Discriminating between Tornadic and Nontornadic Thunderstorms Using Mesoscale Model Output

Abstract

The ability to discriminate between tornadic and nontornadic thunderstorms is investigated using a mesoscale model. Nine severe weather events are simulated: four events are tornadic supercell thunderstorm outbreaks that occur in conjunction with strong large-scale forcing for upward motion, three events are bow-echo outbreaks that also occur in conjunction with strong large-scale forcing for upward motion, and two are isolated tornadic supercell thunderstorms that occur under much weaker large-scale forcing. Examination of the mesoscale model simulations suggests that it is possible to discriminate between tornadic and nontornadic thunderstorms by using the locations of model-produced convective activity and values of convective available potential energy to highlight regions of likely thunderstorm development, and then using the values of storm-relative environmental helicity (SREH) and bulk Richardson number shear (BRNSHR) to indicate whether or not tornadic supercell thunderstorms are likely. Values of SREH greater than 100 m2 s?2 indicate a likelihood that any storms that develop will have a midlevel mesocyclone, values of BRNSHR between 40 and 100 m2 s?2 suggest that low-level mesocyclogenesis is likely, and values of BRNSHR less than 40 m2 s?2 suggest that the thunderstorms will be dominated by outflow. By combining the storm characteristics suggested by these parameters, it is possible to use mesoscale model output to infer the dominant mode of severe convection.