More Like this

1. Disentangling the Influences of Storm-Relative Flow and Horizontal Streamwise Vorticity on Low-Level Mesocyclones in Supercells

   https://doi.org/10.1175/JAS-D-22-0114.1  

   Peters, John M.; Coffer, Brice E.; Parker, Matthew D.; Nowotarski, Christopher J.; Mulholland, Jake P.; Nixon, Cameron J.; Allen, John T. (January 2023, Journal of the Atmospheric Sciences)

   Abstract Sufficient low-level storm-relative flow is a necessary ingredient for sustained supercell thunderstorms and is connected to supercell updraft width. Assuming a supercell exists, the role of low-level storm-relative flow in regulating supercells’ low-level mesocyclone intensity is less clear. One possibility considered in this article is that storm-relative flow controls mesocyclone and tornado width via its modulation of overall updraft extent. This hypothesis relies on a previously postulated positive correspondence between updraft width, mesocyclone width, and tornado width. An alternative hypothesis is that mesocyclone characteristics are primarily regulated by horizontal streamwise vorticity irrespective of storm-relative flow. A matrix of supercell simulations was analyzed to address the aforementioned hypotheses, wherein horizontal streamwise vorticity and storm-relative flow were independently varied. Among these simulations, mesocyclone width and intensity were strongly correlated with horizontal streamwise vorticity, and comparatively weakly correlated with storm-relative flow, supporting the second hypothesis. Accompanying theory and trajectory analysis offers the physical explanation that, when storm-relative flow is large and updrafts are wide, vertically tilted streamwise vorticity is projected over a wider area but with a lesser average magnitude than when these parameters are small. These factors partially offset one another, degrading the correspondence of storm-relative flow with updraft circulation and rotational velocity, which are the mesocyclone attributes most closely tied to tornadoes. These results refute the previously purported connections between updraft width, mesocyclone width, and tornado width, and emphasize horizontal streamwise vorticity as the primary control on low-level mesocyclones in sustained supercells. Significance Statement The intensity of a supercell thunderstorm’s low-level rotation, known as the “mesocyclone,” is thought to influence tornado likelihood. Mesocyclone intensity depends on many environmental attributes that are often correlated with one another and difficult to disentangle. This study used a large body of numerical simulations to investigate the influence of the speed of low-level air entering a supercell (storm-relative flow), the horizontal spin of the ambient air entering the thunderstorm (streamwise vorticity), and the width of the storm’s updraft. Our results suggest that the rotation of the mesocyclone in supercells is primarily influenced by streamwise vorticity, with comparatively weaker connections to storm-relative flow and updraft width. These findings provide important clarification in our scientific understanding of how a storm’s environment influences the rate of rotation of its mesocyclone, and the associated tornado threat. 

   more » « less
2. Low-level Updraft Intensification in Response to Environmental Wind Profiles

   https://doi.org/10.1175/JAS-D-20-0354.1  

   Goldacker, Nicholas A.; Parker, Matthew D. (June 2021, Journal of the Atmospheric Sciences)

   Abstract Supercell storms can develop a “dynamical response” whereby upward accelerations in the lower troposphere amplify as a result of rotationally induced pressure falls aloft. These upward accelerations likely modulate a supercell’s ability to stretch near-surface vertical vorticity to achieve tornadogenesis. This study quantifies such a dynamical response as a function of environmental wind profiles commonly found near supercells. Self-organizing maps (SOMs) were used to identify recurring low-level wind profile patterns from 20,194 model-analyzed, near-supercell soundings. The SOM nodes with larger 0–500 m storm-relative helicity (SRH) and streamwise vorticity ( ω s ) corresponded to higher observed tornado probabilities. The distilled wind profiles from the SOMs were used to initialize idealized numerical simulations of updrafts. In environments with large 0–500 m SRH and large ω s , a rotationally induced pressure deficit, increased dynamic lifting, and a strengthened updraft resulted. The resulting upward-directed accelerations were an order of magnitude stronger than typical buoyant accelerations. At 500 m AGL, this dynamical response increased the vertical velocity by up to 25 m s –1 , vertical vorticity by up to 0.2 s –1 , and pressure deficit by up to 5 hPa. This response specifically augments the near-ground updraft (the midlevel updraft properties are almost identical across the simulations). However, dynamical responses only occurred in environments where 0–500 m SRH and ω s exceeded 110 m 2 s –2 and 0.015 s –1 , respectively. The presence vs. absence of this dynamical response may explain why environments with higher 0–500 m SRH and ω s correspond to greater tornado probabilities. 

