The Relationship between Instantaneous and 3-Second Wind Gust Periods as a Function of a Modified Rankine Vortex

Abstract The ASCE/SEI/AMS Standards Committee on Estimating Wind Speeds in Tornadoes and Other Windstorms has undertaken a long-term effort to develop a multi-method standard for improving estimations of tornado windspeeds. While these methods show promise at discriminating general tornado intensity trends among themselves, at least two significant challenges remain: (1) discriminating finer details of intensity estimates among upper-echelon-intensity tornadoes and (2) comparing across different estimation techniques. Most techniques approach effective limits at high tornado intensities that represent a lower-bound intensity estimate. Furthermore, different intensity estimation techniques are often inherently representing different windspeed variables or characteristics—e.g., the EF-Scale estimates a peak 3-s gust speed at essentially single points, treefall pattern estimation methods estimate peak quasi-instantaneous wind gusts over an area, and gust periods associated with radar measurements can vary based on the radar gate spacing and tornado wind intensity. In this study, we use a simple modified Rankine vortex model to illustrate the sensitivity of the relationship between the peak instantaneous wind gust and peak 3-s-average wind gusts associated with idealized tornadic vortices as functions of vortex intensity, size, translation speed, and ratio of inflow to rotational flow. We illustrate that all of these factors impact the instantaneous-to-3-s gust speed relationship non-linearly, even under the most idealized condition of a modified Rankine vortex. These results highlight the importance of understanding what each tornado-intensity estimation method is actually estimating and the need for improved understanding of which gust periods are most relevant for causing tornado damage to various damage indicators.