Horizontal Vortex Tubes near a Simulated Tornado: Three-Dimensional Structure and Kinematics

Wicker Wicker,Yussouf Yussouf,Xue Xue,Oliveira Oliveira,Roberts Roberts

doi:10.3390/atmos10110716

Abstract

Supercell thunderstorms can produce a wide spectrum of vortical structures, ranging from midlevel mesocyclones to small-scale suction vortices within tornadoes. A less documented class of vortices are horizontally-oriented vortex tubes near and/or wrapping about tornadoes, that are observed either visually or in high-resolution Doppler radar data. In this study, an idealized numerical simulation of a tornadic supercell at 100 m grid spacing is used to analyze the three-dimensional (3D) structure and kinematics of horizontal vortices (HVs) that interact with a simulated tornado. Visualizations based on direct volume rendering aided by visual observations of HVs in a real tornado reveal the existence of a complex distribution of 3D vortex tubes surrounding the tornadic flow throughout the simulation. A distinct class of HVs originates in two key regions at the surface: around the base of the tornado and in the rear-flank downdraft (RFD) outflow and are believed to have been generated via surface friction in regions of strong horizontal near-surface wind. HVs around the tornado are produced in the tornado outer circulation and rise abruptly in its periphery, assuming a variety of complex shapes, while HVs to the south-southeast of the tornado, within the RFD outflow, ascend gradually in the updraft.

Highlights

Supercell thunderstorms produce a wide spectrum of vortical structures with length scales ranging from the large, midlevel mesocyclone (5–10 km in diameter; [1,2]) down to vortices on the order of only a few meters, such as suction vortices [3,4,5]
Given the small scale and transient nature of horizontal vortices, they are hard to observe in Doppler radar data or to resolve in high-resolution numerical simulations; most evidence for their existence relies on videos or photographs [7,8,9]
This suggests that horizontal vortices (HVs) near the surface have horizontal vorticity vectors that point to the left of the horizontal wind vector at large angles, implying considerable crosswise horizontal vorticity [11,13,14,52]

Summary

Introduction

Supercell thunderstorms produce a wide spectrum of vortical structures with length scales ranging from the large, midlevel mesocyclone (5–10 km in diameter; [1,2]) down to vortices on the order of only a few meters, such as suction vortices [3,4,5]. Atmosphere 2019, 10, 716 observations, Wurman and Kosiba [5] provide evidence for large HVs south-southeast of two large tornadoes sampled by the Doppler-on-Wheels (DOW) mobile radar In both cases, the HVs are located outside of the tornadic circulation but their sense of rotation (inferred from radial velocity plan-position indicators at two levels; see their Figure 14) is different; in one case the inferred horizontal vorticity vector points to the southwest while in the other it points to the north-northeast. Surface friction acts on outflow and inflow parcels to produce strong near-surface horizontal vorticity that is abruptly tilted and stretched to produce pre-tornadic vertical vorticity centers that eventually coalesce into a tornado This mechanism was explored in detail by Roberts et al [13] and Roberts and Xue [14] in an idealized, single-sounding simulation of the 3 May 1999 Bridge Creek-Moore tornado.

Numerical Experiments

Simulated Storm Overview

(Figures of

Figures with

Evolution

Near-Ground

Discussion and Concluding