Wind Field of a Nonmesocyclone Anticyclonic Tornado Crossing the Hong Kong International Airport

Karen A Kosiba,P W Chan,Joshua Wurman,Paul Robinson

doi:10.1155/2014/597378

Abstract

A nonmesocyclone tornado traversed the Hong Kong International Airport on September 6, 2004 directly impacting a surface weather station. This allowed for 1-second 10-meter above ground level (AGL) wind observations through the core of the tornado. Integration of these 10-meter AGL wind data with Ground-Based Velocity Track (GBVTD) wind retrievals derived from LIDAR data provided a time history of the three-dimensional wind field of the tornado. These data indicate a progressive decrease in radial inflow with time and little to no radial inflow near the time the tornado crosses the surface weather station. Anemometer observations suggest that the tangential winds approximate a modified-Rankine vortex outside the radius of maximum winds, suggesting that frictionally induced radial inflow was confined below 10 m AGL. The radial-height distribution of angular momentum depicts an increase in low-level angular momentum just prior to the tornado reaching its maximum intensity.

Highlights

Fine-scale observations of tornado structure have occurred more frequently in recent years with the proliferation of mobile radars and focused field experiments (e.g., [1,2,3,4,5,6,7]) with few exceptions (e.g., [8,9,10,11,12,13]) low-level observations, below 15–20 m above ground level (AGL), are comparatively rare
Observations by Wurman et al [9] document little reduction in wind speed between measurements taken at 3 m above ground level (AGL) and those taken at 18 m AGL, possibly suggesting that 3 m AGL is near the top of the boundary layer in that case
Substantial radial inflow at 3 m AGL was documented in a strong tornado by Wurman et al [12], but due to the available data, it was difficult to determine whether the total winds increased with height or were constant between 3 m AGL and 20 m AGL; the depth of the boundary layer may have been near 3 m AGL or much higher

Summary

Introduction

Fine-scale observations of tornado structure have occurred more frequently in recent years with the proliferation of mobile radars and focused field experiments (e.g., [1,2,3,4,5,6,7]) with few exceptions (e.g., [8,9,10,11,12,13]) low-level observations, below 15–20 m above ground level (AGL), are comparatively rare. Surface friction results in an imbalance between the centrifugal and radial pressure gradient forces, which acts to increase near-surface inflow. This leads to an increase in the tangential velocities in the mid and upper portions of the boundary layer. Substantial radial inflow at 3 m AGL was documented in a strong tornado by Wurman et al [12], but due to the available data, it was difficult to determine whether the total winds increased with height or were constant between 3 m AGL and 20 m AGL; the depth of the boundary layer may have been near 3 m AGL or much higher. An accurate assessment of the boundary layer depth in tornadoes is needed to fully understand and model the wind distribution with height

Data and Methodology

Results

Conclusions