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Abstract This study investigates the effects of idealized and realistic terrain on tornado characteristics and behavior. It uses a novel simulation approach, nesting a high-fidelity, ultrafine-resolution, tornado-scale, engineering large-eddy simulation (LES) within a Cloud Model 1 (CM1) simulation of a tornadic supercell. We analyze the effects of terrain on the tornado’s central pressure, horizontal and vertical velocities, vortex shape, and path. Seven idealized terrain configurations are used including 1) a control run with flat ground, 2) and 3) an idealized hill with steep and gradual slopes having the height of 25.4 m, 4) and 5) an idealized escarpment with steep and gradual slopes having the height of 25.4 m, and 6) and 7) an idealized hill having heights of either 50 or 65 m. Furthermore, a real-world, complex terrain configuration of the same height is analyzed as the eighth case. Results suggest that the presence of terrain relief increases the central pressure deficit, the peak wind speed, and the width of the high wind speed region in the tornado swath, enhancing tornado intensity and causing path deviation. Specifically, the horizontal and vertical velocities at 10 m above ground level (AGL) are stronger with terrain and the location of the maximum pressure deficit occurs along the uphill segment for all idealized cases except the steep hill. The precise location of the maximum wind velocities and pressure deficits varies with the terrain shape and slope. The real terrain simulation is similar to the idealized terrain simulations to a certain extent; however, the vertical velocities are lower and the strongest winds occur over a smaller region, demonstrating the complexity of the tornado–terrain relationship. Significance StatementThis high-resolution numerical study investigates the effects that idealized hills and escarpments have on tornadoes and offers a comparison with a real-world, complex terrain configuration. This study is particularly novel for three reasons: 1) The nested simulation approach with an ultrafine inner grid (spacing of 0.01 m) facilitates a high-fidelity vortex simulation which reflects the variability of a real-world tornado in time and space, at a reasonable computation cost; 2) the ultrafine-scale LESs resolve turbulent features to this spatial scale and facilitate better characterization of tornadic winds; and 3) an experiment having real-world, complex terrain is successfully executed for the first time in tornado research (to the authors’ knowledge). Results suggest that terrain generally causes tornadoes to become stronger and wider than they otherwise would have been if the ground were flat. However, another important result is that the effects of the real-world, complex terrain on the tornado are not the same as those from simplified terrains.
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Understanding How Complex Terrain Impacts Tornado Dynamics Using a Suite of High-Resolution Numerical Simulations
Abstract A simulated vortex within a large-eddy simulation is subjected to various surface terrain, implemented through the immersed boundary method, to analyze the effects of complex topography on vortex behavior. Thirty simulations, including a control with zero-height terrain, are grouped into four categories—2D sinusoidal hills, 3D hills, valleys, and ridges—with slight modifications within each category. A medium-swirl-ratio vortex is translated over shallow terrain, which is modest in size relative to the vortex core diameter and with no explicitly defined surface roughness. While domain size restricts results to the very near-field effects of terrain, vortex–terrain interaction yields notable results. Terrain influences act to increase the variability of the near-surface vortex, including a notable leftward (rightward) deflection, acceleration (deceleration), and an expansion (a contraction) of the vortex as it ascends (descends) the terrain owing to changes in the corner flow swirl ratio. Additionally, 10-m track analyses show stronger horizontal wind speeds are found 1) on upslope terrain, resulting from transient subvortices that are more intense compared to the control simulation, and 2) in between adjacent hills simultaneous with strong pressure perturbations that descend from aloft. Composite statistics confirm that the region in between adjacent hills has the strongest horizontal wind speeds, while upward motions are more intense during ascent. Overall, valley (ridge) simulations have the largest horizontal (vertically upward) wind speeds. Last, horizontal and vertical wind speeds are shown to be affected by other terrain properties such as slope steepness and two-dimensionality of the terrain.
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- Award ID(s):
- 1823478
- PAR ID:
- 10192384
- Date Published:
- Journal Name:
- Journal of the Atmospheric Sciences
- Volume:
- 77
- Issue:
- 10
- ISSN:
- 0022-4928
- Page Range / eLocation ID:
- 3277 to 3300
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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- Free Publicly Accessible Full Text
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Accepted Manuscript
- Journal Article:
- https://doi.org/10.1175/JAS-D-19-0321.1
