Austin Dixon - February 4

Convective Meteorology (Mesoscale Dynamics) Seminar

Simulated WSR-88D Observations of the Streamwise Vorticity Current

Austin Dixon

Friday, February 4^th

3:00 pm

Join Google Meet:

https://meet.google.com/iru-ggiv-afj

In recent years, the streamwise vorticity current (SVC) has been linked to supercellular tornado genesis. While this fact is interesting scientifically, it is only beneficial to operational predictability if identification of the SVC is possible in real time. Using the same simulation as Orf et al. 2017, this study converts the simulated reflectivity and wind data to emulated radar WSR-88D images of dBZ, radial velocity, and vertical shear in an effort to gauge operational detectability of the SVC. The analysis focuses on two time steps from the simulation: approximately 22 minutes and 3 minutes before tornado genesis. The early time step is characterized by a significantly weaker SVC than the near-genesis time, with nearly an order of magnitude difference in maximum streamwise vorticity values.

We examine simulated radar signatures of the SVC using three methods: 1) PPI scans using a standard VCP 212 WSR-88D scanning strategy; 2) Simulated RHI reconstructions using available elevation angles; and 3) Plots of the low-level elevation shear calculated as the change in radial velocity between the 0.5 and 1.3 degree elevation angles. Clear differences are noted in radial velocity and elevation shear between the two times. Leading up to the tornado, a significant increase in radial velocity occurs throughout the domain, but especially north of the mesocyclone. This occurs in concert with an increase in warm inflow winds with height along the left flank convergence boundary (LFCB). RHI scans are reconstructed from the VCP 212 PPI scans at varying distances and angles relative to the SVC. At relatively close range, RHIs of radial velocity across the SVC show a signature consistent with a horizontal vortex. This signature becomes more difficult to discern as the beam becomes less orthogonal to the SVC and as distance to the SVC increases. Vertical shear analyses also show a marked increase in magnitude near tornado genesis, with a discernible linear maximum becoming apparent along the location of the SVC. PPI plots are shown for 24 different radar-relative locations to demonstrate how radar characteristics of the SVC might differ between each scenario. In general, there is evidence of the SVC at distances up to 60km away from the radar, especially in locations where the radial component of the wind is maximized.