Low-Level Mesocyclone Evolution of a Cyclic Tornadic Supercell Observed during TORUS on 17 May 2019

Abstract This case study analyzes the 17 May 2019 cyclic, tornadic supercell from southwest Nebraska observed by the Targeted Observation by Radars and UAS of Supercells (TORUS) field experiment. Specifically, 12 multi-Doppler wind syntheses are generated over a 96-min period from 2301 UTC 17 May to 0037 UTC 18 May using two P-3 airborne radars and the ground-based NOXP research radar. Synthesized winds and reflectivity are assimilated into a diabatic Lagrangian analysis for the retrieval of thermodynamic data. The 4D wind fields are found to correlate well with observed tornadic and nontornadic periods, and several storm-scale features related to low-level mesocyclone (LLM) and near-ground rotation processes are documented. This includes vortex line arches that are a defining feature during the first EF2 tornado, followed by an occlusion process and reorganization period. During the most active tornadic period, backward trajectories reveal both inflow parcels and forward-flank parcels participate in the core of the 0–1-km rotation. While tilting of streamwise vorticity into vertical vorticity and subsequent powerful vertical stretching occurs for both inflow and forward-flank parcels, the solenoidal generation of streamwise vorticity is dominant with the latter. This resembles streamwise vorticity currents found within numerical simulations. Last, an intense left-flank convergence boundary develops coincident with the intensification of storm-relative inflow winds, with its formation and dissipation correlated with the final tornado. The 96-min analysis period with 4D kinematic and thermodynamic data makes this study one of the most detailed supercell case studies presented in the literature. Significance Statement A detailed analysis of a supercell that produced nine tornadoes within a 96-min period is presented. The supercell was observed by five radars, which are used to obtain information about the 3D wind, temperature, and moisture fields. Although computer simulations can provide detailed looks into supercell processes, collecting and analyzing observed supercell data of this quality is challenging and rare. We identify features within the supercell that are correlated with periods of strong and weak tornado production. Additionally, we identify the source region of air that is associated with low-level rotation in the supercell and comment on the importance of temperature gradients observed within the supercell, comparing these results to what has been found in simulations.