Preprints
https://doi.org/10.5194/wcd-2021-25
https://doi.org/10.5194/wcd-2021-25

  12 May 2021

12 May 2021

Review status: this preprint is currently under review for the journal WCD.

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

Hélène Barras1,2, Olivia Martius1, Luca Nisi2, Katharina Schroeer3, Alessandro Hering2, and Urs Germann2 Hélène Barras et al.
  • 1Mobiliar Lab for Natural Risks, Institute of Geography, Oeschger Centre for Climate Change Reserach, University of Bern, Bern, Switzerland
  • 2Federal Office of Meteorology and Climatology, MeteoSwiss, Locarno-Monti, Switzerland
  • 3Federal Office of Meteorology and Climatology, MeteoSwiss, Zurich, Switzerland

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.

Hélène Barras et al.

Status: open (until 27 Jun 2021)

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Hélène Barras et al.

Hélène Barras et al.

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Short summary
In Switzerland hail may occur several days in a row. Such multi-day hail events may cause significant damage and understanding and forecasting these events is, thus, important. Using reanalysis data we show that weather systems over Europe move slower before and during multi-day hail events compared to single hail days. Surface temperatures are typically warmer and the air more humid over Switzerland and winds are slower on multi-day hail clusters. These results may be used for hail forecasting.