National Hail Climatology Switzerland

Project start 01.05.2018
Project end 30.04.2021
Topics Climate
Hazards
Region Swiss
Status Current projects

The project National Hail Climatology Switzerland („Hagelklima Schweiz“) aims at developing a consistent and spatially explicit hail climatology for Switzerland. A new generation of radar observations and reference data and modern statistical methods promise a significant improvement over the currently used hazard maps and will therefore facilitate precise risk assessments. The lead partners in the project are the Federal Office for Meteorology and Climatology MeteoSwiss, the Federal Office for the Environment (FOEN), the Federal Office for Civil Protection (FOCP), the Federal Office for Agriculture (FOAG), the Prevention Foundation of the Association of Cantonal Building Insurers (VKG / PSFP), the Swiss Insurance Association (SVV), the Swiss Hail Insurance, and the Swiss Society of Engineers and Architects (sia). All project partners delegate stakeholders, who are closely involved in the project design through regular discussion meetings. This guarantees targeted and user-oriented product development. The University of Bern is an additional associated consulting partner, contributing world-leading scientific expertise on hail research.

Hail is one of the three costliest natural hazards in Switzerland. Every year, hail storms cause millions of Swiss Francs in damage, surpassed only by storms and flooding. Buildings, vehicles and agriculture are assets that are particularly at risk. Forecast and warnings are essential nececcisties for risk management, but also insurers and the building sector require reliable and qualitative information on current and long-term hail risk.

The localized occurrence and short duration of hailstorms make it hard to accurately measure hail on the ground. As a consequence, systematic, spatially comprehensive observations of hail are largely missing in Switzerland. In the past, hail maps have been generated using damage observations from insurance data, hail reports, and point-scale observations. These maps are inconsistent and often based on data which are not quality-controlled, or were calculated with now outdated methods.

The National Hail Climatology Switzerland („Hagelklima Schweiz“)  project aims at developing  consistent, high-quality climatologies of hail frequency and intensity in Switzerland, which are run and updated operationally. Within the frame of the project, the most recent generation of radar data is reprocessed and improved and innovative statistical methodologies are developed and applied. Information on hail tracks, but also products that take a closer look at individual hail events complement the climatological maps and datasets.

A close and regular exchange with the stakeholders from all project partners ensures that deliverables are approached in targeted and user-oriented ways and that they will be ready to be used in practical applications.

Two MeteoSwiss teams, one in Zurich (climatology) and one in Locarno Monti (radar observations), closely collaborate to develop and implement the national hail climatology. The adequate treatment of inevitable uncertainties, such as those inherent to the estimation of recurrence intervals of rare extreme hail events, is a particular requirement within this project.

In addition, it is ensured that all developed products and climate information will continuously be updated and always be based on the latest radar observations, guaranteeing long-term and high-quality operation and provision hosted at MeteoSwiss.

From improved radar observations …

Because hail occurs on very small spatio-temporal scales, it is very difficult to measure on the ground in an accurate and representative way. Direct measurements, such as those by so-called “hailpads” or digital hail sensors (see info box “Parallel projects") would need to be available in very high spatial density over the entire area of Switzerland in order to deliver useful information on hail occurrence over the country.

Observations from the five radar stations of the Swiss radar network cover the entire area of Switzerland and thus present a unique data basis for developing a new Swiss national hail climatology.

Parallel projects

In parallel projects, MeteoSwiss, the Mobiliar Lab for Natural Risks at the University of Bern, the inNET Monitoring AG, and the Mobiliar Insurance Company collaborate on developing new reference data to refine and validate automatic radar hail detection technology. An entirely new kind of information is provided by the so-called “crowd-sourcing” data, i.e. hail reports by the population collected through the MeteoSwiss mobile app.

In addition, within the scope of the “Swiss hail observation network” project, 80 automatic hail sensors are being deployed and operated in the hail hotspots of Switzerland. The observation network will run from 2018 to 2025 and will deliver unique measurements of the number, size and kinetic energy of hail stones reaching the ground.

By using the data from weather radars and applying specifically developed algorithms,  the probability of hail and even the expected size of hailstones can be estimated on a 1 km by 1 km raster covering Switzerland. Such estimates ground on empirically established relations between the altitude of the freezing level and the altitude of specific radar echoes.

In the course of the project, the hail detection algorithms currently implemented at MeteoSwiss will be reviewed and improved where possible. This will provide a new generation of radar data for the statistical estimation of climatological frequencies and intensities of hail storms.

… to new climate data for hail risk assessment

Climatologies, describing the long-term statistics of weather, inform us on where and when hailstorms of certain intensities can be expected. Such information is relevant, for example, for insurance companies calculating portfolios, but also for the building sector to adequately plan prevention measures such as building codes for roofs.

Because of incomplete measurements and short observational records of hail, existing climatological maps are spatially coarse, and the magnitude of uncertainties is largely unknown.

Improved radar data now provide the opportunity to recalculate a hail climatology for Switzerland with unprecedented spatial accuracy. In addition, exceedance probabilities and intensities of hail storms can be analyzed in high spatio-temporal resolution. To deliver robust statements and quantitative estimation of uncertainties, the project builds on internationally renowned expert knowledge and years of expertise in extreme value statistics at MeteoSwiss. The hail hazard information will be made available to the public after the project is finished.      

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