Comparing Observations and Simulations of the Streamwise Vorticity Current in Tornadic and Nontornadic Supercell Storms

Baroclinic zones have long been known to exist and play an important role in the vorticity budget of a supercell, particularly at low levels. While the periphery of the rear flank downdraft (RFD) has been shown to be an important source of baroclinic vorticity generation, the forward flank has not been investigated as thoroughly. Studies have shown that convergence zones may exist within the forward flank in supercells. These convergence zones exist along density gradients, where the storm relative flow is along the boundaries toward the updraft. Therefore, baroclinically generated streamwise vorticity can accumulate and feed into the updraft. Recent high-resolution simulations of supercells (Orf et al. 2017) 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 and vorticity rivers in supercells. One of the Texas Tech Ka band radars was used to capture RHIs from the RFD through the forward flank of a supercell at high resolution (0.33 degree beamwidth). Five datasets were gathered during this project including one tornadic case.

On June 12, 2017 a tornadic supercell was observed northeast of Cheyenne, Wyoming. A sector of RHIs was gathered continuously for around 30 minutes from tornadogenesis through tornado decay on this storm. Prior to storm initiation, a mobile observed sounding was launched from NSSL’s P1 mobile mesonet to sample the pre-convective environment. Using this observed sounding, a supercell was simulated using CM1. Simulated RHIs in the simulated storm are compared to the observed RHIs showing areas of broad horizontal vorticity near the surface. The simulated storm is also visualized with VAPOR to show the SVC present. Using NSSLs mobile mesonet observations through the forward flank, the association of horizontal virtual potential temperature with these resolved areas of vorticity will be discussed. These observations will be contrasted with those of a non-tornadic RiVorS case to give a more complete understanding of the SVC, and its effect on low-level mesocyclone development and tornadogenesis.