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1. 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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2. Supercell low-level mesocyclones: Origins of inflow and vorticity

   https://doi.org/10.1175/MWR-D-22-0269.1  

   Coffer, Brice E.; Parker, Matthew D.; Peters, John M.; Wade, Andrew R. (June 2023, Monthly Weather Review)

   Abstract The development and intensification of low-level mesocyclones in supercell thunderstorms has often been attributed, at least in part, to augmented streamwise vorticity generated baroclinically in the forward flank of supercells. However, the ambient streamwise vorticity of the environment (often quantified via storm-relative helicity), especially near the ground, is particularly skillful at discriminating between nontornadic and tornadic supercells. This study investigates whether the origins of the inflow air into supercell low-level mesocyclones, both horizontally and vertically, can help explain the dynamical role of environmental versus storm-generated vorticity in the development of low-level mesocyclone rotation. Simulations of supercells, initialized with wind profiles common to supercell environments observed in nature, show that the air bound for the low-level mesocyclone primarily originates from the ambient environment (rather than from along the forward flank) and from very close to the ground, often in the lowest 200 - 400 m of the atmosphere. Given that the near-ground environmental air comprises the bulk of the inflow into low-level mesocyclones, this likely explains the forecast skill of environmental streamwise vorticity in the lowest few hundred meters of the atmosphere. The low-level mesocyclone does not appear to require much augmentation from the development of additional horizontal vorticity in the forward flank. Instead, the dominant contributor to vertical vorticity within the low-level mesocyclone is from the environmental horizontal vorticity. This study provides further context to the on-going discussion regarding the development of rotation within supercell low-level mesocyclones. 

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3. Processes Preventing the Development of a Significant Tornado in a Colorado Supercell on 26 May 2010

   https://doi.org/10.1175/MWR-D-19-0288.1  

   Murdzek, Shawn S.; Markowski, Paul M.; Richardson, Yvette P.; Tanamachi, Robin L. (May 2020, Monthly Weather Review)

   null (Ed.)

   Abstract A supercell produced a nearly tornadic vortex during an intercept by the Second Verification of the Origins of Rotation in Tornadoes Experiment on 26 May 2010. Using observations from two mobile radars performing dual-Doppler scans, a five-probe mobile mesonet, and a proximity sounding, factors that prevented this vortex from strengthening into a significant tornado are examined. Mobile mesonet observations indicate that portions of the supercell outflow possessed excessive negative buoyancy, likely owing in part to low boundary layer relative humidity, as indicated by a high environmental lifted condensation level. Comparisons to a tornadic supercell suggest that the Prospect Valley storm had enough far-field circulation to produce a significant tornado, but was unable to converge this circulation to a sufficiently small radius. Trajectories suggest that the weak convergence might be due to the low-level mesocyclone ingesting parcels with considerable crosswise vorticity from the near-storm environment, which has been found to contribute to less steady and weaker low-level updrafts in supercell simulations. Yet another factor that likely contributed to the weak low-level circulation was the inability of parcels rich in streamwise vorticity from the forward-flank precipitation region to reach the low-level mesocyclone, likely owing to an unfavorable pressure gradient force field. In light of these results, we suggest that future research should continue focusing on the role of internal, storm-scale processes in tornadogenesis, especially in marginal environments. 

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4. 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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5. Effects of Roll Vortices on the Evolution of Hurricane Harvey During Landfall

   https://doi.org/10.1175/JAS-D-20-0270.1  

   Li, Xin; Pu, Zhaoxia; Gao, Zhiqiu (April 2021, Journal of the Atmospheric Sciences)

   Abstract Horizontal boundary layer roll vortices are a series of large-scale turbulent eddies that prevail in a hurricane’s boundary layer. In this paper, a one-way nested sub-kilometer-scale large eddy simulation (LES) based on the Weather Research and Forecasting model (WRF) was used to examine the impact of roll vortices on the evolution of Hurricane Harvey around its landfall from 0000z on 25 to 1800z 27 August 2017. The simulation results imply that the turbulence in the LES can be attributed mainly to roll vortices. With the representation of roll vortices, the LES simulation provided a better simulation of hurricane wind vertical structure and precipitation. In contrast, the mesoscale simulation with the YSU PBL scheme overestimated the precipitation for the hurricane over the ocean. Further analysis indicates that the roll vortices introduced a positive vertical flux and thinner inflow layer, whereas a negative flux maintained the maximum tangential wind at around 400 m above ground. During hurricane landfall, the weak negative flux maintained the higher wind in the LES simulation. The overestimated low-level vertical flux in the mesoscale simulation with the YSU scheme led to overestimated hurricane intensity over the ocean and accelerated the decay of the hurricane during landfall. Rainfall analysis reveals that the roll vortices led to a weak updraft and insufficient water vapor supply in the LES. For the simulation with the YSU scheme, the strong updraft combined with surplus water vapor eventually led to unrealistic heavy rainfall for the hurricane over the ocean. 

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