Prevalence of tornado-scale vortices in the tropical cyclone eyewall

Abstract

SignificanceTornado-scale vortices in the intense tropical cyclone eyewall have been speculated upon for more than two decades, but their small horizontal scale, their fast movement, and the associated severe turbulence make them very difficult to observe directly, except for the case of Hurricane Hugo (1989) in the Atlantic basin. Using the Advanced Weather Research and Forecast large eddy simulation framework with the unprecedented horizontal grid size of 37 m, the numerical experiment in this study confirms the existence of simulated tornado-scale vortices similar to the Hugo case in the turbulent eyewall boundary layer and further suggests that tornado-scale vortices are prevalent at the inner edge of the intense eyewall convection.

Tornado-scale vortices in the tropical cyclone boundary layer: numerical simulation with the WRF–LES framework

“…For the thresholds of 20 m s −1 in vertical motion and 0.2 s −1 in vertical relative vorticity, the mean duration is 40 seconds and the longest is 138 s. We can conclude that the identified tornadoscale vortices are not repeatedly counted in the hourly output. The durations of tornado-scale vortices are consistent with observational and numerical studies (Wurman and Kosiba, 2018;Stern and Bryan, 2018). Figure 4a shows the location of the maximum vertical motions of the detected tornado-scale vortices including 89 vortices identified with the threshold of maximum vertical motion of 15 m s −1 .…”

“…The updraft of 22 m s −1 in Isabel was detected by a GPS dropwindsonde at just about 1300 m Stern and Bryan, 2018), while the updraft of 31 m s −1 in Hurricane Felix (2007) was observed at the flight altitude (∼ 3 km). Marks et al (2008) found that the EVM in Hurricane Hugo (1989) was associated with a max- imum vertical motion of 21 m s −1 and a maximum vertical relative vorticity of 0.125 s −1 at the altitude of 450 m. Based on these studies, a small-scale vortex associated with extreme wind speed can be treated as a tornado-scale vortex (Wurman and Kosiba, 2018;Wu et al, 2018). The tornado-scale vortex in the simulated TC is subjectively defined as a small-scale cyclonic circulation with the diameter of 1-2 km below the altitude of 3 km, containing maximum upward motion larger than 20 m s −1 and maximum vertical relative vorticity larger than 0.2 s −1 .…”

“…One of the major TC threats is damaging winds. Uneven damage patterns often show horizontal scales ranging from a few hundred meters to several kilometers (Wakimoto and Black, 1994;Wurman and Kosiba, 2018), suggesting that TC threats are associated with both sustained winds and gusts. The latter are believed to result from small-scale coherent structures in the TC boundary layer (Wurman and Winslow, 1998;Morrison et al, 2005;Lorsolo et al, 2008;Kosiba et al, 2013;Kosiba and Wurman, 2014).…”

“…Another important small-scale feature is the so-called eyewall vorticity maximum (EVM) or tornado-scale vortices in the TCBL (Wurman and Kosiba, 2018;Wu et al, 2018). So far, our understanding is mainly from a few observational analyses based on limited data collected during the research aircraft penetration of hurricane eyewalls.…”

Importance of identifying tropical cyclone tornadoes in typhoon warning and defense systems

“…A recent study has shown that a large number of tornado-scale vortices can be produced at the inner edge of the intense eyewall convection of tropical cyclones [4]. This is in agreement with observations of tropical cyclones, both offshore (e.g., Hurricane Katrina in 2005 [5]) and onshore (e.g., Hurricane Ivan in 2004 [6]).…”