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Quasi-linear convective systems (QLCSs) are responsible for approximately a quarter of all tornado events in the United States, but no field campaigns have focused specifically on collecting data to understand QLCS tornadogenesis. The Propagation, Evolution, and Rotation in Linear Storms (PERiLS) project was the first observational study of tornadoes associated with QLCSs ever undertaken. Participants were drawn from more than 10 universities, laboratories, and institutes, with over 100 students participating in field activities. The PERiLS field phases spanned 2 years, late winters and early springs of 2022 and 2023, to increase the probability of intercepting significant tornadic QLCS events in a range of large-scale and local environments. The field phases of PERiLS collected data in nine tornadic and nontornadic QLCSs with unprecedented detail and diversity of measurements. The design and execution of the PERiLS field phase and preliminary data and ongoing analyses are shown. KEYWORDS: Severe storms; Squall lines; Tornadoes; Mesoscale processes; Storm environments; Mesoscale forecasting, 1. Introduction Over the past 10 years, ~1400 tornadoes occurred annually in the United States, of which roughly 15%-25% are spawned by quasi-linear convective systems (QLCSs) (e.g., Trapp et al. [...]

On 28 May 2019, a tornadic supercell, observed as part of Targeted Observation by UAS and Radars of Supercells (TORUS) produced an EF-2 tornado near Tipton, Kansas. The supercell was observed to interact with multiple preexisting airmass boundaries. These boundaries and attendant air masses were examined using unoccupied aircraft system (UAS), mobile mesonets, radiosondes, and dual-Doppler analyses derived from TORUS mobile radars. The cool-side air mass of one of these boundaries was found to have higher equivalent potential temperature and backed winds relative to the warm-side air mass; features associated with mesoscale air masses with high theta-e (MAHTEs). It is hypothesized that these characteristics may have facilitated tornadogenesis. The two additional boundaries were produced by a nearby supercell and appeared to weaken the tornadic supercell. This work represents the first time that UAS have been used to examine the impact of preexisting airmass boundaries on a supercell, and it provides insights into the influence environmental heterogeneities can have on the evolution of a supercell. [ABSTRACT FROM AUTHOR]

Recent studies have shown how very small differences in the background environment of a supercell can yield different outcomes, particularly in terms of tornado production. In this study, we use a novel convection initiation technique to simulate six supercells with a focus on their early development. Each experiment is identical, except for the strength of thermal forcing for the initial convection initiation. Each experiment yields a mature supercell, but differences in storm-scale characteristics like updraft speed, cold pool temperature deficit, and vertical vorticity development abound. Of these, the time when the midlevel updraft strengthens is most strongly related to initiation strength, with stronger thermal forcing favoring quicker updraft development. The same is true for the low-level updraft, with the additional relationship that stronger thermal forcing also tends to yield stronger low-level updrafts for around the first 2 h of the simulations. The experiments with faster updraft development tend to be associated with more rapid surface vortex intensification; however, cold pool evolution differs between simulations with weaker versus stronger thermal forcing. Stronger thermal forcing also yields deviant rightward storm motion earlier in the supercell's life cycle that remains more consistent for the duration of the simulation. These results highlight the range of supercellular outcomes that are possible across a background environment due to differences in storm-scale initiation strength. They are also of potential importance for predicting the paths and tornado potential of supercells in real time. Significance Statement: Despite a better understanding of processes related to tornado production in supercell thunderstorms, forecasters still have difficulty discriminating between tornadic and nontornadic supercells in close proximity to each other within the same severe weather event. In this study, we use six simulations of supercells to examine how these different outcomes can occur. Our results show that, given the same background environment, a storm that is more strongly initiated will exhibit faster updraft development and, possibly, quicker tornado production. The opposite can be said for storms that are more weakly initiated. Differences in initiation strength are also associated with different storm motions. These findings inspire future work to better relate supercell evolution to characteristics of initiation and the environment. [ABSTRACT FROM AUTHOR]

