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Abstract Present-day simulations of supercell thunderstorms have high resolution and incorporate the physical processes known to be conducive to simulating tornado-like vortices (TLVs). In such supercell simulations, TLVs are identified by the strength, duration, and location of the vertical vorticity in the simulated supercell. To bring the analysis of these supercell-produced TLVs a step closer to observations and theory, the TLV in an advanced supercell simulation is identified as the tornado center, and the Cartesian model velocities are transformed to cylindrical coordinates and azimuthally averaged. The azimuthally averaged TLV exhibits many of the observed and theoretically modeled features of tornadoes, including an end-wall vortex with strong maximum vertical and azimuthal velocities (w_maxandυ_max) close to the ground with transition to the weaker core velocities (w_candυ_c) aloft through vortex breakdown. A theory for the rotating-flow boundary layer with radial and azimuthal inflow velocities modeled on the axisymmetric analysis is shown to produce good qualitative agreement with the analyzed axisymmetric TLVs; however, the theoreticalw_maxandυ_maxare too large as the theory does not account for vortex breakdown. Estimation of the corner flow swirl ratio suggests a limit ofυ_max/υ_cslightly greater than unity; sinceυ_cis a feature of the mesocyclone, there is a limit on the extent to which the amplified velocities of the end-wall velocities can be realized. To augment the diversity of cases, the present analysis is applied to a simplified set of supercell simulations; the present theory explains several features of the axisymmetric vortices. 

Abstract Three relatively high-resolution (75-m horizontal grid spacing), large-eddy simulation (LES) ensembles of tornadic supercell storms are used to investigate the effect of a more realistic treatment of the lower boundary condition on supercell storm simulations. These ensembles vary only in the parameterization of near-surface turbulence: one ensemble uses the semislip scheme, whereas the other two employ versions of the “thin boundary layer equations” (TBLE) approach. Each simulated storm is a long-lived supercell, with an intense mesocyclone and at least one tornado-like vortex (TLV). The primary differences observed across the ensembles are (i) the amplitude of near-surface turbulent eddies in the environmental boundary layer and (ii) the intensity of the TLVs, with both TBLE schemes resulting in stronger turbulent eddies in the environment and stronger TLVs within the simulated storms. Although substantial variations in precipitation distribution and cold pool strength are also found among the simulations, the differences in these larger-scale storm attributes from one ensemble to another are not statistically significant. Significance StatementThe assumptions implicit in the semislip boundary condition, which is the lower boundary condition most commonly used in convective storm simulations that include surface drag, are problematic. An alternative approach to modeling the effects of the lower boundary, borrowed from the engineering large-eddy simulation community, is adopted for numerical simulations of supercell storms. The approach at least avoids some of the thorny assumptions inherent in the semislip condition. Though it is not yet possible to know which storm simulations are ultimately the “best,” an awareness of their sensitivities to the handling of the unresolved/underresolved flow in the near-surface layer is an important first step toward improving their realism. 

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. 

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. 

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. 

The initiation of thunderstorms in environments characterized by strong wind shear presents a forecast challenge because of the complexities of the interactions between growing cumulus clouds and wind shear. Thunderstorms that develop in such environments are often capable of producing high-impact hazards, highlighting the importance of convection initiation in sheared environments. Although recent research has greatly improved understanding of the structure and evolution of rising thermals in unsheared environments, there remains uncertainty in how wind shear influences the convection initiation process. Two large-eddy simulations (75-m horizontal grid spacing) were performed to study this problem. Convection initiation attempts are forced in the simulations through prescribed surface heat fluxes (the initial boundary layers are statistically horizontally homogeneous and quasi–steady state but contain turbulent eddies as a result of random initial temperature perturbations). The only difference between the two simulations is the presence or absence of wind shear above 2 km. Important differences in the entrainment patterns are present between sheared and unsheared growing cumulus clouds. As found in previous research, the overturning circulation associated with rising thermals drives dynamic entrainment in the unsheared clouds. However, in sheared clouds, wake entrainment resulting from the tilting of environmental vorticity is an important dynamic entrainment pathway. This result has implications for both the structure of sheared growing cumulus clouds and for convection initiation in sheared environments. Significance StatementForecasts of thunderstorm hazards such as tornadoes, hail, and strong winds, require the accurate prediction of when and where thunderstorms form. Unfortunately, predicting thunderstorm formation is not easy, as there are a lot of different factors to consider. One such factor is environmental vertical wind shear, which describes how winds change speed and direction with height. The purpose of this study is to better understand how wind shear impacts developing clouds. Our results demonstrate a specific mechanism, called “wake entrainment,” through which wind shear can weaken developing clouds and potentially prevent them from becoming strong thunderstorms entirely. Understanding this mechanism may be useful for thunderstorm prediction in environments characterized by wind shear. 

Abstract Over the last decade, supercell simulations and observations with ever increasing resolution have provided new insights into the vortex-scale processes of tornado formation. This article incorporates these and other recent findings into the existing three-step model by adding an additional fourth stage. The goal is to provide an updated and clear picture of the physical processes occurring during tornadogenesis. Specifically, we emphasize the importance of the low-level wind shear and mesocyclone for tornado potential, the organization and interaction of relatively small-scale pre-tornadic vertical vorticity maxima, and the transition to a tornado-characteristic flow. Based on these insights, guiding research questions are formulated for the decade ahead. 

