Tornado-Scale Vortices in the Tropical Cyclone Boundary Layer: Numerical Simulation with WRF-LES Framework

The 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 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 the similar features as revealed with the limited observational data, including the updraft/downdraft couplet, the sudden jump of wind speeds, the favorable location, and the horizontal scale. It is suggested that the WRF-LES framework can successfully simulate the tornado-scale vortex with the grids at the resolution of 37 m that cover the TC eye and eyewall. The simulated tornado-scale vortex has a cyclonic circulation with a small horizontal scale of ~1 km in the TCBL. It is accompanied by strong updrafts (more than 20 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-θ_e layer, mostly below the altitude of 2 km. Nearly in all the simulated tornado-scale vortices, the narrow intense updraft is coupled with the relatively broad downdraft, constituting one or two updraft/downdraft couplets or horizontal rolling vortices, as observed by the research aircraft. The presence of the tornado-scale vortex also lead to significant gradients in the near surface wind speed and wind gusts.