Search Results
You are looking at 1 - 10 of 15 items for :
- Author or Editor: Morris Weisman x
- Journal of the Atmospheric Sciences x
- Refine by Access: All Content x
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
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.
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.
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
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.
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.
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.
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.
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 RKW, 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 RKW 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 WKR.
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 RKW, 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 RKW 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 WKR.
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.
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.
Abstract
Convection-allowing simulations of two warm seclusion cyclones are used to elucidate the vorticity dynamics that contribute to intensification of these systems. The rapidly intensifying oceanic “bomb” cyclone on 4–5 January 1989 and the super derecho on 8 May 2009 are the subject of this study. While these systems occupy different spatial scales, they both acquire characteristics of a warm seclusion cyclone. The aim of this study is to compare the basic structure and determine the dynamics driving increases in system-scale vertical vorticity during the intensification of these systems. Results from a vorticity budget show that system-scale stretching and the lateral transport of vertical vorticity to the cyclone center contribute to increases of system-scale low-level vertical vorticity during the intensification of the oceanic cyclone. The intercomparison of the oceanic cyclone and the super derecho shows that the relative contributions to increases in system-scale vertical vorticity by stretching and tilting as a function of height differ among the two cases. However, the lateral transport of vertical vorticity to the cyclone center is a key contributor to increases in low-level system-scale vertical vorticity for both cases. We hypothesize that this process may be common among a wide array of intense cyclonic systems across scales ranging from warm seclusion extratropical cyclones to some mesoscale convective systems.
Abstract
Convection-allowing simulations of two warm seclusion cyclones are used to elucidate the vorticity dynamics that contribute to intensification of these systems. The rapidly intensifying oceanic “bomb” cyclone on 4–5 January 1989 and the super derecho on 8 May 2009 are the subject of this study. While these systems occupy different spatial scales, they both acquire characteristics of a warm seclusion cyclone. The aim of this study is to compare the basic structure and determine the dynamics driving increases in system-scale vertical vorticity during the intensification of these systems. Results from a vorticity budget show that system-scale stretching and the lateral transport of vertical vorticity to the cyclone center contribute to increases of system-scale low-level vertical vorticity during the intensification of the oceanic cyclone. The intercomparison of the oceanic cyclone and the super derecho shows that the relative contributions to increases in system-scale vertical vorticity by stretching and tilting as a function of height differ among the two cases. However, the lateral transport of vertical vorticity to the cyclone center is a key contributor to increases in low-level system-scale vertical vorticity for both cases. We hypothesize that this process may be common among a wide array of intense cyclonic systems across scales ranging from warm seclusion extratropical cyclones to some mesoscale convective systems.
Abstract
Simulations of squall lines, using nonhydrostatic convection-resolving models, have been limited to two dimensions or three dimensions with the assumption of along-line periodicity. The authors present 3D nonhydrostatic convection-resolving simulations, produced using an adaptive grid model, where the lines are finite in length and the restriction to along-line periodicity is removed. The base state for the simulations is characterized by weak, shallow shear and high convective available potential energy (CAPE), an environment in which longlived midlatitude mesoscale convective systems (MCSs) are observed. The simulated systems bear strong resemblance to many observed systems, suggesting that large-scale forcing, absent in the horizontally homogeneous environment, is not needed to produce many of the distinguishing features of midlatitude MCSs.
In simulations without Coriolis forcing, the presence of line ends leads to mature symmetric systems characterized by a central region of strong convection, trailing flanks of weaker convection, and a strong, centrally focused rear inflow. Simulations that include Coriolis forcing lead to asymmetric systems with significant system growth and migration to the right (south) of the original system centerline. In both cases the evolution of the leading-line convection is primarily controlled by the surface cold pool expansion, with Coriolis forcing promoting rightward system propagation. In the Coriolis simulation, a midlevel mesoscale convective vortex (MCV) forms in the north, to the rear of the convection, while the outflow region aloft is strongly anticyclonic. The northern location of the MCV is coincident with and influenced by a northward bias in the positive buoyancy anomaly aloft. Midlevel vertical vorticity generation by tilting of horizontal vorticity, both ambient and baroclinically generated, is observed in both the Coriolis and no-Coriolis simulations. On larger scales, the convergence of Coriolis rotation generates significant vorticity and is crucial to the formation of the MCV.
Abstract
Simulations of squall lines, using nonhydrostatic convection-resolving models, have been limited to two dimensions or three dimensions with the assumption of along-line periodicity. The authors present 3D nonhydrostatic convection-resolving simulations, produced using an adaptive grid model, where the lines are finite in length and the restriction to along-line periodicity is removed. The base state for the simulations is characterized by weak, shallow shear and high convective available potential energy (CAPE), an environment in which longlived midlatitude mesoscale convective systems (MCSs) are observed. The simulated systems bear strong resemblance to many observed systems, suggesting that large-scale forcing, absent in the horizontally homogeneous environment, is not needed to produce many of the distinguishing features of midlatitude MCSs.
In simulations without Coriolis forcing, the presence of line ends leads to mature symmetric systems characterized by a central region of strong convection, trailing flanks of weaker convection, and a strong, centrally focused rear inflow. Simulations that include Coriolis forcing lead to asymmetric systems with significant system growth and migration to the right (south) of the original system centerline. In both cases the evolution of the leading-line convection is primarily controlled by the surface cold pool expansion, with Coriolis forcing promoting rightward system propagation. In the Coriolis simulation, a midlevel mesoscale convective vortex (MCV) forms in the north, to the rear of the convection, while the outflow region aloft is strongly anticyclonic. The northern location of the MCV is coincident with and influenced by a northward bias in the positive buoyancy anomaly aloft. Midlevel vertical vorticity generation by tilting of horizontal vorticity, both ambient and baroclinically generated, is observed in both the Coriolis and no-Coriolis simulations. On larger scales, the convergence of Coriolis rotation generates significant vorticity and is crucial to the formation of the MCV.
