Tuesday, 29 August 2023: 2:15 PM
Great Lakes BC (Hyatt Regency Minneapolis)
The TORUS field project successfully deployed on a cyclic tornadic supercell in southwest Nebraska on 17 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. The continuous analysis period runs from 22:57 to 00:36 UTC, during which time nine tornadoes are confirmed. Wind syntheses and gridded reflectivity are ingested into a diabatic Lagrangian algorithm (DLA), which integrates ordinary differential equations along backward trajectories and representing heat and water substance conservation, to obtain gridded θ, θv, and various mixing ratios. Kinematic comparisons to mobile mesonets (MMs) show generally good performance of the wind syntheses and there exists high correlation between thermodynamic analyses compared to the both MMs and two updraft soundings launched around 00:00 UTC.
Low-level analyses show derived wind vectors adequately capture observed supercell behavior, including a transition from a weaker to stronger LLM as the supercell becomes more actively tornadic, especially after 00:00 UTC. Vortex-line arches (VLAs) are predominant with the first EF-2 tornado before 23:03 UTC, and inability to maintain VLAs owing to absence of strong low-level updrafts may have been related to the short-lived nature of the tornado. After a lengthy reorganization period (23:06 to 00:00 UTC), wind syntheses present strengthening of the near-surface circulation coincident with northerly reorientation of the winds within the rear-flank downdraft (RFD), signaling the beginning of a sustained, active tornadic period. This period has four tornadoes, two of which are significant, reflected as persistent and strong near-surface vertical vorticity. Trajectory analyses show LLM strengthening is coincident with parcels obtaining a more curved path through the forward-flank (FF) precipitation region rather than directly through the undisturbed inflow environment. FF influence on LLM parcels remains non-zero through the end of the analysis period, corroborating numerical simulation results showing FF parcels play a large role in LLM modulation. Quantification of 0-1 km vertical vorticity (ζ) stretching shows lack of robust ζ stretching before 00:15 UTC, evidenced by two significant but short-lived tornadoes. After 00:15 UTC, collocation of 0-1 km updrafts with ζ centered on and to the northwest of the circulation show consistent and intense ζ stretching, which likely led to a longer-lived, 18-min EF-1 tornado after 00:15 UTC. Additionally, ζ isosurfaces depict a mesocyclone extending through 9 km AGL ancillary to the robust ζ stretching. Starting from 00:30 UTC mesocyclone occlusion and vortex spin-down is captured by the analyses.
DLA results are presented in the form of evolution of surface θv boundaries. Namely, a FF cool pool surge simultaneous with a surface warm pocket appears at 23:48 UTC, which leads to RFD maturation just before a significant tornado. After 00:00 UTC, more negatively buoyant parcels begin to wrap around the surface circulation, eventually blocking surface inflow to the LLM at 00:30 UTC as occlusion of the LLM begins. Vorticity budgets of LLM parcels at 00:00 UTC show clear evidence of a streamwise vorticity current with positive 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, confirming that SVCs in numerical simulations are indeed rooted in reality.
Low-level analyses show derived wind vectors adequately capture observed supercell behavior, including a transition from a weaker to stronger LLM as the supercell becomes more actively tornadic, especially after 00:00 UTC. Vortex-line arches (VLAs) are predominant with the first EF-2 tornado before 23:03 UTC, and inability to maintain VLAs owing to absence of strong low-level updrafts may have been related to the short-lived nature of the tornado. After a lengthy reorganization period (23:06 to 00:00 UTC), wind syntheses present strengthening of the near-surface circulation coincident with northerly reorientation of the winds within the rear-flank downdraft (RFD), signaling the beginning of a sustained, active tornadic period. This period has four tornadoes, two of which are significant, reflected as persistent and strong near-surface vertical vorticity. Trajectory analyses show LLM strengthening is coincident with parcels obtaining a more curved path through the forward-flank (FF) precipitation region rather than directly through the undisturbed inflow environment. FF influence on LLM parcels remains non-zero through the end of the analysis period, corroborating numerical simulation results showing FF parcels play a large role in LLM modulation. Quantification of 0-1 km vertical vorticity (ζ) stretching shows lack of robust ζ stretching before 00:15 UTC, evidenced by two significant but short-lived tornadoes. After 00:15 UTC, collocation of 0-1 km updrafts with ζ centered on and to the northwest of the circulation show consistent and intense ζ stretching, which likely led to a longer-lived, 18-min EF-1 tornado after 00:15 UTC. Additionally, ζ isosurfaces depict a mesocyclone extending through 9 km AGL ancillary to the robust ζ stretching. Starting from 00:30 UTC mesocyclone occlusion and vortex spin-down is captured by the analyses.
DLA results are presented in the form of evolution of surface θv boundaries. Namely, a FF cool pool surge simultaneous with a surface warm pocket appears at 23:48 UTC, which leads to RFD maturation just before a significant tornado. After 00:00 UTC, more negatively buoyant parcels begin to wrap around the surface circulation, eventually blocking surface inflow to the LLM at 00:30 UTC as occlusion of the LLM begins. Vorticity budgets of LLM parcels at 00:00 UTC show clear evidence of a streamwise vorticity current with positive 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, confirming that SVCs in numerical simulations are indeed rooted in reality.
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