Sack Lunch Seminar (SLS) – Brice Coffer (NC State)

“Forecasting supercell tornadogenesis using the near-ground wind profile”

A defining characteristic of supercell thunderstorms is their mesocyclone, a quasi-steady region of vertical vorticity within the storm’s updraft. The mesocyclone, especially at low altitudes (<1 km above ground level; AGL), plays a crucial role in the tornadogenesis process in supercells. Despite increasing understanding of environments that favor tornadic supercells, it is unclear how differences in the environmental profiles, especially the low-level wind profile, affect the in-storm processes that lead to tornadogenesis. As a result, the false-alarm rate for tornado warning is approximately 75%.

Using composite near-storm environments of nontornadic and tornadic supercells sampled during the second Verification of the Origins of Rotation in Tornadoes Experiment (VORTEX2), a nontornadic and a tornadic supercell were simulated in Cloud Model 1 (CM1). Both environments produce strong supercells with robust midlevel mesocyclones and hook echoes. The nontornadic supercell readily produces subtornadic surface vortices, but these vortices fail to be stretched by the low-level updraft. This is due to a disorganized low-level mesocyclone caused by crosswise vorticity in the lowest 500 m AGL within the nontornadic environment. In contrast, the tornadic supercell ingests streamwise horizontal vorticity, promoting a strong low-level mesocyclone with enhanced stretching of surface vorticity.

Using a larger dataset of supercells compared to those observed during VORTEX2 shows that supercell tornado forecasts are improved by using the storm-relative helicity (SRH) in the lowest few hundred meters of the atmosphere (instead of much deeper layers). Progressively shallower layers for the SRH calculation leads to increasing forecast skill. Replacing the effective-layer SRH with the 0–500 m AGL SRH in the formulation of Significant Tornado Parameter increases the number of correctly predicted events by 8% and decreases the number of missed events and false alarms by 18%.

While the environmental wind-profile shows substantial skill at forecasting tornadoes, conventional wisdom and textbooks attribute low-level mesocyclones to augmented streamwise vorticity generated baroclinically in the forward flank of supercells. Using additional supercell simulations, it is shown that air bound for the low-level mesocyclone primarily originates from the ambient environment (rather than the forward flank) and from very close to the ground, often in the lowest 200–400 m AGL. The low-level mesocyclone does not appear to require much augmentation from additional horizontal vorticity in the forward flank. Instead, the dominant contributor to vertical vorticity within the low-level mesocyclone is from the environmental horizontal vorticity.

About this Series: The Atmospheres, Ocean and Climate Sack Lunch Seminar Series is an informal seminar series within PAOC that focuses on more specialized topics than the PAOC Colloquium. Seminar topics include all research concerning the science of atmospheres, ocean and climate. The seminars usually take place on Wednesdays from 12-1pm. The presentations are either given by an invited speaker or by a member of PAOC and can focus on new research or discussion of a paper of particular interest. Contact: sacklunch-committee@mit.edu for more information and Zoom password