Significant-Hail-Producing Storms in Finland: Convective-Storm Environment and Mode

Abstract

Abstract The environmental characteristics and convective mode of significant hailstorms (those storms producing reported hail 5 cm or larger in diameter) in Finland during 1972–2011 were analyzed. Altogether, 23 significant-hail-day environments were analyzed by modifying radiosonde data from proximity soundings in the observed data archives of the Finnish Meteorological Institute. Convective parameters derived from the environmental soundings were compared between a set of significant-hail soundings and a null set of nonsevere-thunderstorm soundings. A subset of 13 significant-hail days was examined using data from a network of Doppler radars during 1999–2011. Convective-storm mode and storm characteristics (e.g., hook echo, bounded weak-echo region) were determined for the 18 significant-hail-producing storms during these days. Most (78%) of these storms producing significant hail in Finland occurred with supercells. Of the significant-hail days, 39% (9 out of 23) did not have the minimum of 15 m s−1 of deep-layer (0–6 km) shear commonly expected for supercells. Convective parameters of significant-hail and thunderstorm-day environments were substantially different from each other. Specifically, significant-hail environments had a mean most-unstable convective available potential energy (MUCAPE) of 1464 J kg−1 and deep-layer shear of 17.5 m s−1, whereas thunderstorm days had a MUCAPE of 593 J kg−1 and deep-layer shear of 10.2 m s−1. Larger hail was associated with higher values of MUCAPE. The lifetimes and track lengths of significant-hail-producing storms were related to the convective mode and storm environment. Specifically, larger deep-layer shear seemed to support longer lifetimes and track lengths. Nonsupercells had shorter lifetimes, shorter stormtrack lengths, and lower speeds than supercells. The value of deep-layer shear was smaller for nonsupercells than for supercells. Discrete supercells had higher speeds, longer lifetimes, and longer track lengths than cluster supercells.