Ambient conditions prevailing during hail events in central Europe

Kunz, Michael; Wandel, Jan; Fluck, Elody; Baumstark, Sven; Mohr, Susanna; Schemm, Sebastian

doi:10.5194/nhess-20-1867-2020

Cited by 33 publications

(32 citation statements)

References 79 publications

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“…Thus, the link to heavy precipitation events (including flood events) has already been intensively investigated in past years (e.g. Martius et al, 2013;Grams et al, 2014;Piaget et al, 2015;Sousa et al, 2017;Lenggenhager et al, 2018;Lenggenhager and Martius, 2019). A new study by Mohr et al (2019) shows a statistical relationship between convective activity (based on lightning data) and specific blocking situations in the European sector.…”

Section: Introductionmentioning

confidence: 99%

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

Mohr

Wilhelm

Wandel

et al. 2020

Weather Clim. Dynam.

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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.

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Section: Introductionmentioning

confidence: 99%

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

Mohr

Wilhelm

Wandel

et al. 2020

Weather Clim. Dynam.

Self Cite

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“…In the study of Qiang et al [23], an FM acted on the feature extractor to solve the dilemma that high-order features in sparse data were difficult to learn while ensuring the diversity of extracted features and reducing the complexity of the algorithm. This is similar to matrix decomposition in collaborative filtering, and the expression of an FM is ( ) (5) where i v represents the latent vector of feature component i…”

Section: Fm Componentmentioning

confidence: 99%

“…In meteorology, hail is generally forecast by meteorological factor analysis and atmospheric evolution law [5]. Manzato et al [6] conducted diagnostic analysis on 52 2 of 22 meteorological factors, and the results revealed that five sounding factors had good correlation with local hail events.…”

Section: Introductionmentioning

confidence: 99%

XGB+FM for Severe Convection Forecast and Factor Selection

Ding

et al. 2021

Electronics

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In the field of meteorology, radiosonde data and observation data are critical for analyzing regional meteorological characteristics. Because of the high false alarm rate, severe convection forecasting is still challenging. In addition, the existing methods are difficult to use to capture the interaction of meteorological factors at the same time. In this research, a cascade of extreme gradient boosting (XGBoost) for feature transformation and a factorization machine (FM) for second-order feature interaction to capture the nonlinear interaction—XGB+FM—is proposed. An attention-based bidirectional long short-term memory (Att-Bi-LSTM) network is proposed to impute the missing data of meteorological observation stations. The problem of class imbalance is resolved by the support vector machines–synthetic minority oversampling technique (SVM-SMOTE), in which two oversampling strategies based on the support vector discrimination mechanism are proposed. It is proven that the method is effective, and the threat score (TS) is 7.27~14.28% higher than other methods. Moreover, we propose the meteorological factor selection method based on XGB+FM and improve the forecast accuracy, which is one of our contributions, as well as the forecast system.

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“…Multi-day clusters of severe weather and associated high impacts have also been reported from North America (Shafer, C., Doswell III, 2012;Trapp, 2014;Schroder and Elsner, 2020;Gensini et al, 2019). Although the atmospheric conditions associated with hail in Switzerland and central Europe are well studied (e.g., Huntrieser et al, 1997;Madonna et al, 2018;Taszarek et al, 2017;Púčik et al, 2015;Brooks, 2009;Púčik et al, 01 Nov. 2019;Kunz et al, 2020), little is known about the large-scale weather conditions that lead to multi-day clusters of hail days. Such multi-day clusters are likely the result of the extended longevity or repeated re-establishment of particular features of weather situations over Europe.…”

Section: Introductionmentioning

confidence: 97%

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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Ambient conditions prevailing during hail events in central Europe

Cited by 33 publications

References 79 publications

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

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

XGB+FM for Severe Convection Forecast and Factor Selection

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

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