Abstract

Idealized, dry simulations are used to investigate the roles of environmental vertical wind shear and baroclinic vorticity generation in the development of near-surface vortices in supercell-like “pseudostorms.” A cyclonically rotating updraft is produced by a stationary, cylindrical heat source imposed within a horizontally homogeneous environment containing streamwise vorticity. Once a nearly steady state is achieved, a heat sink, which emulates the effects of latent cooling associated with precipitation, is activated on the northeastern flank of the updraft at low levels. Cool outflow emanating from the heat sink spreads beneath the updraft and leads to the development of near-surface vertical vorticity via the “baroclinic mechanism,” as has been diagnosed or inferred in actual supercells that have been simulated and observed.

An intense cyclonic vortex forms in the simulations in which the environmental low-level wind shear is strong and the heat sink is of intermediate strength relative to the other heat sinks tested. Intermediate heat sinks result in the development (baroclinically) of substantial near-surface circulation, yet the cold pools are not excessively strong. Moreover, the strong environmental low-level shear lowers the base of the midlevel mesocyclone, which promotes strong dynamic lifting of near-surface air that previously resided in the heat sink. The superpositioning of the dynamic lifting and circulation-rich, near-surface air having only weak negative buoyancy facilitates near-surface vorticity stretching and vortex genesis. An intense cyclonic vortex fails to form in simulations in which the heat sink is excessively strong or weak or if the low-level environmental shear is weak.

Abstract

In idealized numerical simulations of supercell-like “pseudostorms” generated by a heat source and sink in a vertically sheared environment, a tornado-like vortex develops if air possessing large circulation about a vertical axis at the lowest model levels can be converged. This is most likely to happen if the circulation-rich air possesses only weak negative buoyancy (the circulation-rich air has a history of descent, so typically possesses at least some negative buoyancy) and is subjected to an upward-directed vertical perturbation pressure gradient force. This paper further explores the sensitivity of the development of near-surface vertical vorticity to the horizontal position of the heat sink. Shifting the position of the heat sink by only 2–3 km can significantly influence vortex intensity by altering both the baroclinic generation of circulation and the buoyancy of circulation-rich air. Many of the changes in the pseudostorms that arise from shifting the position of the heat sink would be difficult to anticipate. The sensitivity of the pseudostorms to heat sink position probably at least partly explains the well-known sensitivity of near-surface vertical vorticity development to the microphysics parameterizations in more realistic supercell storm simulations, as well as some of the failures of actual supercells to produce tornadoes in seemingly favorable environments.

Abstract

Severe convective storms are typically simulated using either an idealized, horizontally homogeneous environment (i.e., single sounding) or an inhomogeneous environment constructed using numerous types of observations. Representing opposite ends of the spectrum, the former allows for the study of storm dynamics without the complicating effects of either land surface or atmospheric variability, though arguably at the expense of physical realism, while the latter is especially useful for prediction and data sensitivity studies, though because of its physical completeness, determination of cause can be extremely difficult. In this study, the gap between these two extremes is bridged by specifying horizontal variations in environmental vertical shear in an idealized, controlled manner so that their influence on storm morphology can be readily diagnosed. Simulations are performed using the Advanced Regional Prediction System (ARPS), though with significant modification to accommodate the analytically specified environmental fields. Several steady-state environments are constructed herein that retain a good degree of physical realism while permitting clear interpretation of cause and effect. These experiments are compared to counterpart control simulations in homogeneous environments constructed using single wind profiles from selected locations within the inhomogeneous environment domain. Simulations in which steady-state vertical shear varies spatially are presented for different shear regimes (storm types). A gradient of weak shear across the storm system leads to preferred cell development on the flank with greater shear. In a stronger shear regime (i.e., in the borderline multicell/supercell regime), however, cell development is enhanced on the weaker shear flank while cell organization is enhanced on the strong shear side. When an entire storm system moves from weak to strong shear, changes in cell structure are influenced by local mesoscale forcing associated with the cold pool. In this particular experiment, cells near the leading edge of the cold pool, where gust front convergence occurs along a continuous line, evolve into a bow-echo structure as the shear increases. In contrast, simulated cells that remain relatively isolated on the flank of the cold pool tend to develop supercellular characteristics.

Abstract

The 12 May 2010 supercell thunderstorm intercepted by the Second Verification of the Origins of Rotation in Tornadoes Experiment (VORTEX2) is analyzed during a time period of strong low-level rotation in which dual-Doppler radar observations were collected. Two different cyclonic vortices are documented. The first vortex was “marginally tornadic” before abruptly weakening, following the development of a descending reflectivity core (DRC) similar to those that have been documented in past studies of supercells. The second vortex rapidly developed immediately north of the DRC shortly after the DRC reached low altitudes, and was associated with a tornado that produced damage near Clinton, Oklahoma. The paper explores the possible roles of the first vortex in triggering the DRC, the DRC in the subsequent initiation of a new updraft pulse on its flank, and the updraft pulse on the development of the second, stronger vortex. The Clinton storm case is, unfortunately, a nice example of the challenges in predicting tornadogenesis within supercell storms even in environments understood to be favorable for tornadoes.

Abstract

This paper uses idealized numerical simulations to investigate the dynamical influences of stable boundary layers on the morphology of supercell thunderstorms, especially the development of low-level rotation. Simulations are initialized in a horizontally homogeneous environment with a surface-based stable layer similar to that found within a nocturnal boundary layer or a mesoscale cold pool. The depth and lapse rate of the imposed stable boundary layer, which together control the convective inhibition (CIN), are varied in a suite of experiments.

