Reconciling Updraft Size vs. Mesocyclone Size Arguments in Tornado Formation and Intensity Prediction

Is the size of a supercell updraft an important dynamical feature for tornado peak intensity? Modeling simulation studies have provided differing possibilities. Some work has provided evidence that larger supercell updrafts, at both mid and low levels, are predictive of greater peak tornado intensity via circulation arguments and relationships among the sizes of the updrafts, downdrafts, mesocyclones, and tornadoes; the wider tornadoes are then assumed, generally, to be more intense. In turn, a large body of modeling work now supports that environmental low-level (i.e., 0-1, 0-2 km) storm-relative flow is largely responsible for the wider supercell updrafts important to this idea. However, results from more recent simulations have been used to argue that tornado intensity is not related to updraft width, but rather to the size (and rotational characteristics) of the low-level mesocyclone, and these characteristics are largely driven by environmental horizontal streamwise vorticity rather than low-level storm-relative flow. But complicating matters is that (i) multiple observational studies using different remote sensing proxies for updraft size have provided strong evidence that there is indeed a relationship between midlevel updraft area/width and peak tornado intensity, and (ii) environmental studies have differentiated tornadic from non-tornadic supercell far-field environments using storm-relative flow. Regarding (i), there also is an apparent modest link between midlevel updraft area and tornadogenesis, which is consistent with (ii) if storm-relative flow does drive updraft size.

How to reconcile the apparent differences among all of these works? In this study, we use WSR-88D data from a large number (200+) of tornadic and non-tornadic supercells cases combined with near-storm environmental data from model analysis fields to (1) better isolate the importance of both midlevel updraft area and low-level mesocyclone width to tornado formation and peak intensity, and (2) directly relate the near-storm environment to the same two features. Midlevel updraft area is estimated using a novel algorithm that determines the area of enhanced differential radar reflectivity factor (ZDR) columns and the low-level mesocyclone width is estimated via radial velocity data. With these radar proxy data, we use subsets of cases to setup different parameter spaces of updraft/mesocyclone size (i.e., small vs. large) and determine optimal tornado formation and peak intensity prediction, and how the near-storm environment may differentiate, if at all, among the spaces. Our approach ideally provides clarity as to the robustness of previous observational results. The implications of our results on using radar proxies of updraft and mesocyclone size for nowcasting both tornadogenesis and peak tornado intensity also will be discussed.