Spatial and temporal characteristics of hailstorms over complex orography: a long-term radar-based analysis in the Swiss Alpine area

During the warm season of the year (May-September), intense thunderstorms regularly affect the Alpine area. High impact convective phenomena like severe hailstorms can cause substantial damage to agriculture, forest, buildings, cars and infrastructure. In Switzerland severe summer storms are among the costliest high-impact weather events and it is therefore important to understand in-depth the dynamics and the physics of these phenomena as well as their interaction with the complex orography. A new project focusing on hail storms over the Swiss Alpine and pre-Alpine region recently started thanks the collaboration of the University of Bern and MeteoSwiss. An important first step is to compile a comprehensive radar-based climatology of spatial and temporal characteristics of hail storms in Switzerland for the period 2002-2013. Two weather radar products referred to as Probability Of Hail (POH) and Maximum Expected Severe Hail Size (MESHS) have been reprocessed for the whole period. These products have been analyzed in order to investigate spatial and temporal distribution of hail. A pixel- and an object-based approach have been used to investigate climatological characteristics of hailstorms. An independent, operational radar-based thunderstorms detection and tracking algorithm, called TRT (Thunderstorms Radar Tracking), has been used to track severe hailswaths. The paper will present and discuss the first results of this new radar-based hail climatology for the Alpine and pre-Alpine area. The discussion will rely on the meteorological interpretation of the events and on a preliminary statistical analysis. A comparison with the results of other climatological investigations like lightning, intense precipitation and cold fronts will also be provided. Furthermore, the challenges of using radar-based hail products over complex terrain and an overview of future works will be presented.