TTUKa Single-Doppler Radar Analysis of Low-Level Tornado Structure

To this day, considerable uncertainty exists regarding the low-level wind fields and vertical structure associated with tornado vortices. Unfortunately, ground-clutter contamination and beam blockage make resolving the lowest levels of tornado vortices difficult for many research radars. Higher-frequency radars have proven especially useful in detecting important low-level aspects of tornado vortices. Owing to the higher sensitivity and finer angular resolution, these observations are critical in verifying and expanding upon our current understanding of tornado structure, which prior to now has relied primarily on theory and laboratory simulations.

During the VORTEX2 field campaign, and subsequently in the fall 2011 and spring 2012 seasons, two Texas Tech University Ka-band (TTUKa) mobile Doppler radars deployed to collect high-resolution Plan Position Indicator (PPI) and Range-Height Indicator (RHI) sweeps of tornadoes and the near-tornado environment. Of these deployments, data from one case in particular, 14 April 2012, stand out. During this case, one TTUKa radar successfully captured low-level PPIs of two simultaneous tornadoes, as well as RHI cross-sections through each tornado individually. Data and analyses from the 14 April 2012 case and any data collected during the current 2013 season will be the topic of this study.

Ground-based velocity track display (GBVTD) method is utilized to obtain tangential and radial components of velocity at concentric rings about each tornado vortex center. Although the GBVTD method was originally designed for larger-scale vortices (e.g., tropical cyclones), recent studies show that this method can be modified and applied to tornado-scale vortices as long as a vortex center can be well defined. From these analyses, radial profiles of tangential and radial velocity, as well as derived parameters such as angular momentum, horizontal divergence, and vertical vorticity have been produced. When coupled with simultaneous or neighboring RHI cross-sections, these GBVTD products prove useful in identifying key low-level dynamical structures associated with tornadoes, such as the corner-flow region, inflow/outflow jets, and secondary circulations. By resolving these features, we can better understand the roles they play in the life cycle of a tornado, as well as the hazards they present in the near-surface layer, where impacts to infrastructure and human life are most prevalent.

Preliminary results reveal radial profiles of tangential velocity similar to those produced by simulations assuming Rankine vortex structure, although wind speeds beyond the radius of maximum velocity decrease at a rate more linearly than those presented in simulations, similar to the Burgers-Rott profile. Radial profiles of angular momentum differ from those idealized in Rankine vortex assumptions, in that angular momentum continues to increase beyond the radius of maximum winds instead of remaining constant. Meanwhile, radial profiles of axisymmetric radial velocity and vertical vorticity suggest a near-surface single-celled tornado vortex mode, matching observations made during the event. RHI cross-sections have proven successful thus far in resolving the lowest levels of tornadoes, where radial inflow and outflow jets have already been observed in several sweeps. Tornado-induced secondary circulations have also been resolved, manifesting in the RHI cross-sections as horizontal vorticity maxima in close proximity to the tornado vortices.