Future scenarios of thunderstorm environments in Fennoscandia based on a regional climate model

Abstract. This study investigates how increasing greenhouse gas concentrations may affect environmental conditions favourable for thunderstorms over Fennoscandia. The future scenarios are based on data produced with the HARMONIE-Climate convection-permitting regional climate model forced by two global climate models at the lateral boundaries. The distribution of thunderstorm-favourable days, defined as days with co-occurring conditional instability and precipitation, in model data compares well with observed thunder days in May–September 2002–2018 across the study domain. By 2081–2100, the area-average frequency of favourable days is projected to increase by approximately 40–185 % in Finland, 30–185 % in Sweden, and 15–210 % in Norway relative to 1986–2005 under the RCP4.5 and RCP8.5 emission scenarios. Projected increases are largest over the northern Fennoscandia and in simulations exhibiting stronger warming. Thunderstorm-favourable conditions occurring simultaneously with strong vertical wind shear are also simulated to become more common, suggesting more frequent potential for severe organized convection in a future climate. More research is needed to understand how the changes in general storm-favourable environmental conditions may affect actual hazard occurrence. The results support the use of the HARMONIE-Climate model for convective storm scenario studies and offer valuable context for climate change adaptation in Fennoscandia.