Comparing observations and simulations of the streamwise vorticity current in a tornadic supercell storm
Schueth, Alex J.
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Baroclinic zones have long been known to exist and play an important role in the vorticity budget of a supercell, particularly at low levels. Studies have shown that convergence zones may exist within the forward flank of supercells along density gradients, where the storm relative flow is parallel to the boundaries and directed toward the updraft. Therefore, baroclinically generated streamwise vorticity exists in these convergence zones that can add appreciable vorticity to the mesocyclone. Recent high-resolution simulations of supercells (Orf et al. 2016) have produced intense realizations of this vorticity in the lowest 250 m AGL within the forward flank, a feature identified as the streamwise vorticity current (SVC). During the Rivers of Vorticity in Supercells (RiVorS) project in the spring of 2017, a suite of instrumentation was used to observe these baroclinic zones in supercells. On June 12, 2017, a tornadic supercell was observed northeast of Cheyenne, Wyoming. One of the Texas Tech Ka-band radars was used to capture RHIs from the rear flank downdraft through the forward flank downdraft of the supercell at high resolution (0.33° beamwidth). Using an observed sounding from NSSL’s P1 mobile mesonet, a tornadic supercell was simulated using CM1 at 125 m horizontal grid spacing. Simulated RHIs in the simulated storm are compared to the observed RHIs showing similar areas of broad horizontal vorticity near the surface. Further analysis of the simulation reveals other properties of the SVC. The SVC is proven to be baroclinically driven through a time series of the virtual potential temperature gradient and the streamwise vorticity magnitude. It is primarily situated on the forward flank convergence boundary (FFCB); however, the left flank convergence boundary does sporadically have appreciable streamwise vorticity along it. The evolution of the FFCB is erratic at times showing a bifurcation of the boundary. The maximum of streamwise vorticity occurs at between 250 m and 500 m AGL and follows the erratic boundary placement. Coincident with the maximum in streamwise vorticity is a minimum in pressure perturbation, which extends to the surface.