When compared with a control simulation having little surface-based CIN, each supercell simulated in an environment having a stable boundary layer develops weaker rotation, updrafts, and downdrafts at low levels; in general, low-level vertical vorticity and vertical velocity magnitude decrease as initial CIN increases (changes in CIN are due only to variations in the imposed stable boundary layer). Though the presence of a stable boundary layer decreases low-level updraft strength, all supercells except those initiated over the most stable boundary layers had at least some updraft parcels with near-surface origins. Furthermore, the existence of a stable boundary layer only prohibits downdraft parcels from reaching the lowest grid level in the most stable cases. Trajectory and circulation analyses indicate that weaker near-surface rotation in the stable-layer scenarios is a result of the decreased generation of circulation coupled with decreased convergence of the near-surface circulation by weaker low-level updrafts. These results may also suggest a reason why tornadogenesis is less likely to occur in so-called elevated supercell thunderstorms than in surface-based supercells.

Abstract

During the International H₂O Project, mobile radars collected high-resolution data of several 0.5–2-km-wide vertically oriented vortices (or misocyclones) along at least five mesoscale airmass boundaries. This study analyzes the properties of the misocyclones in three of these datasets—3, 10, and 19 June 2002—to verify findings from finescale numerical models and other past observations of misocyclones and to further the understanding of the role that they play in the initiation of deep moist convection and nonsupercell tornadoes. Misocyclones inflect or disjoint the swath of low-level convergence along each boundary to varying degrees depending on the size of their circulations. When several relatively large misocyclones are next to each other, the shape of low-level convergence along each boundary is arranged into a staircase pattern. Mergers of misocyclones are an important process in the evolution of the vorticity field, as a population of small vortices consolidates into a smaller number of larger ones. Additionally, merging misocyclones may affect the mixing of thermodynamic fields in their vicinity when the merger axis is perpendicular to the boundary. Misocyclones interact with linear and cellular structures in the planetary boundary layers (PBLs) of the air masses adjacent to each boundary. Cyclonic low-level vertical vorticity generated by both types of structures makes contact with each boundary and sometimes is incorporated into preexisting misocyclones. Intersections of either type of PBL structure with the boundary result in strengthened pockets of low-level convergence and, typically, strengthened misocyclones.

Abstract

Recent high-resolution numerical simulations of supercells have identified a feature referred to as the streamwise vorticity current (SVC). Some have presumed the SVC to play a role in tornadogenesis and maintenance, though observations of such a feature have been limited. To this end, 125-m dual-Doppler wind syntheses and mobile mesonet observations are used to examine three observed supercells for evidence of an SVC. Two of the three supercells are found to contain a feature similar to an SVC, while the other supercell contains an antistreamwise vorticity ribbon on the southern fringe of the forward flank. A closer examination of the two supercells with SVCs reveals that the SVCs are located on the cool side of boundaries within the forward flank that separate colder, more turbulent flow from warmer, more laminar flow, similar to numerical simulations. Furthermore, the observed SVCs are similar to those in simulations in that they appear to be associated with baroclinic vorticity generation and have similar appearances in vertical cross sections. Aside from some apparent differences in the location of the maximum streamwise vorticity between simulated and observed SVCs, the SVCs seen in numerical simulations are indeed similar to reality. The SVC, however, may not be essential for tornadogenesis, at least for weak tornadoes, because the supercell that did not have a well-defined SVC produced at least one brief, weak tornado during the analysis period.

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.

Abstract

The severe storms research community lacks reliable, aboveground, thermodynamic observations (e.g., temperature, humidity, and pressure) in convective storms. These missing observations are crucial to understanding the behavior of both supercell storms (e.g., the generation, reorientation, and amplification of vorticity necessary for tornado formation) and larger-scale (mesoscale) convective systems (e.g., storm maintenance and the generation of damaging straight-line winds). This paper describes a novel way to use balloonborne probes to obtain aboveground thermodynamic observations. Each probe is carried by a pair of balloons until one of the balloons is jettisoned; the remaining balloon and probe act as a pseudo-Lagrangian drifter that is drawn through the storm. Preliminary data are presented from a pair of deployments in supercell storms in Oklahoma and Kansas during May 2017. The versatility of the observing system extends beyond severe storms applications into any area of mesoscale meteorology in which a large array of aboveground, in situ thermodynamic observations are needed.

Abstract

A dryline and misocyclones have been simulated in a cloud-resolving model by applying specified initial and time-dependent lateral boundary conditions obtained from analyses of the 22 May 2002 International H₂O Project (IHOP_2002) dataset. The initial and lateral boundary conditions were obtained from a combination of the time–spaced Lagrangian analyses for temperature and moisture with horizontal velocities from multiple-Doppler wind syntheses. The simulated dryline, horizontal dry-convective rolls (HCRs) and open cells (OCCs), misocyclones, and cumulus clouds are similar to the corresponding observed features. The misocyclones move northward at nearly the mean boundary layer (BL) wind speed, rotate dryline gradients owing to their circulations, and move the local dryline eastward via their passage. Cumuli develop along a secondary dryline, along HCR and OCC segments between the primary and secondary drylines, along HCR and OCC segments that have moved over the dryline, and within the dryline updraft. After the initial shearing instability develops, misocyclogenesis proceeds from tilting and stretching of vorticity by the persistent secondary dryline circulation. The resulting misocyclone evolution is discussed.