   more » « less
3. The Influences of Effective Inflow Layer Streamwise Vorticity and Storm-Relative Flow on Supercell Updraft Properties

   https://doi.org/10.1175/JAS-D-19-0355.1  

   Peters, John M.; Nowotarski, Christopher J.; Mulholland, Jake P.; Thompson, Richard L. (September 2020, Journal of the Atmospheric Sciences)

   null (Ed.)

   Abstract The relationship between storm-relative helicity (SRH) and streamwise vorticity ωs is frequently invoked to explain the often robust connections between effective inflow layer (EIL) SRH and various supercell updraft properties. However, the definition of SRH also contains storm-relative (SR) flow, and the separate influences of SR flow and ωs on updraft dynamics are therefore convolved when SRH is used as a diagnostic tool. To clarify this issue, proximity soundings and numerical experiments are used to disentangle the separate influences of EIL SR flow and ωs on supercell updraft characteristics. Our results suggest that the magnitude of EIL ωs has little influence on whether supercellular storm mode occurs. Rather, the transition from nonsupercellular to supercellular storm mode is largely modulated by the magnitude of EIL SR flow. Furthermore, many updraft attributes such as updraft width, maximum vertical velocity, vertical mass flux at all levels, and maximum vertical vorticity at all levels are largely determined by EIL SR flow. For a constant EIL SR flow, storms with large EIL ωs have stronger low-level net rotation and vertical velocities, which affirms previously established connections between ωs and tornadogenesis. EIL ωs also influences storms’ precipitation and cold-pool patterns. Vertical nonlinear dynamic pressure acceleration (NLDPA) is larger at low levels when EIL ωs is large, but differences in NLDPA aloft become uncorrelated with EIL ωs because storms’ midlevel dynamic pressure perturbations are substantially influenced by the tilting of midlevel vorticity. Our results emphasize the importance of considering EIL SR flow in addition to EIL SRH in the research and forecasting of supercell properties. 

   more » « less
4. Examining Right-Moving Supercell Environments in the Great Plains with Composite Doppler Wind Lidar Observations

   https://doi.org/10.1175/MWR-D-25-0177.1  

   Saba, Robert A; Coniglio, Michael C; Gebauer, Joshua G; Bell, Tyler M; Smith, Elizabeth N; Waugh, Sean (April 2026, Monthly Weather Review)

