Abstract

A series of idealized simulations using a nonhydrostatic cloud model is used to investigate the genesis of bow echoes (a bow-shaped system of convective cells that is especially noted for producing long swaths of damaging surface winds). It is hypothesized that severe, long-lived bow echoes represent a dynamically unique form of mesoconvective organization being produced for a restricted range of environmental conditions, including a convective available potential energy (CAPE) of at least 2000 m² s² and vertical wind shears of at least 20 m s⁻¹ over the lowest 2.5–5 km AGL. The key structural features include a 40–100-km-long bow-shaped segment of convective cells, with a strong rear-inflow jet extending to the leading edge of the bow at 2–3 km AGL, and cyclonic and anticyclonic eddies (referred to as “bookend” vortices) on the northern and southern flanks of the bowed segment, respectively. This structure characteristically develops three to four hours into the lifetime of a convective system and may remain coherent for several hours.

The evolution of this coherent structure occurs systematically as the convectively produced cold pool strengthens over time, eventually producing a circulation that overwhelms the ambient shear. This forces the convective cells to advect rearward above the cold air and weaken. The horizontal buoyancy gradients along the back edge of these rearward-advecting cells subsequently generate an elevated rear-inflow jet that extends to near the leading edge of the cold pool. The circulation of this jet helps negate the circulation of the cold pool, reestablishing deep, forced lifting at the leading edge of the system. This elevated rear-inflow jet is also enhanced through the development of bookend vortices. Such vortices are produced at the ends of a convective line segment as vortex lines inherent in the ambient vertically sheared environment are first tilted upward by the convective updrafts and then tilted downward and stretched by the convective downdrafts. The development of these features requires both large amounts of CAPE and strong vertical wind shear in the environment of these systems, as is consistent with the observed environments of many severe, long-lived bow echoes.

Abstract

In this study, the structure of convectively generated rear-inflow jets and their role in the evolution of long-lived mesoconvective systems are investigated through an analysis of idealized three-dimensional simulations using a nonhydrostatic cloud model. Rear-inflow jets are generated within these systems in response to the upshear-tilting of the convective circulation, as the horizontal buoyancy gradients along the back edge of the expanding system create a circulation that draws midlevel air in from the rear. Within this framework, a wide range of rear-inflow strengths and structures are produced, depending on the magnitude of the ambient convective available potential energy (CAPE) and the vertical wind shear. In general, for environments characterized by weak-to-moderate vertical wind shear and weak-to moderate CAPE, the rear-inflow jet descends and spreads along the surface well behind the leading edge of the gust front, and the subsequent convective activity becomes weaker. However, for environments characterized by strong environmental vertical wind shear and strong CAPE, the rear inflow remains elevated to near the leading edge of the system, and strong, upright convective cells are maintained along the gust front. The influence of the rear-inflow jets on the evolution of these systems is examined through an extension of the recent theory of Rotunno et al., whereby the characteristics of the lifting produced at the leading edge of the system are controlled by the relative balance between the horizontal vorticity generated by the cold pool and the horizontal vorticity that is inherent in both the ambient vertical wind shear and the rear-inflow jet.

Bow echoes represent one of the unique and more well-known forms of severe convective organization, often being responsible for the production of long swaths of damaging surface winds and small tornadoes. They are identified by their characteristic bow shape as seen on radar reflectivity displays. Much of what is known about bow echoes originated with T. T. Fujita, whose observational insights and careful analyses two decades ago still guide research and forecasting of bow-echo phenomena today. This paper reviews Fujita's contributions to our understanding of bow echoes, and also summarizes more recent observational and numerical studies that have built on the foundation that he provided. Perhaps not surprisingly, the life cycle of bow echoes as first described by Fujita, consisting of an evolution from a symmetric line of convective cells to a comma-shaped echo with a dominant cyclonic vortex, is now recognized as one of the fundamental modes of mesoconvective evolution, for both severe and nonsevere convective systems alike.

Abstract

Using a three-dimensional numerical cloud model, we investigate the effects of directionally varying wind shear on convective storm structure and evolution over a wide range of shear magnitudes. As with a previous series of experiments using unidirectional wind shear profiles (Weisman and Klemp), the current results evince a spectrum of storm types ranging from short lived single cells at low shears, multicells at intermediate shears, to supercells at high shears. With a clockwise curved hodograph, the supercellular growth is confined to the right flank of the storm system while multicellular growth is favored on the left flank. An analysis of the dynamic structure of the various cells reveals that the quasi-steady supercell updrafts are strongly enhanced by dynamically induced pressure gradients on the right flank of the storm system. We use this feature along with other related storm characteristics (such as updraft rotation) to propose a dynamically based storm classification scheme. Following Browning, this scheme includes two basic storm types: ordinary cells and supercells. Multicell storm systems and squall lines would then be made up of a combination of supercells and ordinary cells. As in the unidirectional shear experiments, a convective bulk Richardson numbercharacterizes the environment conducive to producing particular storm types.

