A 15‐year hail streak climatology for the Alpine region

Abstract

In this study, we present a unique 15‐year hail streak climatology for Switzerland based on volumetric radar reflectivity. Two radar‐based hail detection products and an automatic thunderstorm‐tracking algorithm were reprocessed for the extended convective season (April–September) between 2002 and 2016. More than 1.1 million convective cells were automatically tracked over the full radar domain, and over 191,000 storms and 31,000 hail streaks in the considered subdomain were selected for analysis following consistency and robustness tests. The year‐to‐year variability in the number of hailstorms reveals two types of convective seasons: (a) a few seasons with hail frequency far above the average, and (b) all other years with an average number of hailstorms. A high number of hailstorms in a particular year is not correlated with a higher number of convective storms in general, but is related to a greater fraction of severe storms. Convection initiation, hail initiation, and hail frequency maxima are located along the southern and northern foothills over the pre‐Alpine area and over the Jura mountains. Few hail streaks are present over the Alpine main ridge. Hail streak frequency and location is found to be strongly dependent on the synoptic‐scale weather regimes. This is important for monthly and seasonal outlooks, as well as for climate modelling. Analysis of storm life cycles shows that: (a) the majority of hail swaths contain only a single hail streak, (b) severe storms follow a more rapid evolution during their initial stages than do less severe storms, and (c) severe storms produce more spatially extended hail streaks. Finally, significant seasonal and diurnal cycles are present in most of the considered storm characteristics.

Ambient conditions prevailing during hail events in central Europe

“…3D radar reflectivity (Puskeiler et al, 2016;Lukach et al, 2017), a longer period (Kunz and Kugel, 2015), or overshooting top detections from satellite (Bedka, 2011;Punge et al, 2017). This applies also to the seasonal and diurnal cycles (Nisi et al, 2016;Punge and Kunz, 2016;Nisi et al, 2018). The good quantitative and qualitative agreement is a strong indication of the reliability of our methods and results.…”

“…High-density hailpad networks exist in only a few regions across Europe (e.g., Merino et al, 2014;Hermida et al, 2015) and therefore cannot be used to reproduce entire hailstorm footprints. In order to compensate for this monitoring gap, remote sensing instruments such as satellite (Bedka, 2011;Punge et al, 2017;Ni et al, 2017;Mroz et al, 2017), lightning (Chronis et al, 2015;Wapler, 2017), or radar (Holleman et al, 2000;Puskeiler et al, 2016;Nisi et al, 2018) due to their area-wide observability are used to estimate the frequency and intensity of SCS. In particular, weather radars can give some indications of hail occurrence using either radar reflectivity above a certain threshold (e.g., Mason, 1971;Hohl et al, 2002) or at specific elevations in combination with different height specifications (melting level, -20 • C environmental temperature, top of the storm cell; Waldvogel et al, 1979;Smart and Alberty, 1985;Witt et al, 1998).…”

Investigation of the temporal variability of thunderstorms in central and western Europe and the relation to large‐scale flow and teleconnection patterns

“…These modulations to frequency can lead to extreme differences in thunderstorm frequency from year to year, a pattern that is evident in European severe thunderstorm frequency (e.g. Kunz et al, 2009;Mohr et al, 2015a;Madonna et al, 2018;Nisi et al, 2018;Rädler et al, 2018;Taszarek et al, 2019). The primary drivers for this type of relationship are related to remote sea-surface temperatures (SST), the generation of tropical convection and the resulting influences on atmospheric Rossby waves and synoptic features (Tippett et al, 2015).…”

A 10‐year radar‐based analysis of orographic precipitation growth and decay patterns over the Swiss Alpine region

“…One of the most interesting features is the widespread region of growth going from the Upper Savoy to the Berner Prealps with high HZT, which is not observed with low HZT. This is probably linked to the systematic development and intensification of convective storms in the Prealps during summer months, which is also observed in the radar‐based hail climatology presented in Nisi et al , (; ()). With low HZT values, the region of maximum growth on the southern side of the Alps is located in an area extending from the Aosta Valley to the Monte Rosa and Simplon region (Figure a).…”