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1. Transition of Near-Ground Vorticity Dynamics during Tornadogenesis

   https://doi.org/10.1175/JAS-D-21-0181.1  

   Fischer, Jannick; Dahl, Johannes M. (February 2022, Journal of the Atmospheric Sciences)

   Abstract Although much is known about the environmental conditions necessary for supercell tornadogenesis, the near-ground vorticity dynamics during the tornadogenesis process itself are still somewhat poorly understood. For instance, seemingly contradicting mechanisms responsible for large near-ground vertical vorticity can be found in the literature. Broadly, these mechanisms can be sorted into two classes, one being based on upward tilting of mainly baroclinically produced horizontal vorticity in descending air (here called the downdraft mechanism), while in the other the horizontal vorticity vector is abruptly tilted upward practically at the surface by a strong updraft gradient (referred to as the in-and-up mechanism). In this study, full-physics supercell simulations and highly idealized simulations show that both mechanisms play important roles during tornadogenesis. Pretornadic vertical vorticity maxima are generated via the downdraft mechanism, while the dynamics of a fully developed vortex are dominated by the in-and-up mechanism. Consequently, a transition between the two mechanisms occurs during tornadogenesis. This transition is a result of axisymmetrization of the pretornadic vortex patch and intensification via vertical stretching. These processes facilitate the development of the corner flow, which enables production of vertical vorticity by upward tilting of horizontal vorticity practically at the surface, i.e., the in-and-up mechanism. The transition of mechanisms found here suggests that early stages of tornado formation rely on the downdraft mechanism, which is often limited to a small vertical component of baroclinically generated vorticity. Subsequently, a larger supply of horizontal vorticity (produced baroclinically or via surface drag, or even imported from the environment) may be utilized, which marks a considerable change in the vortex dynamics. 

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2. Favorability of Tornado-Like Vortex Formation and Duration as a Function of Surface Drag in Idealized Simulations

   https://doi.org/10.1175/MWR-D-24-0137.1  

   Jiang, Qin; Dawson, Daniel T; Chavas, Daniel R (September 2025, Monthly Weather Review)

   Abstract Tornado-like vortices (TLVs) are known to be very sensitive to processes in the atmospheric surface layer, where surface drag can play multiple roles. This study is designed to examine the behaviors and dynamics of TLVs in response to a range of surface drag strengths in idealized ensemble simulations and to explore their sensitivities to different storm environments. Despite high variability, a systematic influence of surface drag on the interactions between surface rotation, surface convergence boundaries, and the low-level mesocyclone is found. The inclusion of surface drag facilitates TLV formation by enhancing near-surface vortices and low-level lifting, mitigating the need for an intense updraft gradient developing close to the ground. As surface drag increases, a wider circulation near the surface blocks the inflow from directly reaching the rotating core, leading to a less-tilted structure that allows the TLV to be positioned beneath the pressure minima aloft. A further increase in drag strength discourages TLV intensification due to a downward vertical pressure perturbation gradient force, and it stops benefiting from the support of surrounding convergence boundaries, instead becoming detached from them. We hence propose a favorable condition for TLV formation and duration in which a TLV forms a less-tilted structure directly beneath the low-level mesocyclone but also evolves near surrounding surface boundaries; such a scenario strongly depends on the underlying surface drag strength. Experiments varying convective available potential energy (CAPE) and convective inhibition (CIN) also indicate that the optimum for a simulated TLV depends on how/where these favorable conditions are achieved, but the relationships are more complex than expected. Significance StatementTornado development is very sensitive to near-surface processes, especially those involving converging air and the upward stretching of rotating air close to the ground. We show that surface friction can affect those processes in multiple ways, altering the mechanisms leading to tornadogenesis. This study elucidates how surface drag can both benefit and hinder tornado formation and determine how long they last in simulated supercells. The overall effect in any particular situation depends on the strength of the surface drag. These findings help us better understand the complex factors behind tornado formation in supercell storms, particularly regarding near-surface processes. 

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3. A New Pathway for Tornadogenesis Exposed by Numerical Simulations of Supercells in Turbulent Environments

   https://doi.org/10.1175/JAS-D-23-0161.1  

   Markowski, Paul M. (March 2024, Journal of the Atmospheric Sciences)

   Abstract A simulation of a supercell storm produced for a prior study on tornado predictability is reanalyzed for the purpose of examining the fine-scale details of tornadogenesis. It is found that the formation of a tornado-like vortex in the simulation differs from how such vortices have been understood to form in previous numerical simulations. The main difference between the present simulation and past ones is the inclusion of a turbulent boundary layer in the storm’s environment in the present case, whereas prior simulations have used a laminar boundary layer. The turbulent environment contains significant near-surface vertical vorticity (ζ > 0.03 s⁻¹at z= 7.5 m), organized in the form of longitudinal streaks aligned with the southerly ground-relative winds. The ζ streaks are associated with corrugations in the vertical plane in the predominantly horizontal, westward-pointing environmental vortex lines; the vortex-line corrugations are produced by the vertical drafts associated with coherent turbulent structures aligned with the aforementioned southerly ground-relative winds (longitudinal coherent structures in the surface layer such as these are well known to the boundary layer and turbulence communities). The ζ streaks serve as focal points for tornadogenesis, and may actually facilitate tornadogenesis, given how near-surface ζ in the environment can rapidly amplify when subjected to the strong, persistent convergence beneath a supercell updraft. Significance StatementIn high-resolution computer simulations of supercell storms that include a more realistic, turbulent environment, the means by which tornado-like vortices form differs from the mechanism identified in prior simulations using a less realistic, laminar environment. One possibility is that prior simulations develop intense vortices for the wrong reasons. Another possibility could be that tornadoes form in a wide range of ways in the real atmosphere, even within supercell storms that appear to be similar, and increasingly realistic computer simulations are finally now capturing that diversity. 

