Characteristics and Development of Streamwise Vorticity Currents in Simulated Supercells with Varying Midlevel Shear Vector Orientations

Streamwise vorticity currents (SVCs) have been hypothesized to enhance low-level mesocyclones within supercell thunderstorms and perhaps increase the likelihood of tornadogenesis. Recent observational studies have confirmed the existence of SVCs in supercells and numerical studies have allowed for further investigation of SVCs using high-resolution model output. A suite of 19 idealized supercell simulations, 14 of which exhibit SVCs, is analyzed to determine how SVC formation and characteristics may differ between storms. SVCs are also analyzed to determine if SVCs that precede tornado-like vortex (TLV) production differ from other SVCs.

SVCs develop on the cold side of left-flank convergence boundaries and their updraft-relative positions are partially dependent on outflow surge location. The duration and mean thickness of SVCs do not differ significantly between simulations with varying midlevel shear vector orientations and the duration of SVCs that precede TLVs does not differ from other SVCs. The mean thickness of SVCs preceding TLVs, however, is greater than that of other SVCs. TLV formation does not require the development of an SVC beforehand; 44% of TLVs in the suite of simulations are preceded by SVCs. When an SVC occurs, it is followed by a TLV 53% of the time, indicating not all SVCs lead to TLV formation. It seems plausible that SVCs can sometimes, but not always, enhance low-level mesocyclones and the likelihood of TLV production.

Backward trajectories initialized in SVCs reveal two primary airstreams feeding SVCs. The first is air that flows through the SVC for the entirety or majority of its length. This air descends from 3 km AGL or lower and usually exhibits a significant left turn near the rear or left flank of the storm (where more outflow is concentrated) before the trajectory flows through the SVC. The second is modified inflow air from the forward flank that converges with the SVC, where the air is then entrained into the SVC horizontal circulation.

Vorticity budgets calculated along trajectories reveal that trajectories within the first airstream exhibit significantly greater maximum streamwise vorticity magnitudes than those within the second airstream. The vorticity budgets indicate that stretching of streamwise vorticity is likely the dominate contributor to the large values of streamwise vorticity within SVCs, regardless of airstream. The trajectories within the first airstream also obtain crosswise vorticity through baroclinic generation as they approach the supercell downdraft, which is then exchanged or tilted to streamwise vorticity during the left turn, and then the streamwise vorticity is stretched as the air flows through the SVC. The streamwise vorticity is finally tilted into the vertical within the updraft where it may be subsequently stretched.