Observed Relationship between Tornado Intensity and Pretornadic Mesocyclone Characteristics

Trapp et al. (2017, JAS) used idealized model simulations of supercell thunderstorms to demonstrate support of their hypothesis that wide, intense tornadoes should form more readily out of wide, rotating updrafts. Observational data were used herein to test the generality of this hypothesis, especially to tornado-bearing convective morphologies such as quasi-linear convective systems (QLCSs), and within environments such as those found in the southeastern U.S. during boreal spring and autumn. A new radar dataset was assembled that focuses explicitly on the pre-tornadic characteristics of the mesocyclone, such as width and differential velocity: the pre-tornadic focus allows us to eliminate the effects of the tornado itself on the mesocyclone characteristics. GR2Analyst was used to manually analyze 80 tornadic events during the period 1 April 2011 to 1 May 2019. The corresponding tornadoes had damage (EF) ratings ranging from EF0 to EF5, and all were within 100 km of a WSR-88D. A key finding is that the linear regression between the mean, pre-tornadic mesocyclone width and the EF rating of the corresponding tornado yields a coefficient of determination (R2) value of 0.72. This linear relationship is the same for discrete (supercell) cases (R2=0.72), and lower for QLCS cases (R2=0.35). Overall, we have found that pre-tornadic mesocyclone width tends to be a persistent, relatively time-invariant characteristic that is a good predictor of potential tornado intensity. In contrast, the pre-tornadic mesocyclone intensity (differential velocity) tends to exhibit considerable time variability, and thus would offer less reliability in anticipating tornado intensity. Additionally, the environmental characteristics of each case were analyzed and compared to the pre-tornadic mesocyclone characteristics to better understand the environmental controls on the pre-tornadic mesocyclone width and intensity. Initial findings support a stronger relationship between the pre-tornadic mesocyclone and measures of vertical wind shear than with measures of instability.