The life cycle of the 2 June 1995 Dimmitt, Texas, tornado cyclone, observed during the Verification of the Origins of Rotation in Tornadoes Experiment (VORTEX), is described. The tornado cyclone here is defined as a significantly axisymmetric flow larger than the visible tornado and characterized by increasing angular momentum with increasing radius. Its life cycle included three phases with somewhat differing evolution of angular momentum, herein called intensifying, transition, and weakening. During the intensifying stage, the funnel and debris cloud gradually increased in size. The azimuthally averaged secondary circulation of the larger-scale tornado cyclone, as determined using high-resolution single-Doppler data obtained by a mobile radar, was primarily inward and upward, consistent with the presence of a wall cloud outside the tornado. The azimuthally averaged angular momentum increased monotonically away from the tornado, so inward advection allowed the angular momentum to increase slowly with time in part of the tornado cyclone. During the transition phase, downdrafts began to occur within the tornado cyclone. The transport of angular momentum by the secondary circulation nearly was offset by eddy flux convergence of angular momentum so that the azimuthally averaged angular momentum tendency was only weakly negative at most radii. The tornado was visually impressive during this stage, featuring a 400-m diameter debris cloud extending to cloud base, while the surrounding wall cloud shrank and eroded. During the weakening phase, the funnel and debris cloud gradually shrank, and the funnel went through a rope stage prior to disappearing. The weakening phase was characterized by extensive downdrafts at all radii outside the tornado, and large-scale near-ground outflow as observed by mobile mesonet systems in a portion of the tornado cyclone. The secondary circulation acted to transport smaller angular momentum downward from aloft, and outward along the ground. All terms of the angular momentum budget became negative throughout most of the low-level (0-800-m AGL) tornado cyclone during the weakening phase. Several hypotheses for this evolution are evaluated, including changes in water loading in the tornado cyclone, cooling of the near-ground air, and the distribution of tangential velocity with height with its concomitant influence on the nonhydrostatic vertical pressure gradient force.

Idealized numerical simulations are conducted in which an axisymmetric, moist, rotating updraft free of rain is initiated, after which a downdraft is imposed by precipitation loading. The experiments are designed to emulate a supercell updraft that has rotation aloft initially, followed by the formation of a downdraft and descent of a rain curtain on the rear flank. In the idealized simulations, the rain curtain and downdraft are annular, rather than hook-shaped, as is typically observed. The downdraft transports angular momentum, which is initially a maximum aloft and zero at the surface, toward the ground. Once reaching the ground, the circulation-rich air is converged beneath the updraft and a tornado develops. The intensity and longevity of the tornado depend on the thermodynamic characteristics of the angular momentum-transporting downdraft, which are sensitive to the ambient low-level relative humidity and precipitation character of the rain curtain. For large low-level relative humidity and a rain curtain having a relatively small precipitation concentration, the imposed downdraft is warmer than when the low-level relative humidity is small and the precipitation concentration of the rain curtain is large. The simulated tornadoes are stronger and longer-lived when the imposed downdrafts are relatively warm compared to when the downdrafts are relatively cold, owing to a larger amount of convergence of circulation-rich downdraft air. The results may explain some recent observations of the tendency for supercells to be tornadic when their rear-flank downdrafts are associated with relatively small temperature deficits.

This paper describes the Verification of the Origins of Rotation in Tornadoes Experiment planned for 1994 and 1995 to evaluate a set of hypotheses pertaining to tornadogenesis and tornado dynamics. Observations of state variables will be obtained from five mobile mesonet vehicles, four mobile ballooning laboratories, three movie photography teams, portable Doppler radar teams, two in situ tornado instruments deployment teams, and the T-28 and National Atmospheric and Oceanic Administration P-3 aircraft. In addition, extensive use will be made of the new generation of observing systems, including the WSR-88D Doppler radars, demonstration wind profiler network, and National Weather Service rawinsondes.