Abstract Unsteadiness and horizontal heterogeneities frequently characterize atmospheric motions, especially within convective storms, which are frequently studied using large-eddy simulations (LES). The models of near-surface turbulence employed by atmospheric LES, however, predominantly assume statistically steady and horizontally homogeneous conditions (known as the equilibrium approach). The primary objective of this work is to investigate the potential consequences of such unrealistic assumptions in simulations of tornadoes. Cloud Model 1 (CM1) LES runs are performed using three approaches to model near-surface turbulence: the “semi-slip” boundary condition (which is the most commonly used equilibrium approach), a recently proposed nonequilibrium approach that accounts for some of the effects of turbulence memory, and a nonequilibrium approach based on thin boundary layer equations (TBLE) originally proposed by the engineering community for smooth-wall boundary layer applications. To be adopted for atmospheric applications, the TBLE approach is modified to account for the surface roughness. The implementation of TBLE into CM1 is evaluated using LES results of an idealized, neutral atmospheric boundary layer. LES runs are then performed for an idealized tornado characterized by rapid evolution, strongly curved air parcel trajectories, and substantial horizontal heterogeneities. The semi-slip boundary condition, by design, always yields a surface shear stress opposite the horizontal wind at the lowest LES grid level. The nonequilibrium approaches of modeling near-surface turbulence allow for a range of surface-shear-stress directions and enhance the resolved turbulence and wind gusts. The TBLE approach even occasionally permits kinetic energy backscatter from unresolved to resolved scales. Significance Statement The traditional approach of modeling the near-surface turbulence is not suitable for a tornado characterized by rapid evolution, strongly curved air parcel trajectories, and substantial horizontal heterogeneities. To understand the influence of statistically unsteady and horizontally heterogeneous near-surface conditions on tornadoes, this work adopts a fairly sophisticated approach from the engineering community and implements it into a widely used atmospheric model with necessary modifications. Compared to the traditional approach, the newly implemented approach produces more turbulent near-surface winds, more flexible surface-drag directions, and stronger wind gusts. These findings suggest a simulated tornado is very sensitive to the modeling approach of near-surface turbulence. 

Abstract The national upgrade of the operational weather radar network to include polarimetric capabilities has lead to numerous studies focusing on polarimetric radar signatures commonly observed in supercells. One such signature is the horizontal separation of regions of enhanced differential reflectivity ( Z DR ) and specific differential phase ( K DP ) values due to hydrometeor size sorting. Recent observational studies have shown that the orientation of this separation tends to be more perpendicular to storm motion in supercells that produce tornadoes. Although this finding has potential operational utility, the physical relationship between this observed radar signature and tornadic potential is not known. This study uses an ensemble of supercell simulations initialized with tornadic and nontornadic environments to investigate this connection. The tendency for tornadic supercells to have a more perpendicular separation orientation was reproduced, although to a lesser degree. This difference in orientation angles was caused by stronger rearward storm-relative flow in the nontornadic supercells, leading to a rearward shift of precipitation and, therefore, the enhanced K DP region within the supercell. Further, this resulted in an unfavorable rearward shift of the negative buoyancy region, which led to an order of magnitude less baroclinic generation of circulation in the nontornadic simulations compared to tornadic simulations. 

Abstract This study analyzes aboveground thermodynamic observations in three tornadic supercells obtained via swarms of small balloon-borne sondes acting as pseudo-Lagrangian drifters; the storm-relative winds draw the sondes through the precipitation, outflow, and baroclinic zones, which are believed to play key roles in tornado formation. Three-dimensional thermodynamic analyses are produced from the in situ observations. The coldest air is found at the lowest analysis levels, where virtual potential temperature deficits of 2–5 K are observed. Air parcels within the forward-flank outflow are inferred from their equivalent potential temperatures to have descended only a few hundred meters or less, whereas parcels within the rear-flank outflow are inferred to have downward excursions of 1–2 km. Additionally, the parcels following paths toward the low-level mesocyclone pass through horizontal buoyancy gradients that are strongest in the lowest 750 m and estimated to be capable of baroclinically generating horizontal vorticity having a magnitude of 6–10 × 10 −3 s −1 . A substantial component of the baroclinically generated vorticity is initially crosswise, though the vorticity subsequently could become streamwise given the leftward bending of the airstream in which the vorticity is generated. The baroclinically generated vorticity could contribute to tornado formation upon being tilted upward and stretched near the surface beneath a strong, dynamically forced updraft. Significance Statement Swarms of balloon-borne probes are used to produce the first-ever, three-dimensional mappings of temperature from in situ observations within supercell storms (rotating storms with high tornado potential). Temperature has a strong influence on the buoyancy of air, and horizontal variations of buoyancy generate spin about a horizontal axis. Buoyancy is one of the primary drivers of upward and downward motions in thunderstorms, and in supercell storms, horizontally oriented spin can be tipped into the vertical and amplified by certain arrangements of upward and downward motions. Unfortunately, the long-standing lack of temperature observations has hampered scientists’ ability to evaluate computer simulations and the tornadogenesis theories derived from them. We find that significant spin could be generated by the horizontal buoyancy variations sampled by the probes.