Analyses of a Simulated Severe MCS and Tornadic Mesovortex Observed by PECAN on 5-6 July 2015

Understanding and forecasting nocturnal thunderstorms and their hazards remain elusive goals. To this end, an expansive network of fixed and mobile observing systems deployed in the summer of 2015 for the Plains Elevated Convection at Night (PECAN) field experiment to intercept and observe nighttime atmospheric phenomena. On 5-6 July 2015, several mobile radars and numerous ground-based surface and upper-air profiling systems sampled a severe quasi-linear convective system (QLCS) as it moved through southeastern South Dakota at night. The QLCS was responsible for several severe wind reports, including one over 80 mph, and an EF-0 tornado. The QLCS and its environment leading up to the mesovortex (MV) that produced this tornado was well-observed by the PECAN observing network.

In this study, observations from mobile radiosonde vehicles, Doppler radars, and aircraft are assimilated into an ensemble analysis and forecasting system to analyze this event with a focus on processes influencing the tornadic MV. All ensemble members simulated low-level MVs with one in particular generating two MVs within the MCS in conditions very similar to those observed. Forecasts from this member were analyzed to examine the processes leading up to the development of the­ MVs. Although the MV was embedded within a QLCS and occurred to the north of a bowing line segment, a supercell-like mechanism appeared responsible. Cyclonic vertical vorticity was traced to three separate airstreams: the first from southerly inflow that was characterized by tilting of predominantly crosswise horizontal vorticity along the gust front, the second from the north that imported streamwise horizontal vorticity directly into the low-level updraft, and the third from a localized rear-inflow jet in which the horizontal vorticity became streamwise during descent. The cyclonic vertical vorticity then intensified rapidly through intense stretching as the parcels entered the low-level updraft of the developing MV.