Abstract
Abstract. A tornado-scale vortex in the tropical cyclone (TC) boundary layer (TCBL) has been observed in intense hurricanes and the associated intense turbulence poses a severe threat to the manned research aircraft when it penetrates hurricane eyewalls at a lower altitude. In this study, a numerical experiment in which a TC evolves in a large-scale background over the western North Pacific is conducted using the Advanced Weather Research and Forecast (WRF) model by incorporating the large-eddy simulation (LES) technique. The simulated tornado-scale vortex shows features similar to those revealed with limited observational data, including the updraft–downdraft couplet, the sudden jump of wind speeds, the location along the inner edge of the eyewall, and the small horizontal scale. It is suggested that the WRF–LES framework can successfully simulate the tornado-scale vortex with grids at a resolution of 37 m that cover the TC eye and eyewall. The simulated tornado-scale vortex is a cyclonic circulation with a small horizontal scale of ∼1 km in the TCBL. It is accompanied by strong updrafts (more than 15 m s−1) and large vertical components of relative vorticity (larger than 0.2 s−1). The tornado-scale vortex favorably occurs at the inner edge of the enhanced eyewall convection or rainband within the saturated, high-θe layer, mostly below an altitude of 2 km. In nearly all the simulated tornado-scale vortices, the narrow intense updraft is coupled with the relatively broad downdraft, constituting one or two updraft–downdraft couplets, as observed by the research aircraft. The presence of the tornado-scale vortex also leads to significant gradients in the near-surface wind speed and wind gusts.
Highlights
Tropical cyclones (TCs) pose a severe risk to life and property in tropical cyclone (TC)-prone areas and the risk will increase due to the rapidly rising coastal population and number of buildings (Pielke et al, 2008; Zhang et al, 2009)
Since the Richardson number is calculated as the ratio of the moist static stability to the vertical wind shear in the TC boundary layer (TCBL), we speculate that the strong vertical wind shear in the inward side of the intense eyewall convection is an important factor for the development of tornado-scale vortices
A tornado-scale vortex or eyewall vorticity maximum (EVM) in the TCBL has been observed in intense hurricanes and is always associated with strong turbulence
Summary
Tropical cyclones (TCs) pose a severe risk to life and property in TC-prone areas and the risk will increase due to the rapidly rising coastal population and number of buildings (Pielke et al, 2008; Zhang et al, 2009). The TCBL is known to play a critical role in transporting energy and controlling TC intensity (Braun and Tao, 2000; Rotunno et al, 2009; Smith and Montgomery, 2010; Bryan, 2012; Zhu et al, 2013; Green and Zhang, 2015) Another important small-scale feature is the so-called eyewall vorticity maximum (EVM) (Marks et al, 2008) or tornado-scale vortices in the TCBL (Wurman and Kosiba, 2018; Wu et al, 2018). By simulating tornadoscale vortices in the TCBL, this study will focus on the spatial distribution of occurrence of the tornado-scale vortices and the features of their 3-D structures
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