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

Atmos. Chem. Phys., 19, 2477–2487, 2019 https://doi.org/10.5194/acp-19-2477-2019 © Author(s) 2019. This work is distributed under the Creative Commons Attribution 4.0 License. Tornado-scale vortices in the tropical cyclone boundary layer: numerical simulation with the WRF–LES framework Liguang Wu1,2 , Qingyuan Liu1 , and Yubin Li1 1 Pacific Typhoon Research Center and Key Laboratory of Meteorological Disaster of Ministry of Education, Nanjing University of Information Science and Technology, Nanjing, China 2 Department of Atmospheric and Oceanic Sciences and Institute of Atmospheric Sciences, Fudan University, Shanghai, China Correspondence: Liguang Wu () Received: 31 July 2018 – Discussion started: 8 October 2018 Revised: 6 February 2019 – Accepted: 16 February 2019 – Published: 27 February 2019 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. 1 Introduction 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). 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). The small-scale coherent structures may have significant implications for the vertical transport of energy in TCs and thus TC intensity and structure (Zhu, 2008; Rotunno et al., 2009; Zhu et al., 2013; Green and Zhang, 2014, 2015; Gao et al., 2017). While understanding of the coherent structure is very important for mitigating TC damage and understanding of TC intensity and structure changes, for now direct in situ observation and remote-sensing measurements can only provide very limited information. In the TC boundary layer (TCBL), observational analyses suggest that horizontal streamwise roll vortices prevail with sub-kilometer to multi-kilometer wavelengths (Wurman and Winslow, 1998; Katsaros et al., 2002; Morrison et al., 2005; Lorsolo et al., 2008; Ellis and Businger, 2010; Foster, 2013). Studies found that the rolls can result from the inflection point instability of the horizontal wind profiles in the TCBL (Foster, 2005; Gao and Ginis, 2014) and have sig- Published by Copernicus Publications on behalf of the European Geosciences Union. 2478 L. Wu et al.: Tornado-scale vortices in the tropical cyclone boundary layer nificant influences on the vertical transport of energy in TCs (Zhu, 2008; Rotunno et al., 2009; Zhu et al., 2013; Green and Zhang, 2014, 2015; Gao et al., 2017). 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). So far, our understanding is mainly from a few observational analyses based on limited data collected during the research aircraft penetration of hurricane eyewalls. A WP-3D research aircraft from National Oceanic and Atmospheric Administration (NOAA) encountered three strong updraft–downdraft couplets within 1 min while penetrating the eyewall of category 5 Hurricane Hugo (1989) at 450 m in altitude (Marks et al., 2008). The severe turbulence caused the failure of one of the four engines and the people on board were at a severe risk. The aircraft finally escaped with the help of a U.S. Air Force reconnaissance WC-130 aircraft, which found a safe way out through the eyewall on the northeast side of Hugo. Since then aircraft missions have been prohibited in the boundary layer of the TC eyewall. Later analysis indicated that the dangerous turbulence was associated with a tornado-scale vortex, which is comparable to a weak tornado in terms of its diameter of about 1 km and the estimated peak cyclonic vorticity of 0.125 s−1 (Marks et al., 2008). Such strong turbulence was also observed in Hurricanes Isabel (2003) and Felix (2007) below 3 km (Aberson et al., 2006, 2017). So far, little is known about the structure and evolution of the tornado-scale vortex. With advances in numerical models and computational capability, the large-eddy simulation (LES) technique has been incorporated into the Advanced Weather Research and Forecast (WRF) model (Mirocha et al., 2010) and an increasing number of TC simulations have been conducted with horizontal grid spacing of less than 1 km (Zhu, 2008; Rotunno et al., 2009; Bryan et al., 2014; Stern and Bryan, 2014; Rotunno and Bryan, 2014; Green and Zhang, 2015). In LES, the energy-producing scales of three-dimensional (3-D) atmospheric turbulence in the planetary boundary layer (PBL) are explicitly resolved, while the smaller-scale portion of the turbulence is parameterized (Mirocha et al., 2010). Effort has been made to simulate the structure (...truncated)