This article presents a 13-year hail climatology for Switzerland based on volumetric radar reflectivity. Two radar-based hail detection products that are used operationally at MeteoSwiss, namely the Probability of Hail (POH) and the Maximum Expected Severe Hail Size (MESHS), have been reprocessed for the extended convective season (April-September) between 2002 and 2014. The result of these two products is a comprehensive hail distribution map, which highlights regional and local-scale hail characteristics. The map of the annual number of hail days shows a high spatial variability and several maxima over the foothills north and south of the Alps as wells as over the Jura mountains. Directly over the Alps hail frequency exhibits a minimum. Annual hail anomalies show a pronounced variability, which suggests that hail occurrence is strongly controlled by large-scale weather patterns. Furthermore, hail probability exhibits a strong seasonal and diurnal cycle with a maximum in July in the late afternoon. The hail peak over the northern pre-alpine region occurs approximately two hours earlier compared to the south. A possible explanation is the trigger mechanism between the cold pool initiated by early convective cells over the Jura mountains and the development of cells on the northern slope of the Alps. Since radar-based hail signals are only indirect measurements, statistical verification of the hail detection algorithms is crucial. Damage reports from an automobile insurance company are used as an independent dataset. Verification results confirm that radar-based hail algorithms provide valuable information on hail probability. Finally, the synoptic-scale hail-driving weather conditions are investigated using a weather type classification based on upper-air flow direction and mean pressure from a numerical weather prediction model. The results show that six out of nine main synoptic-scale patterns favour the development of hailstorms in Switzerland.
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.
International audienceDemonstration of probabilistic hydrological and atmospheric simulation of flood events in the Alpine region (D-PHASE) is made by the Forecast Demonstration Project in connection with the Mesoscale Alpine Programme (MAP). Its focus lies in the end-to-end flood forecasting in a mountainous region such as the Alps and surrounding lower ranges. Its scope ranges from radar observations and atmospheric and hydrological modeling to the decision making by the civil protection agents. More than 30 atmospheric high-resolution deterministic and probabilistic models coupled to some seven hydrological models in various combinations provided real-time online information. This information was available for many different catchments across the Alps over a demonstration period of 6 months in summer/ fall 2007. The Web-based exchange platform additionally contained nowcasting information from various operational services and feedback channels for the forecasters and end users. D-PHASE applications include objective model verification and intercomparison, the assessment of (subjective) end user feedback, and evaluation of the overall gain from the coupling of the various components in the end-to-end forecasting system
This paper studies a damaging hail storm that occurred on 6 June 2015 in the complex topography of Switzerland. The storm persisted for several hours and produced large hail resulting in significant damage.Storms of comparable severity occur on average only three times per year within the entire Swiss radar domain, but are rare events at this exact location, according to a set of over 400,000 automatically identified storms. A multitude of datasets, partly novel for central Europe, is now available to study the storm in great detail capturing its impacts, severity and development. The data we use include radar-based hail products, crowd-sourced hail reports, and insurance loss data. These independent datasets permitted a verification of both hail occurrence and hail size estimations by radar. The crowd-sourced reports agree well with radar-based hail observations and insurance data. Model data (ERA-Interim reanalysis, regional COSMO-2 analysis and WRF simulations) and radio-sounding data showed, that conditions were favourable for thunderstorm development due to an unstable and moist atmosphere over Switzerland, brought about by an interplay of large-scale pattern and local processes. Advection ahead of a cold front west of Switzerland and local evapotranspiration lead to high lower-tropospheric moisture. The large-scale flow and topographically induced Alpine pumping resulted in strong directional wind shear, and contributed to the longevity and severity of this storm. The cold front was not relevant for the vertical lifting. Using model simulations with very high resolution, we identified mountain wind systems and cold-air outflow as possible triggering and propagation mechanisms of this hail storm.
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