Climate change impacts and vulnerability in Europe 2016

Changes in the climate system
3.2.7 Hail
Key messages
• Hail events are among the most costly weather-related extreme events in several European regions, causing substantial
damage to crops, vehicles, buildings and other infrastructure.
• The number of hail events is highest in mountainous areas and pre-Alpine regions. Since 1951, increasing hail trends have
been noted in southern France and Austria, and decreasing (but not statistically significant) trends have been noted in
parts of eastern Europe.
• Future projections of hail events are subject to large uncertainties, because small-scale hail events cannot be directly
represented in global and regional climate models. However, model-based studies for central Europe show some
agreement that hailstorm frequency will increase in this region.
Relevance
Hailstorms are most common in mid-latitudes with high
surface temperature and humidity, as these conditions
promote the required instability associated with strong
thunderstorms and the temperature in the upper
atmosphere is sufficiently low to support ice formation.
The occurrence of hail over Europe is not uniform
over space and time (Punge and Kunz, 2016). Most hail
events occur in summer or nearby mountain areas
where convective energy and trigger mechanisms for
convection are highest (Punge et al., 2014).
Hail is responsible for significant damage. For example,
three hailstorm events in Germany in July and August
2013 caused around EUR 4.2 billion of combined
damages to buildings, crops, vehicles, solar panels,
greenhouses and other infrastructure (Munich RE,
2014).
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 (Punge and Kunz, 2016). 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
(Kunz et al., 2009). 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 (Mohr and Kunz, 2013; Punge
and Kunz, 2016). 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 (Mohr,
Kunz, and Geyer, 2015).
Recently, European hail climatology for the period
1951–2010 was analysed using a combination of
various meteorological parameters relevant for
thunderstorms and hail (Mohr, Kunz, and Geyer, 2015).
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
(Map 3.12, 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
(Map 3.12, 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 (Brooks, 2013).
88 Climate change, impacts and vulnerability in Europe 2016 | An indicator-based report