Does Ambient Deep-Tropospheric Vertical Wind Shear Influence Tornadic Supercells in Tropical Cyclones?

Tropical cyclones (TCs) are capable of producing tornadoes that can compound the damage inflicted by other storm hazards (e.g., flooding). These tornadoes typically occur in miniature supercells embedded within the rainbands. While current conceptual models suggest that these tornadoes and their attendant supercells should occur near the coast in the eastern half of the TC, they do not completely explain why tornadoes form inland or to the west of the TC center. One previously hypothesized, yet understudied factor that may provide a greater understanding of tornadoes during landfalling TCs is ambient deep-tropospheric (i.e., 850–200 hPa) vertical wind shear. Such shear has been shown to: 1) influence the symmetry and strength of deep convection as a whole, then by extension, likely affecting tornadic supercell occurrence in TCs, and 2) provide favorable kinematic environments for supercells within the downshear semicircle (i.e., half of the TC in same direction as the shear vector). Building upon this prior work, the present study statistically analyzes ambient deep-tropospheric vertical wind shear impacts upon the frequency, location, and atmospheric environments of tornadic supercells in TCs.

Our analysis uses 1995‒2018 Storm Prediction Center (SPC) TC tornado (TCTOR) data as a proxy for tornadic supercells, and radiosonde data to analyze the associated atmospheric environments. Our results demonstrate that tornadic supercells (especially with EF1/F1+ events) occur more frequently in TCs in strong shear (>11 m s^-1) compared to weak shear (<6 m s^-1). As shear increases, tornadic supercells concentrate more in the downshear half of TCs, such that 97% of tornadoes during strongly sheared TCs occur downshear. A statistical analysis of radiosonde data confirms that the downshear quadrants are associated with enhanced 0–3 km convective-cell-relative helicity, especially between 200–500 km radii from the TC center in the downshear right quadrant, coincident with the region of enhanced tornadic supercell occurrence. This same region of the TC also is associated with enhanced 0–3 km CAPE. Ongoing work is examining the sensitivity of the full vertical profile of winds, temperature, and moisture in TCs to ambient deep-tropospheric vertical wind shear.