资源环境科技发展态势分析平台

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 37m that cover the TC eye and eyewall.

The simulated tornado-scale vortex is a cyclonic circulation with a small horizontal scale of similar to 1 km in the TCBL. It is accompanied by strong updrafts (more than 15 ms(-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 rain-band within the saturated, high-theta(e) 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.