Search Results

You are looking at 1 - 10 of 49 items for

  • Author or Editor: Morris Weisman x
  • Refine by Access: All Content x
Clear All Modify Search
Morris L. Weisman

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.

Full access
Morris L. Weisman

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.

Full access
Morris L. Weisman

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.

Full access
Morris L. Weisman and Richard Rotunno
Full access
Morris L. Weisman and Richard Rotunno

Abstract

A series of idealized simulations of supercell storms are presented for environments representing straight through circular hodographs to clarify the character of the storm dynamics over the large spectrum of hodograph shapes commonly observed. The primary emphasis is on comparing and contrasting recent theories of supercell dynamics, based on updraft–shear interactions, storm-relative environmental helicity (SREH), and Beltrami-flow solutions, to help clarify the degree to which each theory can represent the essential storm dynamics. One of the particular questions being addressed is whether storm dynamics are significantly different for straight versus curved hodographs, which has become a point of some controversy over recent years.

In agreement with previous studies, the authors find that the physical processes that promote storm maintenance, rotation, and propagation are similar for all hodograph shapes employed, and are due primarily to nonlinear interactions between the updraft and the ambient shear, associated with the localized development of rotation on the storm’s flank. Significant correlations between the updraft and vertical vorticity are also observed across the shear spectrum, and, in agreement with predictions of linear theories associated with SREH, this correlation increases for increasing hodograph curvature. However, storm steadiness and propagation must already be known or inferred for such concepts to be applied, thus limiting the applicability of this theory as a true predictor of storm properties. Tests of the applicability of Beltrami solutions also confirm reasonable agreement for purely circular hodographs, for which the analytical solutions are specifically designed. However, analysis of the model results indicates that the terms ignored for such solutions, representing the nonlinear effects associated with storm rotation, are more significant than those retained over most of the hodograph spectrum, which severely limits the general applicability of such analyses.

Full access
Morris L. Weisman and Richard Rotunno

Abstract

Based on the analysis of idealized two- and three-dimensional cloud model simulations, Rotunno et al. (hereafter RKW) and Weisman et al. (hereafter WKR) put forth a theory that squall-line strength and longevity was most sensitive to the strength of the component of low-level (0–3 km AGL) ambient vertical wind shear perpendicular to squall-line orientation. An “optimal” state was proposed by , based on the relative strength of the circulation associated with the storm-generated cold pool and the circulation associated with the ambient shear, whereby the deepest leading edge lifting and most effective convective retriggering occurred when these circulations were in near balance. Since this work, subsequent studies have brought into question the basic validity of the proposed optimal state, based on concerns as to the appropriate distribution of shear relative to the cold pool for optimal lifting, as well as the relevance of such concepts to fully complex squall lines, especially considering the potential role of deeper-layer shears in promoting system strength and longevity. In the following, the basic interpretations of the theory are reconfirmed and clarified through both the analysis of a simplified two-dimensional vorticity–streamfunction model that allows for a more direct interpretation of the role of the shear in controlling the circulation around the cold pool, and through an analysis of an extensive set of 3D squall-line simulations, run at higher resolution and covering a larger range of environmental shear conditions than presented by .

Full access
Morris L. Weisman and Christopher A. Davis

Abstract

Previous idealized simulations of convective systems have demonstrated that the development of mesoscale vortices within quasi-linear convective systems may be a natural consequence of the finite extent of the convective line, as horizontal vorticity is tilted into the vertical at the line ends. However, the source of this horizontal vorticity has not yet been clearly established, either being associated with the ambient shear or else generated within the system. In this paper, results are presented from a series of idealized simulations that demonstrate that the source, strength, and scale of these vortices depends on the strength of the ambient vertical wind shear, the strength of the system-generated cold pool, the scale of the convective line segments, as well as the phase within the life cycle of the convective system. In particular, for systems that develop in an environment with weak-to-moderate shear, a line-end vortex pair is generated primarily via the tilting of horizontal vorticity generated within the system-scale cold pool, as the associated vortex lines are lifted within the laterally finite front-to-rear ascending current. Similar mechanisms also operate in environments with stronger or deeper shear, but subsystem-scale vortices can also originate via the tilting of the ambient horizontal vorticity within supercell updraft–downdraft couplets. In all cases, convergence at midlevels enhances Coriolis rotation over the longer term, leading to the preferred development of a cyclonic vortex, as is frequently observed in asymmetric convective systems.

Full access
Christopher A. Davis and Morris L. Weisman

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.

Full access
David B. Parsons and Morris L. Weisman

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.

Full access
Eugene W. McCaul Jr. and Morris L. Weisman

Abstract

Numerical simulations of the convective storms that form in tornado-producing landfalling hurricanes show that shallow supercells are possible, even though buoyancy is limited because ambient lapse rates are close to moist adiabatic. Updrafts generally reach peak intensity at low levels, often around 2 km above the surface. By comparison, a simulated midlatitude supercell typical of the Great Plains of the United States exhibits a pronounced increase in storm size, both horizontally and vertically. At low levels, however, the hurricane-spawned storms may contain updrafts that rival or exceed in intensity those of Great Plains supercells at similar levels. Simulations made using a tornado-proximity sounding from the remnants of Hurricane Danny in 1985 produce a small but intense supercell, a finding consistent with the available observational evidence.

Although the amplitude of parcel buoyancy is often small in hurricane environments, its concentration in the strongly sheared lower troposphere promotes the development of perturbation pressure minima comparable to those seen in simulated Great Plains supercells. In a typical simulated hurricane-spawned supercell, the upward dynamic pressure gradient force contributes at least three times as much to the maximum updraft speed as does explicit buoyancy. Tilting and stretching of ambient horizontal vorticity by the strong low-level updrafts promotes production of substantial vertical vorticity aloft in the hurricane-spawned storms. However, the weakness of their surface cold pools tends to restrict surface vorticity development, a fact that may help explain why most hurricane-spawned tornadoes are weaker than their Great Plains counterparts.

Full access