Hail cannot be measured on the ground over a widespread comprehensive area. All the products presented are based on radar hail data, which are derived from the radar measurements with the help of algorithms. The radar hail data are compared against local ground observations such as hailstone findings or damage. The availbale radar data series from 2002 onwards is relatively short for climatological purposes. Thus statistical approaches (so-called resampling) are used to estimate rare, more severe hazards in order to make statements about extreme events that are only expected, for example, once every 50 years.
Data from MeteoSwiss weather radars serve as the basis for the climatological calculations. In contrast to other types of measurement data, radar data are more suitable for calculating a hail climatology, as radar data cover the entire area of interest and have a high temporal and spatial resolution. This is an indispensable prerequisite for the observation of comparatively small and short-lived phenomena such as hailstorms.
The Swiss weather radar network
Since 2002, the raw data of the Swiss radar network have been available in high quality. These data cover the whole of Switzerland and neighbouring regions. Between 2002 and the end of May 2014, a total of three radars were in operation, at the sites Monte Lema in Ticino, Albis near Zurich and La Dôle near Geneva (3rd radar generation of the Swiss monitoring network). In 2011 and 2012, the three radars were completely renewed in the Rad4Alp project and equipped with the latest technological features (4th radar generation of the Swiss monitoring network). The generation change resulted in another massive improvement in data quality. Two more radars in mountain regions were added in mid-2014 and early 2016: One on the Pointe de la Plaine Morte in Valais and the other on the Weissfluh peak near Davos. The new radars provide better visibility of partially shaded mountain regions and guarantee coverage of Switzerland if one of the other radars should ever fail. In the Swiss radar network, a new 3D measurement is taken every 5 minutes and stored with a resolution of 1 km2.
Radar data are measurements of reflectivity signals at altitude, for example of raindrops or ice particles in clouds. These signals must first be converted in a complex way to obtain information on the weather at the ground level. The hail estimates are based on the difference between the height of the zero degree isotherm in the thunderstorm environment, which is obtained from the weather model, and the height of the so-called EchoTop, obtained from radar measurements. The EchoTop is related to the active core of a thundercloud. The greater the distance between the EchoTop and the zero degree isotherm, the greater the probability of hail and the expected hailstone size. MeteoSwiss has been using two main hail algorithms since 2002:
- POH (Probability of Hail), indicates the probability of hail on the ground per 1 km2 (Waldvogel et al. 1979 and Foote et al. 2005). The algorithm is based on the 45 dBZ EchoTop height, i.e. the greatest height within a vertical column at which a 45 dBZ reflectivity signal is measured by the radar, and the height of the zero degree isotherm.
- MESHS (Maximum Expected Severe Hail Size), indicates the largest possible hailstone size that could occur per 1km2 (Treloar 1998 and Joe et al. 2004). The algorithm is based, similar to POH, on the relationship between the 50 dBZ EchoTop height and the height of the zero degree isotherm. MESHS is an estimate of how large a hailstone could be that could grow if it remained in the observed thundercloud for a long time, eventually falling out of the cloud and hitting the ground. The fact that a hailstone of the size MESHS is found by someone on the ground within the square kilometre concerned is likely to be rare. Firstly, this maximum conceivable diameter is often not reached in reality, or only just. Secondly, the probability is small that a person will find the largest hailstone impacting the ground within one square kilometre before it melts. Since there are not many of the largest hailstones within the square kilometre, the probability is small that one will fall, for example, exactly on the roof of a house, and cause damage.
Both POH and MESHS are calculated over the whole of Switzerland and nearby regions in neighbouring countries with a mesh size of 1 kilometre and a temporal resolution of 5 minutes. Both data fields are available one minute after each radar measurement.
Quality check and improvement of the data basis
Although radar data are well suited for hail observations, their application to create a climatology presents some challenges.
- In the weather sector, radar data are typically used for observation and short-term forecasting (so-called nowcasting) and not for climatological purposes. This is why, for example, the temporal homogeneity of the data series is not given. Furthermore, this also means that seemingly small errors in the measurements, which are no obstacles in "normal" real-time applications and are simply filtered out by the human eye, can pose problems when aggregating data.
- In the period from 2002 to the present, the radar systems as well as the weather model, whose data are used in the hail algorithms, have been steadily improved. While these improvements increased the quality of observations and forecasts, these changes have had an impact on the long-term comparability of measurements.
Because of this, an elaborate control and preparation is indispensable before further use of the data for climatological applications. In developing the data basis for the new hail climatology of Switzerland, the effects of the technical changes on the many-year data series have been, for the first time, documented, quantified and corrected where possible. The aim is to obtain a data set that is as homogeneous as possible over time in order to be able to make robust statements about hail occurrence in the long term.
The prerequisites for calculating return periods are very long measurement series. The time series of radar measurements since 2002, is short compared to typical climatological time scales which comprise at least 30 years. In addition, hail is a sporadic phenomenon, it occurs very rarely in relation to a single location, and usually lasts only a few minutes. The figure of the total sum of days with hail per location shows that, with the exception of the hotspot regions, there are only a few locations with sufficient data for a robust statistical evaluation of the extremes in the observational data. Spatial climatological evaluations are therefore often dominated by individual observed thunderstorm cells. Due to the nature of hailstorms, it can, however, be assumed that the long-term occurrence probabilities are spatially more homogeneous. On the next thunderstorm day, a thunderstorm can also take a slightly different path - even if the variability in the mountainous country of Switzerland is limited by the terrain.