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Indicator Assessment
Observed annual median and trend of the Mean Potential Hail Index (PHI) over the period 1951-2010
Note: Based on the logistic hail model (Mohr, Kunz, and Geyer, 2015) and reanalysis data from NCEP-NCAR (Kalnay, et al., 1996). Trends with significance below the 5% level are cross-hatched. Note that significant trends are only found for values below -5 PHI over the period.
Past trends
Trends in days with hail have been calculated using surface-based observations, but are unreliable owing to the limited number of stations and the stochastic nature of hailstorms [i]. Trends in hail observations are sometimes analysed using reports of damage as a proxy (e.g. insurance claims), although damage is also a function of the vulnerability of the impacted area to damage. Several European regions show an increase in the convective conditions that can potentially form hail. In some areas (such as south-west Germany), an increase in damage days is observed [ii]. However, these changes are not uniform across Europe, with large regional differences mostly related to topography.
A study of hailstorm frequencies over the period 1978–2009 in Germany and eastern Europe shows general increases in convective available potential energy (CAPE) and increases in evaporation, which have been attributed to rising temperatures, but the changes in these weather variables do not necessary modify the numbers and intensities of severe convective storms [iii]. The atmosphere has become more unstable, and thus more suitable for hail, especially in southern and central Europe, where the temperature increase in summer has been particularly large[iv].
Recently, European hail climatology for the period 1951–2010 was analysed using a combination of various meteorological parameters relevant for thunderstorms and hail[v]. This has been expressed as the potential hail index (PHI), which quantifies the atmospheric potential for hailstorms. The climatology shows the highest values of the mean PHI for the areas north and south of the Alps, the eastern Adriatic coast and parts of eastern Europe (Figure 1 left). Increasing hail trends (with a PHI over 3 in the period 1951–2010) are found in southern France and Spain, and decreasing trends (with a PHI lower than –5 in the period 1951–2010) in eastern Europe (Figure 1 right). However, trends are not significant (at the 5 % significance level) in most grid boxes.
Projections
Much of the published work relevant to future hail projections is based upon developing the relationships between large-scale atmospheric environments and small-scale severe weather events, such as severe thunderstorms, hailstorms and tornadoes. Available projections suggest increases in CAPE, which result in conditions that favour severe thunderstorms becoming more frequent, and decreases in wind shear, which reduces the likelihood of hailstorms [vi].
Different RCMs have been used for assessing changes in hailstorms at the national and sub-national scales. A statistically significant downwards trend for hailstones with diameters between 21 and 50 mm was projected for the United Kingdom [vii]. An increase in hailstorm frequency between 7 and 15 % for the period 2031–2045 compared with 1971–2000 was projected for south-west Germany based on large-scale weather patterns [viii]. Using the PHI and an ensemble of seven RCMs, an increase in hail probability over most areas of Germany was projected for the period 2021–2050 compared with 1971–2000 [ix]. The projected changes are largest in southern Germany (values of almost 7 PHI). However, the results are subject to large uncertainties, mainly owing to low spatial resolution and convective parametrisation schemes in regional climate models [x]. Improving the convective parametrisation schemes and increasing the spatial resolution of models would improve the accuracy of future hail projections.
[i] H. J. Punge and M. Kunz, “Hail Observations and Hailstorm Characteristics in Europe: A Review,”Atmospheric Research 176–77 (August 1, 2016): 159–84, doi:10.1016/j.atmosres.2016.02.012.
[ii] M. Kunz, J. Sander, and Ch. Kottmeier, “Recent Trends of Thunderstorm and Hailstorm Frequency and Their Relation to Atmospheric Characteristics in Southwest Germany,”International Journal of Climatology 29, no. 15 (December 1, 2009): 2283–97, doi:10.1002/joc.1865.
[iii] S. Mohr and M. Kunz, “Recent Trends and Variabilities of Convective Parameters Relevant for Hail Events in Germany and Europe,”Atmospheric Research, 6th European Conference on Severe Storms 2011. Palma de Mallorca, Spain, 123 (April 1, 2013): 211–28, doi:10.1016/j.atmosres.2012.05.016; Punge and Kunz, “Hail Observations and Hailstorm Characteristics in Europe.”
[iv] S. Mohr, M. Kunz, and B. Geyer, “Hail Potential in Europe Based on a Regional Climate Model Hindcast,”Geophysical Research Letters (submitted) (2015).
[v] S. Mohr, M. Kunz, and B. Geyer, “Hail Potential in Europe Based on a Regional Climate Model Hindcast,”Geophysical Research Letters (submitted) (2015)
[vi] H. E. Brooks, “Severe Thunderstorms and Climate Change,”Atmospheric Research, 6th European Conference on Severe Storms 2011. Palma de Mallorca, Spain, 123 (April 1, 2013): 129–38, doi:10.1016/j.atmosres.2012.04.002.
[vii] M. G. Sanderson et al., “Projected Changes in Hailstorms during the 21st Century over the UK,”International Journal of Climatology 35, no. 1 (January 1, 2015): 15–24, doi:10.1002/joc.3958.