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4. Will a Worm Turn? Observations of Coherent Surface-Layer Vertical Vorticity in Supercell Inflow

   https://doi.org/10.1175/BAMS-D-24-0350.1  

   Gebauer, Joshua G; Coniglio, Michael C; Saba, Robert A; Bell, Tyler M; Smith, Elizabeth N; Bodine, David J; Frost, Robby M; Markowski, Paul M (March 2026, Bulletin of the American Meteorological Society)

   Abstract A recent supercell modeling study showed that coherent turbulent structures associated with significant near-surface vertical vorticity in the ambient environment may serve as seeds for tornadoes. These structures, colloquially coined “vorticity worms” owing to their elongated and meandering appearance, develop within the surface layer in strong flow and are distinct from the larger-scale horizontal convective rolls often seen in the mixed layer. Elongated coherent turbulent structures are a common feature of the atmosphere and have been heavily studied by the boundary layer community, but before the recent simulation, they have not been previously implicated as facilitators of tornadogenesis. Observations of these structures around supercells have been limited, and historically, the observing platforms most often used in storm-focused data collection have not prioritized the precipitation-free inflow surface layer especially at the spatial and temporal resolution required to capture these structures and their interactions with storms. In recent field deployments, the National Severe Storms Laboratory used a truck-mounted pulsed Doppler lidar to perform near-ground scans to search for vorticity worms in real supercell inflow to compare with the simulation findings. Multiple deployments revealed vorticity worms and near-surface horizontal vorticity in supercell inflow that were consistent with the recently published supercell simulations that motivated this work. In two presented cases, vorticity worms were associated with the development of pretornadic, near-ground vortices, although neither circulation reached tornadic strength. These observations provide credibility to the recent simulations and suggest that continued observations of these coherent structures and research into their role in tornadogenesis are worth pursuing. Significance StatementNew simulations suggest that a source of rotation for tornadoes may originate in the ambient environment in the form of streak-like coherent structures near the ground. We used a mobile Doppler lidar to document the existence of these streak-like structures around storms and present cases where these structures developed into regions of enhanced rotation when they moved near the storm’s updraft. These observations provide credibility to the recent simulations and suggest that more research and observations of these structures around storms are needed to confirm if these structures play a role in tornado formation. 

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5. Cold Pool Forcing of the Streamwise Vorticity Current and Low-Level Mesocyclone in the 9 June 2009 Supercell during VORTEX2

   https://doi.org/10.1175/MWR-D-25-0025.1  

   Ziegler, Conrad L; Markowski, Paul M; Richardson, Yvette P; Weiss, Christopher C; Waugh, Sean M (April 2026, Monthly Weather Review)

   Abstract This case study analyzes a nontornadic supercell observed on 9–10 June 2009 in southwest Kansas during the Verification of the Origins of Rotation in Tornadoes Experiment 2 (VORTEX2). Time-series multi-Doppler radar analyses and diabatic Lagrangian analysis retrievals document the kinematic and thermal–microphysical evolution of the storm’s strengthening low-level mesocyclone during the period 2342–2351 UTC, an apparent “tornadogenesis failure” event around 2351 UTC, and subsequent storm decay through 0024 UTC. An analyzed current of low- and midlevel streamwise vorticity enters the supercell updraft, appearing similar to the streamwise vorticity current (SVC) identified in previous supercell simulations. However, the present SVC primarily feeds into the midlevel updraft and mesocyclone, with relatively limited inflow to the low-level occlusion updraft and mesocyclone. Lagrangian vector vorticity dynamical calculations demonstrate that baroclinity, differential hydrometeor loading, and horizontal stretching all play significant roles in the generation and amplification of streamwise vorticity associated with this SVC. The origins of concentrated vertical vorticity in this mature low-level mesocyclone are consistent with off-trajectory separation of streamwise vorticity in downdraft previously identified in supercell simulations. However, the off-trajectory vorticity vector displacement in the present case is forced by differential hydrometeor loading instead of thermal gradients, since the cold pool is penetrating the low-level mesocyclone core. Another unique feature of this study is the detailed validation of surface radar-derived airflow and retrieved thermal fields with observations from a dense array of surface in situ measurement platforms in the storm. Significance StatementThis study investigates the origins of strongly rotating low-altitude winds via airflow, temperature, and precipitation analyses of a nontornadic supercell (a long-lived thunderstorm with rotating updrafts). Although computer simulations provide highly detailed process understanding of complicated supercell evolutions, these simulated processes have been difficult to quantify in real supercells owing to a lack of required observations. We identify “currents” of horizontal vorticity—rotating wind in a vertical plane—that develop along the edges of the supercell’s rain-cooled outflow and precipitation core, similar to previous simulated rotation development processes. These vorticity currents are subsequently tipped in the observed storm’s updraft to impart its vertical rotation, although ingesting cold outflow likely prevents the observed rotating updraft from forming a tornado. 

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