. Mohr et al (2020) investigate a series of severe thunderstorms in May-June 2018 in central Europe and find a blocking anticyclone that trapped moist and warm air over western and central Europe and several cut-offs on the block's southern fringe to provide exceptionally persistent low-stability conditions. Madonna et al (2018) compared large-scale conditions during June 2006, a month with above-average hail days (12, average 9.2) with June 2004, where only 2 hail days occurred in northern Switzerland.…”

Section: Introductionmentioning

confidence: 99%

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Multi-day hail clusters and isolated hail days in Switzerland – large-scale flow conditions and precursors

Barras

Martius

Nisi

et al. 2021

Preprint

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Abstract. In Switzerland, hail regularly occurs in multi-day hail clusters. The atmospheric conditions prior to and during multi-day hail clusters are described and contrasted to the conditions prior to and during isolated hail days. The analysis focuses on hail days that occurred between April and September 2002–2019 within 140 km of the Swiss radar network. Hail days north and south of the Alps are defined using a minimum area threshold of a radar-based hail product. Multi-day clusters are defined as 5-day windows containing 4 or 5 hail days and isolated hail days as 5-day windows containing a single hail day. The reanalysis ERA-5 is used to study the large-scale flow in combination with objectively identified cold fronts, atmospheric blocking events, and a weather type classification. Both north and south of the Alps, isolated hail days have frequency maxima in May and August-September whereas clustered hail days occur mostly in July and August. Composites of atmospheric variables indicate a more stationary and meridionally amplified atmospheric flow both north and south of the Alps during multi-day hail clusters. On clustered hail days north of the Alps, blocks are more frequent over the North Sea, and surface fronts are located farther from Switzerland than on isolated hail days. Clustered hail days north of the Alps are also characterized by significantly higher convective available potential energy (CAPE) values, warmer daily maximum surface temperatures, and higher atmospheric moisture content than isolated hail days. Hence, both stationary flow conditions and anomalous amounts of moisture are necessary for multi-day hail clusters on the north side. In contrast, differences in CAPE on the south side between clustered hail days and isolated hail days are small. The mean sea level pressure south of the Alps is significantly deeper, the maximum temperature is colder, and local moisture is significantly lower on isolated hail days. Both north and south of the Alps, the upper-level atmospheric flow over the eastern Atlantic is meridionally more amplified three days prior to clustered hail days than prior to isolated days. Moreover, Moreover blocking occurs prior to more than 10 % of clustered hail days over Scandinavia, but no blocks occur prior to isolated hail days. Half of the clustered hail days south of the Alps are also clustered north of the Alps. On hail days clustering only south of the Alps, fronts are more frequently located on the Alpine ridge, and local low- level winds are stronger. The temporal clustering of hail days is coupled to specific synoptic- and local- scale flow conditions, this information may be exploited for short to medium-range forecasts of hail in Switzerland.

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Statistical relevance of meteorological ambient conditions and cell attributes for nowcasting the life cycle of convective storms

Wilhelm

Wapler

Blahak

et al. 2023

Quart J Royal Meteoro Soc

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The usually short lifetime of convective storms and their rapid development during unstable weather conditions makes forecasting these storms challenging. It is necessary, therefore, to improve the procedures for estimating the storms' expected life cycles, including the storms' lifetime, size, and intensity development. We present an analysis of the life cycles of convective cells in Germany, focusing on the relevance of the prevailing atmospheric conditions. Using data from the radar‐based cell detection and tracking algorithm KONRAD of the German Weather Service, the life cycles of isolated convective storms are analysed for the summer half‐years from 2011 to 2016. In addition, numerous convection‐relevant atmospheric ambient variables (e.g., deep‐layer shear, convective available potential energy, lifted index), which were calculated using high‐resolution COSMO‐EU assimilation analyses (0.0625°), are combined with the life cycles. The statistical analyses of the life cycles reveal that rapid initial area growth supports wider horizontal expansion of a cell in the subsequent development and, indirectly, a longer lifetime. Specifically, the information about the initial horizontal cell area is the most important predictor for the lifetime and expected maximum cell area during the life cycle. However, its predictive skill turns out to be moderate at most, but still considerably higher than the skill of any ambient variable is. Of the latter, measures of midtropospheric mean wind and vertical wind shear are most suitable for distinguishing between convective cells with short lifetime and those with long lifetime. Higher thermal instability is associated with faster initial growth, thus favouring larger and longer living cells. A detailed objective correlation analysis between ambient variables, coupled with analyses discriminating groups of different lifetime and maximum cell area, makes it possible to gain new insights into their statistical connections. The results of this study provide guidance for predictor selection and advancements of nowcasting applications.

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The role of large-scale dynamics in an exceptional sequence of severe thunderstorms in Europe May–June 2018

Cited by 39 publications

References 92 publications

Relationship between atmospheric blocking and warm‐season thunderstorms over western and central Europe

Relationship between atmospheric blocking and warm‐season thunderstorms over western and central Europe

Multi-day hail clusters and isolated hail days in Switzerland – large-scale flow conditions and precursors

Statistical relevance of meteorological ambient conditions and cell attributes for nowcasting the life cycle of convective storms

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