Fine-Scale Structure of a Supercell Forward-Flank Convergence Boundary: Observations and Modeling

The forward flank convergence boundary (FFCB) in supercells has been discussed in many observational and modeling studies. It is theorized that the FFCB can be a focal point in baroclinic generation of vorticity that is streamwise in nature, and therefore important for the generation of mid-level (and possible low-level) vertical vorticity. Simulations of supercells with varying grid spacing are consistent in showing this feature. However, higher resolution simulations tend to resolve larger magnitudes of vorticity present immediately behind the FFCB. Orf et al. (2017) simulated a particularly strong and coherent realization of this vorticity and dubbed it the streamwise vorticity current (SVC). We aim to corroborate and extend on these findings through further simulation and concentrated mobile radar observations.

On June 12, 2017 as part of the Rivers of Vorticity in Supercells (RiVorS) project, a mobile observed sounding was gathered from NSSL’s P1 mobile mesonet to sample the pre-convective environment of a tornadic supercell near Cheyenne, WY. Based on this sounding, a tornadic supercell was modeled using CM1 at 125-m horizontal grid spacing, and a coherent SVC was indeed shown to be present. Simulated range-height indicator (RHI) data show the strongest vorticity in the SVC was concentrated in Kelvin-Helmholtz billows associated with the vertical wind shear characteristic of the region immediately behind the FFCB. Further simulations were run to test dependence of SVC magnitude and FFCB structure on grid spacing. Parcel trajectories and vorticity budgets were also analyzed to identify parcel paths relative to the baroclinic zone.

Throughout the spring of 2017 and 2018, the TTUKa radars were involved in two field campaigns: The Rivers of Vorticity in Supercells (RiVorS) and NSF National Robotics Initiative (NRI) projects, each seeking to learn more about supercell thunderstorm dynamics through intensive multi-platform observations. On June 12, 2018, a long-duration coordinated deployment involving both TTUKa radars took place on a nearly stationary supercell near Gate, OK. This supercell initially exhibited a strong boundary within the forward flank that was observed with both plan-position indicator and RHI strategies. Strong similarities between the modeled and radar-observed FFCB structure will be reported.