HAILSTORMS IN SWITZERLAND - LEFT MOVERS, RIGHT MOVERS, AND FALSE HOOKS

In the central region of Switzerland, lying between the Jura Mountains to the north and the Alps to the south, severe hailstorms are a commo n summertime phenomenon. Eight years of data on these hailstorms show that they are nearly equally divided between left- and right-moving st orms. Depending on the exact environmental conditions, the severe hail storms consist variously of left- or right-moving ordinary-cell storms , left- or right-moving supercell storms, and left-moving storms of an intermediary type (i.e., supercellular in some but not all respects). The left movers of the intermediary type sometimes exhibit a cyclonic ally rotating echo appendage on the right-rear flank of the storm. Thi s appendage to the left mover resembles a hook echo associated with a classic supercell. It is dubbed a false hook, since it has a dynamical configuration substantially different from that of a classic supercel l. This difference is demonstrated by the fact that the false hook app ears on the wrong side of the left mover for it to be a mirror image o f a classic right-moving supercell. Sounding data show that at bulk Ri chardson numbers less than 30-50, the right-moving severe hailstorms i n central Switzerland tend to be stronger and are more likely to be su percellular, though they are almost never tornadic. The hodograph of t he wind in the environment of the storms shows that the winds are abou t one-half to two-thirds the strength of the winds associated with tor nadic storms over the central United States. The wind-shear vector tur ns generally clockwise with increasing height through the lowest 5-6 k m, with a maximum south-westerly wind at about the 3-km level. On days when left-moving storms occur, the shear vector in the lowest 2-3 km of the generally clockwise-turning layer tends to exhibit a slight cou nterclockwise turning with height. Model calculations have been carrie d out for a day on which slight counterclockwise shear was present in the lowest 2-3 km and on which both a right-moving supercell and a lef t-moving false-hook storm occurred. In addition to rawinsonde data, ob servations were obtained by three radars, surface stations, and a hail pad network. The model produces splitting storms. The right- and left- moving model storms match the observed storms quite well. The left-hoo k mover was a false-hook storm, since the separate, cyclonically rotat ing updraft in the false-hook region does not separate from the left-m oving storm. The false-hook appendage is found to consist of updraft a nd precipitation advected westward and southward in the cyclonically r otating south rear flank of the storm. It bounds a cyclonically rotati ng downdraft on the south side of the storm. When the model simulation is repeated after modifying the environment wind hodograph by reversi ng the sense of the turning of the shear vector at low levels, so that the environment wind-shear vector turned in the clockwise sense with increasing height throughout the entire lowest 5-6 km, the second spli t of the left mover occurs much sooner. Consequently, the southern ech o appendage is only a transitory feature, and a long-lived false-hook storm is not maintained. The model simulations indicate that the basic characteristics of thunderstorms in central Switzerland can be realis tically reproduced in a numerical model with a flat lower boundary. He nce, the environmental wind and thermodynamic stratification are infer red to be the primary factors determining storm structure. However, th e environment supports multiple storm structures, and those storm mode s selected by nature at a specific time and location may be determined by very subtle local effects, such as whether the low-level wind hodo graph exhibits a slight clockwise or counterclockwise perturbation. Su ch local variability of the winds is likely related, directly or indir ectly, to orography. Such variability is evidently random, though, res ulting in the even climatological distribution between left- and fight -moving storms.