Hail is one of the most costly local hazards resulting from storms, representing a major threat to agriculture, property and people. Nevertheless, uncertainties remain regarding its predictions regarding with climate change. New computational capabilities and Convection-Permitting Models (CPMs) have made it possible to better assess the future threat of hail on a continental scale. A study using pan-European CPM simulations with a grid spacing of 2.2 km examined future changes in the Severe Hail Potential (SHP) in Europe by the end of the century (around 2100) under the RCP8.5 emissions scenario.

The results of these simulations contradicts findings from coarser resolution models, which typically predicted future increases in hail frequency based on environmental indicators.

Surprising decrease in severe hail potential

New research shows that the potential for severe hail (defined as hailstones 2 cm or larger in diameter) overall decreases under the RCP8.5 scenarioThis decline occurs despite an expected increase in the number of storms producing many small ice particles that act as hail embryos.

In continental Europe, SHP cases are expected to in the summer they will reduce by more than half. Overall, a negative signal dominates for most of continental Europe, with a small increase in northern regions. Even the simulation for mid-century (2040–2049, with a more moderate warming of +2.3 K compared to the pre-industrial period, as opposed to +5.2 K at the end of the century) shows a decrease in SHP compared to the current climate.

This decline is partly due to the fact that Hail forms at higher altitudes as the atmosphere warmsThis shift affects the strength of the updraft in the Hail Growth Zone (HGZ) and the extent to which hail melts before it reaches the surface. The increase in melting height is an important factor controlling changes in the frequency of hail reaching the surface.

The overall reduction in SHP frequency is the result of several factors: weaker rising current in HGZ, increased melting associated with higher freezing levels, and weaker deep-layer wind shear. However, instability overall increases with warming.

Formation of warm-type storms and regional risks

Although overall SHP is declining, the study found evidence of the emergence of the warm-type storm category in the future. These storms are similar to hail-producing storms in the tropics, where the largest hailstones can still hit the surface, despite high freezing levels.

These warm storms will be most common in the future Southern Europe (SEU), leading to a regional increase in the frequency of heavy hail. Potential warm storms occur throughout the warm season and are most common in September. They are expected to occur mainly in the SEU, especially in and around Italy in the summer and autumn.

An important finding is that although overall SHP is decreasing, Significant Severe Hail Potential (SSHP) does not show such a significant decline and is even growing regionally. In the SEU, the ratio of SSHP to SHP will double, which indicates a shift towards conditions favorable for the formation of the largest hailstones in warmer climates. Since the impact of hail increases nonlinearly with hailstone size, even a small increase in size can reverse the effects of a decreasing signal in frequency.

In conclusion, even though SHP is generally projected to decline, the company may need to prepare for (rare, but) stronger storms with greater impact in the futureFurther research is needed to better understand the potential of these warm storm clusters to produce very large and destructive hail at the surface. JRi