A Study on Oscillations in Low-Level Tornado Couplet Intensity

In recent years, mobile radar observations of tornadoes have proven invaluable for assessing tornado characteristics. Rapid-scan mobile radars are typically able to provide higher spatiotemporal resolution compared to traditional, fixed-site radars and are better suited for capturing the rapidly evolving nature of tornadoes. In particular, features which evolve on the order of a few minutes or less are of interest, as such features may not be well-understood currently. In this study, data from a mobile radar platform are used to assess oscillations in low-level tornado strength with periods on the order of 60-90 s. These oscillations have been previously observed at low elevations with the Atmospheric Imaging Radar (AIR), a W-band radar, and the Doppler on Wheels (DOW) radars. The goal of this study is to use data of a violent tornado that occurred on 24 May 2011 near El Reno, OK, as collected by the Rapid X-band Polarimetric (RaXPol) mobile radar to (1) assess periodicities in the strength of the tornado, (2) examine possible causes for the observed periodicities, and (3) compare these results to findings of previous mobile radar studies. Possible causes of tornado strength periodicity could include tornado width oscillations about a stable point at constant angular momentum, oscillatory changes in low-level angular momentum, or periodic debris loading and debris shedding. Additionally, upward-propagating centrifugal waves could contribute to these oscillations. To assess these possibilities, Fourier analysis of the tornado couplet intensity and the radius of maximum winds are conducted, along with a correlation analysis between tornadic debris signature (TDS) characteristics and tornado couplet intensity.