Super High-Resolution Simulation of Tornado-Vortex Structure

Detailed structure of a tornado still remains unclear because of difficulties in observing it with fine spatial and temporal resolutions. The time-varying three-dimensional wind field near the surface is directly linked to tornado damages, and therefore it is crucially important to understand the near-surface structure of a tornado and its associated dynamics.

Most of previous studies have been conducted by photogrammetric analyses, laboratory experiments and idealized large eddy simulations, which contain calculational errors or some unrealistic aspects. In this study, we conducted a super high-resolution simulation with 10 m horizontal grid spacing to resolve the fine-scale tornado structure for the Tsukuba supercell tornado (2012), Japan. The experiment was conducted under realistic condition including the surface friction, downscaled from the 50-m resolution simulation of Mashiko (2016, MWR). The model contains 4001 x 3001 grid points in the horizontal and 250 vertical levels with grid intervals of 10 m near the surface.

Minimum pressure of a simulated tornado reaches 937 hPa (pressure deficit; 65 hPa), and maximum ground-relative surface wind speeds exceed 70 m/s. During the rapid intensifying stage, the vortex core region accompanying large vertical vorticity contracted and was gradually occupied by downdraft. After that, the central downdraft intensified, and multiple vortices formed with an increase of horizontal dimension of a tornado. It is evident that the simulated tornado evolved from one-celled to two-celled tornado and subsequently exhibited multiple vortices, which are consistent with a tornado-like vortex evolution in laboratory experiments. There exist three prominent cyclonic subvortices when most significant multiple vortices formed. Although subvortices locally intensify winds owing to the superposition of the velocity field associated with the small-scale subvortex and the larger-scale tornado, the strongest winds including updraft are found at the shrinking stage just prior to multiple vortices.

 The maximum updraft at 10 m height is about 50 m/s at the shrinking stage. To understand the cause of the strong updraft near the surface, the pressure gradient force was diagnosed using Boussinesq momentum equation for simplicity. As a result, the strong upward pressure gradient force associated with pressure deficit at about 25m height was dynamically caused by the frictional convergence and large vertical vorticity a little above the surface.