Derecho-favoring atmospheric environments in Finland: characteristics, identification criteria, and frequency trends

Abstract. In this study, we investigate the atmospheric environments during eight summertime derechos and two derecho-type events in Finland, as well as trends in frequency of such environments during 1940–2022, using ERA5 reanalysis. We find that derechos in Finland tend to occur near the right entrance region of an upper-level jet streak, where deep-layer vertical wind shear is moderate, and deep-layer flow with a southerly component advects warm, moist air into the area. In all 10 studied cases, equivalent potential temperature at 850 hPa exceeds 325 K along the trajectory of the system. We define three sets of criteria to capture atmospheric environments favoring derecho formation in Finland. Two of the criteria sets are largely based on literature, whereas the third one is more adapted to the atmospheric conditions observed along the trajectories of derechos in Finland. The criteria sets consist particularly of kinematic parameters, including deep-layer (0–6 and 0–10 km) vertical wind shear and 6–10 km layer mean wind speed, but also contain thermodynamic variables, including convective available potential energy (CAPE) and equivalent potential temperature at 850 hPa. For each criteria set, we require that threshold values for combinations of these parameters are satisfied at the same grid point and over a sufficiently large grid area to classify the day as favorable for derecho formation. By applying the criteria sets, it appears that derecho-favoring environments in Finland have become more frequent, particularly in the period 1980–2022, which was characterized by both accelerated warming and a negative trend in the summer mean Eastern Atlantic/Western Russia teleconnection index. As such, it is plausible that derechos may become more common in the future, at least in Finland.