Supercell Environments in Finland

Thunderstorms are a common cause of severe weather in the summer season in Finland. Weather phenomena associated with thunderstorms in Finland - such as lightning, torrential rain, wind gusts, significant hail, and even tornadoes – have been associated with extensive economic damage and occasional loss of life. Some of these damaging events have been identified as supercells – a type of thunderstorm that, although uncommon at high latitudes, does occur in Finland.

In an effort to learn more about the atmospheric environments in which supercells occur in Finland, we examined radar images on dates during which significant hail (hail with a diameter >5 cm) or tornadoes were observed in the country. Thunderstorms on those dates were classified as supercells based on the subjectively identified presence of hook echoes, provided these signatures persisted across multiple consecutive radar timesteps. The dataset resulting from this procedure forms the basis for a comparative analysis of the meteorological environments associated with supercells and ordinary thunderstorms. Specifically, we compare kinematic and thermodynamic parameters from proximity soundings to identify environmental differences between the two storm types. Additionally, we examine the difference in meteorological environments of significant hail-producing supercells (acronym HAIL) and tornado-producing supercells (acronym TOR) environments.

The results indicate that bulk wind shear in various levels, as well as effective bulk wind shear (the bulk wind shear over the unstable layer), are strong discriminators between supercell and ordinary thunderstorm environments in Finland. Composite parameters such as the Energy Helicity Index (EHI) and Supercell Composite Parameter (SCP) also show some utility in distinguishing supercell and ordinary thunderstorm environments in Finland.

Equilibrium Level (EL) and low-level Convective Available Potential Energy (CAPE) stand out as significant discriminators between significant hail-producing and tornado-producing supercell environments, while Lifting Condensation Level (LCL) and low-level humidity show some utility in differentiating between significant hail- and tornado-producing supercell environments. In contrast, composite parameters and Storm Relative Helicity (SRH) offer very limited ability to distinguish between the two.