   Abstract This study utilizes mobile Doppler wind lidar (DWL) observations collected by the National Severe Storms Laboratory throughout recent collaborative field experiments. Using data from 50 deployments on 36 supercells between 2017 and 2023, this work compares the kinematic environments of tornadic and nontornadic right-moving supercells. The DWL observations provide the unique opportunity to sample the subhourly evolution of the storm inflow wind profile, especially below 500 m. DWL observations are combined with surface and radiosonde observations using optimal estimation to create complete vertical wind profiles for the quantification of severe weather forecasting parameters. Supplemental Rapid Refresh model profiles are used to estimate the near-storm environmental perturbations. The tornadic supercells show larger ground-relative winds, and ground-relative wind perturbations, than nontornadic supercells. Differences in storm-relative wind are less consistent but are larger above 1 km in tornadic environments with a signal for larger winds 10–20 min after tornadogenesis. Low-level storm-relative helicity (SRH) discriminates tornadic from nontornadic cases better than deep-layer SRH but does not show a tendency to increase leading up to tornadogenesis, nor does the wind shear or storm-relative wind (SRW). The tornadic environments have greater near-ground horizontal vorticity (both streamwise and crosswise) with vorticity maximized very close to the ground (≤100 m), indicating the potential importance of frictionally generated vorticity and related processes. Overall, this work demonstrates the utility of DWL for sampling near the ground in storm environments while providing clarity on results from recent literature. Significance StatementForecasters and researchers continue to search for ways to differentiate between supercells that produce tornadoes and those that do not. Most previous studies have used vertical profiles of the atmosphere derived from models or radiosonde observations. Mobile deployments of Doppler wind lidars allow for new observations not widely used in severe storm research. Analysis of the resulting wind profiles provides a more detailed understanding of supercell environments. Findings from this work support many past results including the best layers over which to diagnose various severe parameters. A detailed look into near-ground observations reveals how tornadic storms modify their environments differently from nontornadic storms and supports studies that show how the stronger near-ground wind speeds may contribute to the rotation supporting tornado potential in the storm. 

   more » « less
5. Relation between Baroclinity, Horizontal Vorticity, and Mesocyclone Evolution in the 6–7 April 2018 Monroe, Louisiana, Tornadic Supercell during VORTEX-SE

   https://doi.org/10.1175/MWR-D-22-0313.1  

   Hosek, Michael J; Ziegler, Conrad L; Biggerstaff, Michael I; Murphy, Todd A; Wang, Zhien (November 2023, Monthly Weather Review)

   Abstract This case study analyzes a tornadic supercell observed in northeast Louisiana as part of the Verification of the Origins of Rotation in Tornadoes Experiment Southeast (VORTEX-SE) on 6–7 April 2018. One mobile research radar (SR1-P), one WSR-88D equivalent (KULM), and two airborne radars (TAFT and TFOR) have sampled the storm at close proximity for ∼70 min through its mature phase, tornadogenesis at 2340 UTC, and dissipation and subsequent ingestion into a developing MCS segment. The 4D wind field and reflectivity from up to four Doppler analyses, combined with 4D diabatic Lagrangian analysis (DLA) retrievals, has enabled kinematic and thermodynamic analysis of storm-scale boundaries leading up to, during, and after the dissipation of the NWS-surveyed EF0 tornado. The kinematic and thermodynamic analyses reveal a transient current of low-level streamwise vorticity leading into the low-level supercell updraft, appearing similar to the streamwise vorticity current (SVC) that has been identified in supercell simulations and previously observed only kinematically. Vorticity dynamical calculations demonstrate that both baroclinity and horizontal stretching play significant roles in the generation and amplification of streamwise vorticity associated with this SVC. While the SVC does not directly feed streamwise vorticity to the tornado–cyclone, its development coincides with tornadogenesis and an intensification of the supercell’s main low-level updraft, although a causal relationship is unclear. Although the mesoscale environment is not high-shear/low-CAPE (HSLC), the updraft of the analyzed supercell shares some similarities to past observations and simulations of HSLC storms in the Southeast United States, most notably a pulse-like updraft that is maximized in the low- to midlevels of the storm. Significance StatementThe purpose of this study is to analyze the airflow and thermodynamics of a highly observed tornado-producing supercell. While computer simulations can provide us with highly detailed looks at the complicated evolution of supercells, it is rare, due to the difficulty of data collection, to collect enough data to perform a highly detailed analysis on a particular supercell, especially in the Southeast United States. We identified a “current” of vorticity—rotating wind—that develops at the intersection of the supercell’s rain-cooled outflow and warm inflow, similar to previous simulations. This vorticity current develops and feeds the storm’s updraft as its tornado develops and the storm intensifies, although it does not directly enter the tornado. 

   more » « less