Abstract

This two-part study proposes fundamental explanations of the genesis, structure, and implications of low-level meso-γ-scale vortices within quasi-linear convective systems (QLCSs) such as squall lines and bow echoes. Such “mesovortices” are observed frequently, at times in association with tornadoes.

Idealized simulations are used herein to study the structure and evolution of meso-γ-scale surface vortices within QLCSs and their dependence on the environmental vertical wind shear. Within such simulations, significant cyclonic surface vortices are readily produced when the unidirectional shear magnitude is 20 m s⁻¹ or greater over a 0–2.5- or 0–5-km-AGL layer. As similarly found in observations of QLCSs, these surface vortices form primarily north of the apex of the individual embedded bowing segments as well as north of the apex of the larger-scale bow-shaped system. They generally develop first near the surface but can build upward to 6–8 km AGL. Vortex longevity can be several hours, far longer than individual convective cells within the QLCS; during this time, vortex merger and upscale growth is common. It is also noted that such mesoscale vortices may be responsible for the production of extensive areas of extreme “straight line” wind damage, as has also been observed with some QLCSs. Surface vortices are also produced for weaker shears but remain shallow, weak, and short-lived.

Although similar in size and strength to mesocyclones associated with supercell storms, and also sometimes producing similar hooklike structures in the rain field, it is also shown that the present vortices are quite distinct, structurally and dynamically. Most critically, such vortices are not associated with long-lived, rotating updrafts at midlevels and the associated strong, dynamically forced vertical accelerations, as occur within supercell mesocyclones.

Abstract

This two-part study proposes a fundamental explanation of the genesis, structure, and implications of low-level, meso-γ-scale vortices within quasi-linear convective systems (QLCSs) such as squall lines and bow echoes. Such “mesovortices” are observed frequently, at times in association with tornadoes.

Idealized experiments with a numerical cloud model show that significant low-level mesovortices develop in simulated QLCSs, especially when the environmental vertical wind shear is above a minimum threshold and when the Coriolis forcing is nonzero. As illustrated by a QLCS simulated in an environment of moderate vertical wind shear, mesovortexgenesis is initiated at low levels by the tilting, in downdrafts, of initially crosswise horizontal baroclinic vorticity. Over a 30-min period, the resultant vortex couplet gives way to a dominant cyclonic vortex as the relative and, more notably, planetary vorticity is stretched vertically; hence, the Coriolis force plays a direct role in the low-level mesovortexgenesis. A downward-directed vertical pressure-gradient force is subsequently induced within the mesovortices, effectively segmenting the previously (nearly) continuous convective line.

In moderate-to-strong environmental shear, the simulated QLCSs evolve into bow echoes with “straight line” surface winds found at the bow-echo apex and additionally in association with, and in fact induced by, the low-level mesovortices. Indeed, the mesovortex winds tend to be stronger, more damaging, and expand in area with time owing to a mesovortex amalgamation or “upscale” vortex growth. In weaker environmental shear—in which significant low-level mesovortices tend not to form—damaging surface winds are driven by a rear-inflow jet that descends and spreads laterally at the ground, well behind the gust front.

Abstract

Supercells in the southern plains are often localized, forming as cells along a convective line, even though the environment may support supercell formation over a much broader, mesoscale region. A set of numerical experiments is devised in which it is demonstrated that the evolution of supercells in a homogeneous environment depends upon the orientation of the vertical-shear profile with respect to the orientation of the line along which convection is initiated (the “line of forcing”). This work is motivated by the observations that the nature and consequences of the interaction of neighboring cells depend upon differential cell motion, which in turn is a function of the characteristics and orientation of the vertical-shear profile and its impact on the behavior of outflow boundaries.

