Using convection-permitting simulations to assess future changes in lightning and hail in Europe

How lightning and hail will change in the future with global warming is highly uncertain, mainly due to the small space and time scales of these phenomena, which requires high-resolution climate simulations for their accurate representation. Recently km-scale climate simulations have become available that are ‘convection-permitting’ and give the possibility of studying how convective storms may change. We use a pan-European convection-permitting (2.2km grid spacing) model to analyse future changes in deep moist convection, lightning, and hail across Europe under a high emissions-scenario. This includes a physically based lightning prediction scheme, that uses graupel and ice-fluxes that are explicitly represented within the model. It also provides convective-scale outputs, not previously available from conventional climate models, exploited here to study future changes using an ingredients-based approach inherited from severe storm forecasting.

For hail, we derive a new severe hail potential metric that represents the likelihood of hail >2cm at the surface, accounting for hail growth and subsequent melting. Overall, we find an increasing frequency of deep moist convection across Europe mainly due to more instability with warming, but partially limited by greater convective inhibition. However, the increase in storms does not directly translate to an increase in lightning, mainly due to much higher melting level heights resulting in less cloud ice and hence less electrification. Weaker circulation and a northward shift of favourable weather regimes in the future simulation also affects the results, which vary regionally. While lightning increases in future over the mountains (due to enhanced Alpine Pumping) and in the north, lightning decreases over much of central Europe and over the sea. The potential for severe hail generally decreases in future, although storms locally produce higher amounts of graupel in the south. We also identify a new, warmer type of thunderstorms in the future, which may have important implications for changes to the frequency of the largest hailstones.