Radar Updraft Proxies for Supercell Tornadogenesis Prediction

The most fundamental feature of a convective storm is its updraft. The updraft plays a crucial role in the life cycle of a storm and the production of its hazards, including tornadoes. Recent studies, mostly using numerical models, have proposed direct links between updraft properties and tornado formation. However, it is difficult, outside of specialized field campaigns, to directly sample the vertical velocities that define a storm’s updraft. A lack of observations complicates efforts to verify proposed connections from theory and modeling studies between characteristics of storm updrafts and tornadogenesis.

An alternative to direct observations of updrafts is to leverage features in radar data that result from vigorous updrafts. In this way, characteristics of a remote sensing updraft proxy may be used as an estimate of the characteristics of the storm updraft. For our work, we use a polarimetric radar signature, the Z_DR column, as a proxy for supercell updrafts. We aim to study the theorized connection between tornadogenesis and both updraft size and updraft vertical alignment. The former link results from a 2021 paper that found evidence that storms with larger Z_DR column areas were more likely to be tornadic than non-tornadic. The latter inquiry is based on studies that have found that supercell updrafts that are more vertically aligned (upright) are more likely to produce tornadoes.  

In this study, we analyze a large sample of Z_DR column areas and vertical alignments in tornadic and non-tornadic supercells. For the updraft area part of the study, we examine a large number of WSR-88D volumes with a focus on whether Z_DR column areas ≥ 40 km², which we call “immense” updrafts, are only present in tornadic supercells. If so, the implication is that an immense updraft serves as a sufficient condition for imminent tornado formation. For the updraft alignment part of the study, we use the centroid of the Z_DR column at storm midlevels and the storm hook echo at low levels to estimate updraft tilt in supercells; we also use radial velocity data to estimate mesocyclone tilt. The main objective is to identify if there is a difference in updraft (or mesocyclone) tilt between tornadic and non-tornadic supercells. In addition, we determine if there are tilt “cutoff” values large enough to significantly lower the probability of tornado formation. We also discuss study limitations in using radar proxy data and the implications of study results on operational nowcasting of imminent tornadogenesis.