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Bruce D. Lee
and
Robert B. Wilhelmson

, while Part III reports on model parameter studies investigating the role of convective available potential energy (CAPE) and ambient vertical and horizontal shear in the evolution of landspouts. Until this past decade most attention in tornado research has been placed on understanding supercell tornadogenesis due to the severity of this type of tornado. NSTs have attracted more recent attention as they affect geographical areas of increasing population density such as the High Plains just east of

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R. Jeffrey Trapp
and
Robert Davies-Jones

1. Introduction The successful tornadogenesis paradigm must explain why tornadic vortex signatures (TVSs) and embryonic tornadoes are sometimes, but not always, observed aloft prior to tornadogenesis. A TVS is a large value (typically >1 × 10 −2 s −1 ) of azimuthal shear between two adjacent sampling volumes in a Doppler-radar radial-velocity field and usually forms within (but not necessarily in the center of) a mesocyclone (e.g., Brown et al. 1978 ). It is presumed that a TVS is a degraded

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Wei Huang
and
Ming Xue

. , and M. Xue , 2017 : The role of surface drag in mesocyclone intensification leading to tornadogenesis within an idealized supercell simulation . J. Atmos. Sci. , 74 , 3055 – 3077 , https://doi.org/10.1175/JAS-D-16-0364.1 . Roberts , B. , M. Xue , A. D. Schenkman , and D. T. Dawson II , 2016 : The role of surface drag in tornadogenesis within an idealized supercell simulation . J. Atmos. Sci. , 73 , 3371 – 3395 , https://doi.org/10.1175/JAS-D-15-0332.1 . Roberts

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Richard Rotunno
,
Paul M. Markowski
, and
George H. Bryan

rain curtain in a supercell instigate tornadogenesis barotropically? J. Atmos. Sci. , 65 , 2469 – 2497 , doi: 10.1175/2007JAS2516.1 . 10.1175/2007JAS2516.1 Davies-Jones , R. , 2015 : A review of supercell and tornado dynamics . Atmos. Res. , 158–159 , 274 – 291 , doi: 10.1016/j.atmosres.2014.04.007 . 10.1016/j.atmosres.2014.04.007 Davies-Jones , R. , and H. E. Brooks , 1993 : Mesocyclogenesis from a theoretical perspective. The Tornado: Its Structure, Dynamics, Prediction, and

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Paul M. Markowski
,
Jerry M. Straka
, and
Erik N. Rasmussen

rotation near the ground ( Ludlam 1963 ; Fujita 1975 ; Burgess et al. 1977 ; Barnes 1978 ; Lemon and Doswell 1979 ), their precise role in the tornadogenesis process remains unclear. A lengthy review of observational, numerical modeling, and theoretical findings pertinent to hook echoes and RFDs recently has been completed by Markowski (2002) . In a companion paper by Markowski et al. (2002) , it was observed that the air parcels at the surface within the RFDs of tornadic supercells tend to be

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Nicholas A. Goldacker
and
Matthew D. Parker

; Markowski et al. 2008 , 2012 ; Schenkman et al. 2014 ; Dahl 2015 ; Parker and Dahl 2015 ; Markowski 2016 ; Roberts et al. 2016 ; Roberts and Xue 2017 ; Roberts et al. 2020 ). However, Coffer and Parker (2017) showed that both nontornadic and tornadic supercells generate ample pretornadic ζ sfc . The step that “makes or breaks” the mechanism of tornadogenesis is likely step 3: the ability for ζ sfc to be contracted into a tornado. Step 3 is most strongly favored when overlying rotation

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Johannes M. L. Dahl
,
Matthew D. Parker
, and
Louis J. Wicker

. Discussion An interesting question is how the above analysis relates to tornadogenesis. As discussed in section 3b , we have rather limited faith in the treatment of near-ground trajectories once they descend below the bottom scalar model level, so we cannot describe vortex genesis faithfully. However, when animated, the vertical vorticity field at the lowest scalar model level clearly shows how the rivers and lobes of positive ζ move downstream and feed into the developing vortex (see also Fig. 5

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Richard Rotunno
,
George H. Bryan
,
David S. Nolan
, and
Nathan A. Dahl

Abstract

This study is the first in a series that investigates the effects of turbulence in the boundary layer of a tornado vortex. In this part, axisymmetric simulations with constant viscosity are used to explore the relationships between vortex structure, intensity, and unsteadiness as functions of diffusion (measured by a Reynolds number Re r ) and rotation (measured by a swirl ratio S r ). A deep upper-level damping zone is used to prevent upper-level disturbances from affecting the low-level vortex. The damping zone is most effective when it overlaps with the specified convective forcing, causing a reduction to the effective convective velocity scale W e . With this damping in place, the tornado-vortex boundary layer shows no sign of unsteadiness for a wide range of parameters, suggesting that turbulence in the tornado boundary layer is inherently a three-dimensional phenomenon. For high Re r , the most intense vortices have maximum mean tangential winds well in excess of W e , and maximum mean vertical velocity exceeds 3 times W e . In parameter space, the most intense vortices fall along a line that follows , in agreement with previous analytical predictions by Fiedler and Rotunno. These results are used to inform the design of three-dimensional large-eddy simulations in subsequent papers.

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Lawrence R. Frank
,
Vitaly L. Galinsky
,
Leigh Orf
, and
Joshua Wurman

1. Introduction The mechanisms by which tornadoes are formed within supercell thunderstorms (tornadogenesis) remain a significant scientific mystery. This is a problem of both great scientific interest and social impact, as the ability to understand tornado formation is critical in reducing the time between issuing of tornado warnings to potentially affected populations and tornado formation. However, the interplay of spatiotemporally varying processes within the highly dynamic supercell

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D. C. Lewellen
and
W. S. Lewellen

evolution leading to a dynamic corner flow collapse (CFC) can naturally produce a very intense near-surface vortex from a much weaker larger-scale swirling flow, and it was argued that this might sometimes play a critical role in tornadogenesis and/or tornado variability. We begin by addressing important simulation concerns in section 2 , particularly the grid resolution requirements and interaction with the top boundary condition. We then sample the range of behavior encountered and address what

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