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1. Computer modelling of close-to-ground tornado wind-fields for different tornado widths

   Kashefizadeh, M.H; Verma, S; Selvam, R.P. (August 2019, Journal of wind engineering and industrial aerodynamics)

   Tangential velocity (Vt) of tornadoes is the major parameter that causes building damage. In-field tornado measurements are less reliable at less than 20 m above ground level (AGL). Laboratory tornado simulators suggest that swirl ratio (S) and radius (ro) are the major tornado parameters that influence the Vt. However, due to scaling problems, the laboratory simulators also report the Vt at greater than 20 m AGL. Well-refined computational fluid dynamics (CFD) models can evaluate the Vt at less than 10 m AGL. However, the CFD models are limited to ro = 1.0 km, and the effect of ro on Vt is not investigated. The aim of this study is to investigate the maximum Vt for different ro close to ground. Simulation results show that increasing ro decreases the maximum Vt with respect to Vro. Moreover, by increasing ro, the corresponding elevation of occurrence of maximum Vt (zmax) will increase. However, for all tornado radii, the zmax is between 20 m and 64 m AGL. In addition, results show that for all ro, the radial Vt profile has two peaks at z < 10 m AGL due to strong shear force close to the ground and at higher elevation the profile transits to Rankine Combined Vortex Model (RCVM). 

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2. CFD model validation with experimental tornado wind field & comparison of wind field in different tornado chambers

   https://doi.org/https://doi.org/10.12989/was.2021.33.5.367  

   Verma, Sumit.; Selvam, Rathinam P. (January 2021, Wind and Structures)

   Validation of CFD tornado wind field with experimental or field measurements is limited to comparison of tangential velocity profile at certain elevations above the ground level and few studies are based on comparison of pressure profile. However, important tornado vortex features such as touchdown swirl ratio (ST), core radius (rc), maximum tangential velocity (Vtmax), elevation of maximum tangential velocity (zc) and pressure distribution over a range of varying swirl ratios which strongly influences tornado forces on a building have not been accounted for validation of tornado wind field. In this study, important tornado vortex features are identified and validated with experimental measurements; the important tornado features obtained from the CFD model are found to be in reasonable agreement with experimental measurements. Besides, tornado chambers with different geometrical features (such as different outlet size and location and total heights) are used in different works of literature; however, the effect of variation of those key geometrical features on tornado wind field is not very well understood yet. So, in this work, the size of outlet and total height are systematically varied to study the effect on important tornado vortex parameters. Results indicate that reducing outlet diameter in a tornado chamber increases ST, Vtmax and zc and decreases rc. Similarly, increasing total height of tornado chamber decreases ST, Vtmax and rc whereas zc remains nearly constant. Overall, it is found that variation of outlet diameter has a stronger effect on tornado wind field than the variation in total height of tornado chamber. 

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3. An Analysis of an Ostensible Anticyclonic Tornado from 9 May 2016 Using High-Resolution, Rapid-Scan Radar Data

   https://doi.org/10.1175/WAF-D-20-0055.1  

   Snyder, Jeffrey C.; Bluestein, Howard B.; Wienhoff, Zachary B.; Kuster, Charles M.; Reif, Dylan W. (October 2020, Weather and Forecasting)

   null (Ed.)

   Abstract Tornadic supercells moved across parts of Oklahoma on the afternoon and evening of 9 May 2016. One such supercell, while producing a long-lived tornado, was observed by nearby WSR-88D radars to contain a strong anticyclonic velocity couplet on the lowest elevation angle. This couplet was located in a very atypical position relative to the ongoing cyclonic tornado and to the supercell’s updraft. A storm survey team identified damage near where this couplet occurred, and, in the absence of evidence refuting otherwise, the damage was thought to have been produced by an anticyclonic tornado. However, such a tornado was not seen in near-ground, high-resolution radar data from a much closer, rapid-scan, mobile radar. Rather, an elongated velocity couplet was observed only at higher elevation angles at altitudes similar to those at which the WSR-88D radars observed the strong couplet. This paper examines observations from two WSR-88D radars and a mobile radar from which it is argued that the anticyclonic couplet (and a similar one ~10 min later) were actually quasi-horizontal vortices centered ~1–1.5 km AGL. The benefits of having data from a radar much closer to the convective storm being sampled (e.g., better spatial resolution and near-ground data coverage) and providing more rapid volume updates are readily apparent. An analysis of these additional radar data provides strong, but not irrefutable, evidence that the anticyclonic tornado that may be inferred from WSR-88D data did not exist; consequently, upon discussions with the National Weather Service, it was not included in Storm Data. 

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4. An Ultrafine-Resolution Numerical Investigation of the Influence of Terrain on Tornado Behavior

   https://doi.org/10.1175/MWR-D-24-0268.1  

   Dang, Jiamin; Houser, Jana; Orf, Leigh; Yue, Peng; Yan, Guirong Grace (April 2026, Monthly Weather Review)

   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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5. Statistical and Empirical Relationships between Tornado Intensity and Both Topography and Land Cover Using Rapid-Scan Radar Observations and a GIS

   https://doi.org/10.1175/MWR-D-19-0407.1  

   Houser, Jana B.; McGinnis, Nathaniel; Butler, Kelly M.; Bluestein, Howard B.; Snyder, Jeffrey C.; French, Michael M. (October 2020, Monthly Weather Review)

   null (Ed.)

   Abstract This study presents an investigation into relationships among topographic elevation, surface land cover, and tornado intensity using rapid scan, mobile Doppler radar observations of four tornadoes from the U.S. Central Plains. High spatiotemporal resolution observations of tornadic vortex signatures from the radar’s lowest elevation angle data (in most cases ranging from ~100 to 350 m above ground level) are coupled with digital elevation model (DEM) and 2011 National Land Cover Database (NLCD) data using a geographic information system (GIS). The relationships between 1) tornado intensity and topographic elevation or surface roughness and 2) changes in tornado intensity and changes in topographic elevation or surface roughness are investigated qualitatively, and statistical relationships are quantified and analyzed using a bootstrap permutation method for individual case studies and all cases collectively. Results suggest that there are statistically significant relationships for individual cases, but the relationships defy generalization and are different on a case-by-case basis, which may imply that they are coincidental, indicating a null correlation. 

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