Non-mesocyclone Tornadoes

Abstract

Knowledge of environments favorable to the formation of tornadoes, even weak ones, is important to the soaring weather forecaster.  Accordingly, most violent tornadoes develop usually from supercell thunderstorms.  But, in Hungary, there have been damaging tornadoes which developed in environments not favorable for supercell formation (low vertical wind shear, small storm-relative Helicity).  A total of 31 visual vortices from 2005 to 2009 were studied using data obtained from reports of storm spotters and the ESWD database.  Eight of these vortices were classified as tornadoes (five F0, two F1, one F2), the others were funnels.  To investigate these cases, several analysis fields (MSL, moisture convergence, 0-2 km vertical temperature gradient, 0-3 km section of surface based CAPE, LCL) and vertical profiles were generated from the hydrostatic run of WRF ARW 3.1.1 with horizontal resolution of 10 km and the assimilation of surface information (provided by an automatic measurement network including approximately 100 instruments operating in Hungarian Meteorological Service).  The studied parameters in the environment of the cases appeared to be not significant.  So, it may be assumed that these types of tornadoes are the majority of relatively weak tornadoes in Hungary.  All tornadoes developed directly on stationary wind shift boundaries that generated strong convergence and pre-existing vertical vorticity circulations.  These boundaries could be occluded fronts developed ahead or behind the surface cold front, along a weak quasi-stationary front or in a flat pressure area (no pressure gradient).  The parent clouds of these tornadoes were usually developing cumulonimbus.  The intense precipitation started usually a short time after the observation of the intense vortices.  So, it may be assumed that the smaller scale thunderstorm outflow boundaries can enhance the pre-existing vertical circulations along wind shift boundaries.