[viii] M.-L. Kapsch et al., “Long-Term Trends of Hail-Related Weather Types in an Ensemble of Regional Climate Models Using a Bayesian Approach,”Journal of Geophysical Research: Atmospheres 117, no. D15 (August 16, 2012): D15107, doi:10.1029/2011JD017185.
[ix] S. Mohr, M. Kunz, and K. Keuler, “Development and Application of a Logistic Model to Estimate the Past and Future Hail Potential in Germany,”Journal of Geophysical Research: Atmospheres 120, no. 9 (May 16, 2015): 2014JD022959, doi:10.1002/2014JD022959.
[x] E. M. Fischer et al., “Models Agree on Forced Response Pattern of Precipitation and Temperature Extremes,”Geophysical Research Letters 41, no. 23 (December 16, 2014): 2014GL062018, doi:10.1002/2014GL062018.
Hail is commonly classified according to diameter of the hailstones; for example, hail >=2cm diameter.
Hailstorm intensity scale classifies hail on a scale from H0, being hard hail with diameter 5 mm causing no damage to H10, being super hailstorms with diameter >100 mm and causing extensive structural damage with risk of severe or fatal injuries to people.
Hail is here defined with the potential hail index (PHI), which quantifies the atmospheric potential for hailstorms and can be derived from atmospheric numerical models.
PHI (unitless)
In April 2013 the European Commission presented the EU Adaptation Strategy Package (http://ec.europa.eu/clima/policies/adaptation/what/documentation_en.htm). This package consists of the EU Strategy on adaptation to climate change /* COM/2013/0216 final */ and a number of supporting documents. One of the objectives of the EU Adaptation Strategy is Better informed decision-making, which should occur through Bridging the knowledge gap and Further developing Climate-ADAPT as the ‘one-stop shop’ for adaptation information in Europe. Further objectives include Promoting action by Member States and Climate-proofing EU action: promoting adaptation in key vulnerable sectors. Many EU Member States have already taken action, such as by adopting national adaptation strategies, and several have also prepared action plans on climate change adaptation.
The European Commission and the European Environment Agency have developed the European Climate Adaptation Platform (Climate-ADAPT, http://climate-adapt.eea.europa.eu/) to share knowledge on observed and projected climate change and its impacts on environmental and social systems and on human health; on relevant research; on EU, national and subnational adaptation strategies and plans; and on adaptation case studies.
In September 2016, the EC presented an indicative roadmap for the evaluation of the EU Adaptation Strategy by 2018.
In November 2013, the European Parliament and the European Council adopted the 7th EU Environment Action Programme (7th EAP) to 2020, ‘Living well, within the limits of our planet’. The 7th EAP is intended to help guide EU action on environment and climate change up to and beyond 2020. It highlights that ‘Action to mitigate and adapt to climate change will increase the resilience of the Union’s economy and society, while stimulating innovation and protecting the Union’s natural resources.’ Consequently, several priority objectives of the 7th EAP refer to climate change adaptation.
not applicable
Hail forms within deep convective clouds with observations recorded only by ground based hail pad networks. Proxies for hail events can be also derived from satellite temperature imagery and radar reflectivity.
The occurrence of hail is related to atmospheric instability so its likelihood is related indices such as the convective instability index (CI) and the potential hail index (PHI). These indices are usually considered in combination with mesoscale factors such as wind flow, specific humidity and water vapour flux.
Proxies for hail events can be also derived from satellite temperature imagery and radar reflectivity. These are supplemented with eye witness and media reports which are collected by organisations such as the Tornado and Storm Research organisation (TORRO), the European Severe Storm Laboratory (ESSL) which maintains the European Severe Weather Database (ESWD), and Schweizer Hagel (an agricultural cooperative). These databases provide information about the spatial distribution and the frequency of severe convection. However, observational databases are limited in spatial or temporal extent and biased towards population centres where there are more observers.
No methodology references available.
See under "Methodology"
The occurrence of hail over Europe is not uniform as most hail events occur in the summer over Central Europe where convective energy is greatest. Trends in hail observations are sometimes made by using damage as a proxy although damage is also a function of hail type (size, density, accompanying horizontal wind speed and kinetic energy) and vulnerability of the impacted area to damage. The uneven distribution of hail pads across Europe makes trends difficult to detect by using only in-situ based observations.
Hail occurrences are also closely related to specific lightning signals with lightning detection data available from different sources. Radar data is another important proxy for hail events with a very high temporal and spatial resolution. However, radar reflectivity for most of European regions is only available since the mid-2000 and hence limited to assess the trends.
European MSG (SEVIRI) satellite data were used to develop a catalogue of hail events in Europe based on overshooting top data (OT).
Another method is to use combined different meteorological parameters relevant for hailstorm formation using a logistic model. Applied to different reanalysis data sets, the logistic model estimates the number of days with an increased potential of hail occurrence, denoted to as potential hail index.
see under methodology
For references, please go to https://www.eea.europa.eu/data-and-maps/indicators/hail/assessment or scan the QR code.
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