Results for various orientations of the vertical-shear vector with respect to the line along which cells are initiated are described and interpreted physically. It is found that in idealized numerical simulations, shear oblique to (45° from) the line of forcing is most apt to support neighboring cyclonic supercells within the line, but also supports an anticyclonic supercell at the downshear end of the line; shear normal to the line of forcing is favorable for the maintenance of a squall line with isolated supercells at either end; shear parallel to the line of forcing is favorable for isolated supercells only on the downshear side of the line. The effects of low-level clockwise curvature in the hodograph vary from case to case, depending upon the orientation of the leading edges of the system cold pool with respect to the low-level shear. Differences in low-level static stability and the dryness of air aloft affect storm behavior less than differences in the orientation of the vertical shear. The process of storm collision is examined in detail.

Abstract

A positive-definite transport scheme for moisture is tested in a nonhydrostatic forecast model using convection-permitting resolutions. Use of the positive-definite scheme is found to significantly reduce the large positive bias in surface precipitation forecasts found in the non-positive-definite model forecasts, in particular at high precipitation thresholds. The positive-definite scheme eliminates spurious sources of water arising from the clipping of negative moisture values in the non-positive-definite model formulation, leading to the bias reduction.

Abstract

Long-lived, mesoscale convective systems are known to occasionally produce mesoscale convective vortices (MCVs) in the lower to middle troposphere with horizontal scales averaging 100–200 km. The formation of MCVs is investigated using fully three-dimensional cloud model simulations of idealized, mesoscale convective systems (MCSs), initialized with a finite length line of unstable perturbations. In agreement with observations, the authors find that environmental conditions favoring MCV formation exhibit weak vertical shear confined to roughly the lowest 3 km, provided the Coriolis parameter (f) is chosen appropriate for midlatitudes. With f = 0, counterrotating vortices form on the line ends, positive to the north and negative to the south with westerly environmental shear.

The MCV and end vortices are synonymous with anomalies of potential vorticity (PV). Using PV inversion techniques, the authors show that the vortices are nearly balanced, even with f = 0. However, the formation of mesoscale vortices depends upon the unbalanced, sloping, front-to-rear and rear inflow circulations of the mature squall line. End vortices form partly from the tilting of ambient shear but more from the tilting of the perturbation horizontal vorticity inherent in the squall line circulation. With the addition of earth's rotation, an asymmetric structure results with the cyclonic vortex dominant on the northern end of the line. The key to this MCV formation is organized convergence above the surface cold pool and associated mesoscale ascent and latent heating. A simulated MCV can even form in an environment with no ambient shear.

Using a balanced model, the authors perform extended time integrations and show that the MCV produced in a sheared environment remains largely intact because the shear is confined to low levels and is relatively weak. In addition, the interaction of the vortex with the shear produces sufficient, mesoscale vertical motion on the downshear side of the vortex to trigger convection in typical, observed thermodynamic environments.

Results suggest that balanced dynamical arguments may elucidate the long-term behavior of mesoscale vortices. However, because the balance equations neglect the irrotational velocity contribution to the horizontal vorticity, the formation of the mesoscale updraft that leads to an MCV and the generation of vertical vorticity through vortex tilting are both treated improperly. Thus, the authors believe that existing balanced models will have serious difficulty simulating MCS evolution and mesoscale vortex formation unless mesoscale environmental forcing determines the behavior of the convective system.

Abstract

Previous studies have revealed that convective storms often contain intense small-scale downdrafts, termed “downbursts,” that are a significant hazard to aviation. These downbursts sometimes possess strong rotation about their vertical axis in the lower and middle levels of the storm, but studies of how this rotation is produced and how it impacts downdraft strength are lacking. In this study a three-dimensional cloud model was used to simulate a rotating downburst based on conditions observed on a day that produced rotating downbursts. It was found that rotating downbursts may occur when the direction of the wind shear vector in the middle levels of the troposphere varies with height. In the early stages of the convective system, vertical vorticity is generated from tilting of the ambient vertical shear by the updraft, resulting in a vertical vorticity couplet on the flanks of the updraft. Later, the negative buoyancy associated with precipitation loading causes the updraft to collapse and to be eventually replaced by a downdraft downshear of the midlevel updraft. When the direction of the vertical shear vector varies with height, a correlation may develop between the location of the vertical vorticity previously produced by the updraft at midlevels and the location of the developing downdraft. This mechanism causes downbursts to rotate cyclonically when the vertical shear vector veers with height and to rotate anticyclonically when the vertical shear vector backs with height. The rotation associated with the downburst, however, does not significantly enhance the peak downdraft magnitude. The mechanism for the generation of vorticity in a downburst is different from that found for supercell downdrafts, and, for a given vertical shear vector, downbursts and supercell downdrafts will rotate in the opposite sense.