Determining the Vertical Sense of Evolution of Rotation during Tornadogenesis Using Rapid-Scan, Polarimetric Radar Data

Multiple studies over the past decade have sought to identify the spatio-temporal evolution of rotation just prior to and during tornadogenesis in order to answer the question: Does a tornado form from the top-down, bottom up, or simultaneously over between the cloud and the ground? (French et al. 2013; Houser et al; 2015, 2022; etc.) Traditionally, the answer to this question has been illuminated by studying the time-height progression of the tornadic vortex signature (TVS). However, owing to a plethora of non-meteorological considerations (e.g. distance between the radar and the tornado, beam width of the radar, height of the beam, etc.) there is no quantitative definition of what constitutes a small-scale vortex as a TVS. Indeed, velocity differential values ( |vin – vout| ) defining tornadoes in the literature is found to range between 12 m s-1 for WSR-88D’s (Trapp study of 88Ds) to upwards of 55 m s-1 for close-proximity mobile radars (Weinhoff et al 2020)
In this study, radial velocity data from the Rapid-scan X-band polarimetric (RaXPol) radar are used to investigate the time-height evolution of 8 tornadoes, many of which have data collected down to heights O (10 m above radar level) using five different metrics including three different magnitudes of “TVS’s” to quantify a “tornado”. The onset time of the tornadic debris signature at the lowest elevation angle available is incorporated to provide insight as to when debris becomes lofted and is visible in the data. It is found that in some cases, the time-height evolution is consistent regardless of the parameter used. However, for a minority of cases, the vertical sense of evolution is actually dependent upon which parameter is used. Thus, caution should be exercised when analyzing results based only upon a single parameter.