TY - JOUR
T1 - Tornado vortex structure, intensity, and surface wind gusts in large-eddy simulations with fully developed turbulence
AU - Nolan, David S.
AU - Dahl, Nathan A.
AU - Bryan, George H.
AU - Rotunno, Richard
N1 - Funding Information:
The authors thank D. Lewellen and two anonymous reviewers for their comments that led to significant improvements of this manuscript. This work was supported in part by the National Science Foundation through Grant AGS-1265899. We acknowledge high-performance computing support from Yellowstone (ark:/85065/d7wd3xhc) provided by the Computational and Information Systems Laboratory of the National Center for Atmospheric Research.
Publisher Copyright:
© 2017 American Meteorological Society.
Copyright:
Copyright 2017 Elsevier B.V., All rights reserved.
PY - 2017/5/1
Y1 - 2017/5/1
N2 - A large-eddy simulation (LES) framework with an "eddy injection" technique has been developed that ensures a majority of turbulent kinetic energy in numerically simulated tornado-like vortices is represented by resolved eddies. This framework is used to explore the relationships between environmental forcing mechanisms, surface boundary conditions, and tornado vortex structure, intensity, and wind gusts. Similar to previous LES studies, results show that the maximum time- and azimuthal-mean tangential winds (V)max can be well in excess of the "thermodynamic speed limit," which is 66 m -1 for most of the simulations. Specifically, (V)max exceeds this speed by values ranging from 21% for a large, high-swirl vortex to 59% for a small, low-swirl vortex. Budgets of mean and eddy angular and radial momentum are used to show that resolved eddies in the tornado core act to reduce the wind speed at the location of (V)max, although they do transport angular momentum downward into the lowest levels of the boundary layer, increasing low-level swirl. Three measures of tornado intensity are introduced: maximum time-azimuthal-mean surface (10 m) horizontal wind speed ((S10)max), maximum 3-s gusts of S10 (S10-3s), and maximum vertical 3-s gusts at 10 m (W10-3s). While (S10)max is considerably less than (V)max, transient features in the boundary layer can generate S10-3s in excess of 150 m -1, and W10-3s in excess of 100 m s-1. For high-swirl vortices, the extreme gusts are confined closer to the center, well inside the radius of maximum azimuthal-mean surface winds. For the low-swirl vortex, both the strongest mean winds and the extreme gusts are restricted to a very narrow core.
AB - A large-eddy simulation (LES) framework with an "eddy injection" technique has been developed that ensures a majority of turbulent kinetic energy in numerically simulated tornado-like vortices is represented by resolved eddies. This framework is used to explore the relationships between environmental forcing mechanisms, surface boundary conditions, and tornado vortex structure, intensity, and wind gusts. Similar to previous LES studies, results show that the maximum time- and azimuthal-mean tangential winds (V)max can be well in excess of the "thermodynamic speed limit," which is 66 m -1 for most of the simulations. Specifically, (V)max exceeds this speed by values ranging from 21% for a large, high-swirl vortex to 59% for a small, low-swirl vortex. Budgets of mean and eddy angular and radial momentum are used to show that resolved eddies in the tornado core act to reduce the wind speed at the location of (V)max, although they do transport angular momentum downward into the lowest levels of the boundary layer, increasing low-level swirl. Three measures of tornado intensity are introduced: maximum time-azimuthal-mean surface (10 m) horizontal wind speed ((S10)max), maximum 3-s gusts of S10 (S10-3s), and maximum vertical 3-s gusts at 10 m (W10-3s). While (S10)max is considerably less than (V)max, transient features in the boundary layer can generate S10-3s in excess of 150 m -1, and W10-3s in excess of 100 m s-1. For high-swirl vortices, the extreme gusts are confined closer to the center, well inside the radius of maximum azimuthal-mean surface winds. For the low-swirl vortex, both the strongest mean winds and the extreme gusts are restricted to a very narrow core.
KW - Large eddy simulations
KW - Tornadoes
KW - Turbulence
KW - Vorticity
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U2 - 10.1175/JAS-D-16-0258.1
DO - 10.1175/JAS-D-16-0258.1
M3 - Article
AN - SCOPUS:85018334817
VL - 74
SP - 1573
EP - 1597
JO - Journals of the Atmospheric Sciences
JF - Journals of the Atmospheric Sciences
SN - 0022-4928
IS - 5
ER -