Doppler radar observations that establish common patterns in the evolution of the reflectivity and flow structures of squall lines are described. A number of squall lines have been analyzed with unprecedented time resolution in order to identify these patterns. All of the squall lines appeared to be approximately two-dimensional and featured a solid leading edge at some time during their life cycle, instead of being composed of discrete cells separated by echo-free regions. A large variety of intensities and evolution time scales was observed. It is shown that squall lines of this type evolve through identifiable stages of reflectivity structure. This evolution appears to be strongly related to changes that occur in the kinematic structure. As a typical system evolves, a rearward-sloping zone of horizontal vorticity, predominantly associated with vertical shear, develops on the scale of the system, presumably driven by the horizontal buoyancy gradients across the system. The vorticity that is generated allows further generation to take place by causing the superposition of a saturated, precipitating anvil cloud aloft over potentially cooler air below in the trailing region. The rearward-sloping vorticity zone gradually tilts toward the horizontal. The rate at which this zone tilts seems to be the primary difference between the systems studied. To a first approximation, the inflow streamlines parallel the sloping vorticity zone, so as it approaches a horizontal slope, vertical motion becomes smaller. Eventually, convective-scale ascent ceases, giving the impression that the gust front has surged out ahead of the precipitation.

All of the 0000 UTC soundings from the United States made during the year 1992 that have nonzero convective available potential energy (CAPE) are examined. Soundings are classified as being associated with nonsupercell thunderstorms, supercells without significant tornadoes, and supercells with significant tornadoes. This classification is made by attempting to pair, based on the low-level sounding winds, an upstream sounding with each occurrence of a significant tornado, large hail, and/or 10 or more cloud-to-ground lightning flashes. Severe weather wind parameters (mean shear, 0-6-km shear, storm-relative helicity, and storm-relative anvil-level flow) and CAPE parameters (total CAPE and CAPE in the lowest 3000 m with buoyancy) are shown to discriminate weakly between the environments of the three classified types of storms. Combined parameters (energy-helicity index and vorticity generation parameter) discriminate strongly between the environments. The height of the lifting condensation level also appears to be generally lower for supercells with significant tornadoes than those without. The causes for the very large false alarm rates in the tornadic/nontornadic supercell forecast, even with the best discriminators, are discussed. [ABSTRACT FROM AUTHOR]

The processes that force the initiation of deep convection along the dryline are inferred from special mesoscale observations obtained during the 1991 Central Oklahoma Profiler Studies project, the Verification of the Origins of Rotation in Tornadoes Experiment 1994 (VORTEX-94), and the VORTEX-95 field projects. Observations from aircraft, mobile CLASS soundings, and mobile mesonets define the fields of airflow, absolute humidity, and virtual temperature in the boundary layer across the dryline on the 15 May 1991, 7 June 1994, and 6 May 1995 case days. Film and video cloud images obtained by time-lapse cameras on the NOAA P-3 are used to reconstruct the mesoscale distribution of cumulus clouds by photogrammetric methods, permitting inferences concerning the environmental conditions accompanying cloud formation or suppression. The results of the present study confirm the classical notion that the dryline is a favored zone for cumulus cloud formation. The combined cloud distributions for the three cases are approximately Gaussian, suggesting a peak expected cloud frequency 15 km east of the dryline. Deep mesoscale moisture convergence is inferred in cloudy regions, with either subsidence or a lack of deep convergence in cloud-free regions. The results document the modulating effect of vertical wind shear and elevated dry layers in combination with the depth and strength of mesoscale updrafts on convective initiation, supporting the notion that moist boundary layer air parcels must be lifted to their lifted condensation level and level of free convection prior to leaving the mesoscale updraft to form deep convection. By relaxing the overly restrictive assumptions of parcel theory, it is suggested that a modification of proximity soundings to account for mesoscale lift and westerly wind shear effects can improve the diagnosis of the mesoscale dryline environment and the prediction of convective initiation at the dryline. Conversely, proximity environmental... [ABSTRACT FROM AUTHOR]