The May 27, 2019 Imperial, NE Supercell during TORUS: Origins, Observations, and Impacts of an SVC

During the 2019 field season of the Targeted Observation by Radars and UAS of Supercells (TORUS) project, teams sampled 17 supercells, both tornadic and nontornadic, over 14 days. On May 27, 2019, teams had multiple coordinated deployments on two tornadic supercells in northeast Colorado and southwest Nebraska, with three and a half hours of data collected on the second tornadic supercell between 22:00 and 01:30 UTC. Towards the end of the observational period, the supercell took on a high-precipitation structure, produced a few brief tornadoes, and had a very apparent streamwise vorticity current (SVC) both visually and with radar observations. In investigating the origins of this SVC, it is found that the rear-flank gust front of the northern supercell favorably phased into the forward-flank of the southern supercell, and was modified by the forward-flank precipitation to become a storm-scale boundary with appreciable streamwise vorticity flowing into the updraft. An evolution of this transition was observed by the NSSL NOXP X-band mobile radar and the TTU Ka-band mobile radars.

Between 01:12 and 01:23 UTC, high-resolution radar data were captured by one of the TTU Ka-band mobile radars of a spatially and temporally coherent SVC using both range height indicators (RHI)s and plan position indicators (PPI)s. Similar to past observational and modeling studies, a single steady-state rotor was present in the center of a Kelvin-Helmholtz billow, and boundary-layer separation was kinematically observed near the surface. Shortly after the last scan by the TTUKa radar, three NSSL mobile mesonets directly sampled the storm-scale boundary and associated cold pool. The kinematic, thermodynamic, and barometric structure will be discussed in detail. Furthermore, a few windsondes were released in the vicinity of the SVC, allowing for an estimate on the proportion of streamwise versus crosswise vorticity of the SVC.

Finally, continuous sampling of the supercell between 22:00 and 01:30 UTC by the P3 tail-doppler radars (TDR) with an approximate 8 minute cadence between successive volumes allowed for temporally resolute updraft and mesocyclone measurements. Normalizing for the environment, the poorly understood relationship between the SVC and updraft metrics, low-level mesocyclone strength, and tornadogenesis will be discussed.