A Kinematic and Thermodynamic Analysis of the 17 May 2019 McCook / Farnam, Nebraska Tornadic Supercell

Abstract

The Targeted Observation by Radars and UAS of Supercells field project successfully deployed on a cyclic tornadic supercell in southwest Nebraska on 17–18 May 2019. This case study uses data from the two airborne radars mounted onto the P3 along with the ground-based NOXP radar for a triple-Doppler wind synthesis from 17 May 22:57 UTC to 18 May 00:36 UTC. Wind syntheses and gridded reflectivity are ingested into a diabatic Lagrangian algorithm (DLA) to obtain gridded thermodynamic information including θ, θv, and various hydrometeor mixing ratios.

Low-level analyses in Chapter 2 show that the triple-Doppler winds capture general supercell behavior, including a transition from a weaker to stronger low-level mesocyclone (LLM) as the supercell becomes more actively tornadic, especially after 23:57 UTC. Vortex-line arches (VLAs) are shown to be associated with the first EF-2 tornado, but weak low-level updrafts likely resulted in the short-lived nature of the first tornado. After a clear reorganization period from 23:06 to 23:57 UTC, strengthening of near-surface rotation is coincident with northerly reorientation of the winds within the RFD. LLM strengthening is also coincident with a shift of LLM parcels originating from the forward-flank precipitation region rather than directly from the undisturbed inflow environment, matching well with numerical simulation results showing forward-flank parcels play a large role in LLM modulation. ζ stretching within the 0–1 km layer is relatively weak until 00:15 UTC, evidenced by two significant but short-lived (3 min and 4 min) tornadoes. After 00:15 UTC, however, collocation of 0–1 km updrafts with ζ centered on and to the northwest of the circulation show consistent stretching, which likely contributed to a longer-lived, 18 min EF-1 tornado after 00:15 UTC. Isosurfaces of ζ show a deep, continuous mesocyclone at this time with the 0.05 s−1 isosurface connected from the ground to 9 km prior to occlusion at 00:30 UTC.

Evolution of surface baroclinic boundaries from the DLA in Chapter 3 reveal key changes as the supercell strengthens around 23:57 UTC — namely, a cold pool surge within the forward-flank region simultaneous with a surface warm pocket appearance at 23:48 UTC that leads to rear-flank downdraft maturation just before a significant tornado. Vorticity budgets along parcels terminating within the LLM at 00:00 UTC show clear evidence of baroclinic generation of streamwise horizontal vorticity along θv gradients. While observed SVCs have been noted in previous literature via cross-sectional or RHI analyses, this study is the first to explicitly compute vorticity budgets along parcel trajectories within a long-lived tornadic supercell, providing evidence that SVCs in numerical simulations are indeed physical.