Search Results
You are looking at 11 - 20 of 50 items for
- Author or Editor: Morris Weisman x
- Refine by Access: All Content x
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
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.
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.
Abstract
Convective storm simulations are conducted using varying thermal and wind profile shapes, subject to the constraints of strict conservation of convective available potential energy (CAPE) and hodograph trace. Small and large CAPE regimes and straight and curved hodographs are studied, each with a matrix of systematically varying thermal and wind profile shapes having identical levels of free convection and bulk Richardson numbers favorable to supercell development. Differences in storm intensity and morphology resulting from changes in the profile shapes can be profound, especially in the small CAPE regime, where, for the moderate shears studied here, storms are generally weak except when the buoyancy is concentrated at low levels. In stronger CAPE regimes, less dramatic relative enhancements of storm updraft intensity are found when both the buoyancy and shear are concentrated at low levels.
Peak midlevel vertical vorticity correlates roughly with peak updraft speed in the small CAPE regime, but it shows less sensitivity to buoyancy and shear stratification at larger CAPE. Although peak low-level vertical vorticity can be large in either CAPE regime, it is generally larger in the large CAPE regime, where evaporation of rain leads to the formation of stronger surface cold pools, zones of enhanced horizontal shear, and baroclinic production of horizontal vorticity that can be tilted onto the vertical by storm updrafts. The present parameter space study strongly suggests that, while bulk CAPE and shear are important determinants of gross storm morphology and intensity, significant modulation is possible within a given bulk CAPE and shear class by changing only the shapes of the profiles of buoyancy and shear, either alone or in combination.
Abstract
Convective storm simulations are conducted using varying thermal and wind profile shapes, subject to the constraints of strict conservation of convective available potential energy (CAPE) and hodograph trace. Small and large CAPE regimes and straight and curved hodographs are studied, each with a matrix of systematically varying thermal and wind profile shapes having identical levels of free convection and bulk Richardson numbers favorable to supercell development. Differences in storm intensity and morphology resulting from changes in the profile shapes can be profound, especially in the small CAPE regime, where, for the moderate shears studied here, storms are generally weak except when the buoyancy is concentrated at low levels. In stronger CAPE regimes, less dramatic relative enhancements of storm updraft intensity are found when both the buoyancy and shear are concentrated at low levels.
Peak midlevel vertical vorticity correlates roughly with peak updraft speed in the small CAPE regime, but it shows less sensitivity to buoyancy and shear stratification at larger CAPE. Although peak low-level vertical vorticity can be large in either CAPE regime, it is generally larger in the large CAPE regime, where evaporation of rain leads to the formation of stronger surface cold pools, zones of enhanced horizontal shear, and baroclinic production of horizontal vorticity that can be tilted onto the vertical by storm updrafts. The present parameter space study strongly suggests that, while bulk CAPE and shear are important determinants of gross storm morphology and intensity, significant modulation is possible within a given bulk CAPE and shear class by changing only the shapes of the profiles of buoyancy and shear, either alone or in combination.
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
Herein, an analysis of a 3-km explicit convective simulation of an unusually intense bow echo and associated mesoscale vortex that were responsible for producing an extensive swath of high winds across Kansas, southern Missouri, and southern Illinois on 8 May 2009 is presented. The simulation was able to reproduce many of the key attributes of the observed system, including an intense [~100 kt (51.4 m s−1) at 850 hPa], 10-km-deep, 100-km-wide warm-core mesovortex and associated surface mesolow associated with a tropical storm–like reflectivity eye. A detailed analysis suggests that the simulated convection develops north of a weak east–west lower-tropospheric baroclinic zone, at the nose of an intensifying low-level jet. The system organizes into a north–south-oriented bow echo as it moves eastward along the preexisting baroclinic zone in an environment of large convective available potential energy (CAPE) and strong tropospheric vertical wind shear. Once the system moves east of the low-level jet and into an environment of weaker CAPE and weaker vertical wind shear, it begins an occlusion-like phase, producing a pronounced comma-shaped reflectivity echo with an intense warm-core mesovortex at the head of the comma. During this phase, a deep strip of cyclonic vertical vorticity located on the backside of the bow echo consolidates into a single vortex core. A notable weakening of the low-level convectively generated cold pool also occurs during this phase, perhaps drawing parallels to theories of tropical cyclogenesis wherein cold convective downdrafts must be substantially mitigated for subsequent system intensification.
Abstract
Herein, an analysis of a 3-km explicit convective simulation of an unusually intense bow echo and associated mesoscale vortex that were responsible for producing an extensive swath of high winds across Kansas, southern Missouri, and southern Illinois on 8 May 2009 is presented. The simulation was able to reproduce many of the key attributes of the observed system, including an intense [~100 kt (51.4 m s−1) at 850 hPa], 10-km-deep, 100-km-wide warm-core mesovortex and associated surface mesolow associated with a tropical storm–like reflectivity eye. A detailed analysis suggests that the simulated convection develops north of a weak east–west lower-tropospheric baroclinic zone, at the nose of an intensifying low-level jet. The system organizes into a north–south-oriented bow echo as it moves eastward along the preexisting baroclinic zone in an environment of large convective available potential energy (CAPE) and strong tropospheric vertical wind shear. Once the system moves east of the low-level jet and into an environment of weaker CAPE and weaker vertical wind shear, it begins an occlusion-like phase, producing a pronounced comma-shaped reflectivity echo with an intense warm-core mesovortex at the head of the comma. During this phase, a deep strip of cyclonic vertical vorticity located on the backside of the bow echo consolidates into a single vortex core. A notable weakening of the low-level convectively generated cold pool also occurs during this phase, perhaps drawing parallels to theories of tropical cyclogenesis wherein cold convective downdrafts must be substantially mitigated for subsequent system intensification.
Abstract
The representation of convective processes within mesoscale models with horizontal grid sizes smaller than 20 km has become a major concern for the simulation of mesoscale weather systems. In this paper, the authors investigate the effects of grid resolution on convective processes using a nonhydrostatic cloud model to help clarify the capabilities and limitations of using explicit physics to resolve convection in mesoscale models. By varying the horizontal grid interval between 1 and 12 km, the degradation in model response as the resolution is decreased is documented and the processes that are not properly represented with the coarser resolutions are identified.
Results from quasi-three-dimensional squall-line simulations for midlatitude-type environments suggest that resolutions of 4 km are sufficient to reproduce much of the mesoscale structure and evolution of the squall-line-type convective systems produced in 1-km simulations. The evolution at coarser resolutions is characteristically slower, with the resultant mature mesoscale circulation becoming stronger than those produced in the 1-km case. It is found that the slower evolution in the coarse-resolution simulations is largely a result of the delayed strengthening of the convective cold pool, which is crucial to the evolution of a mature, upshear-tilted convective system. The relative success in producing realistic circulation patterns at later times for these cases occurs because the cold pool does eventually force the system to grow upscale, allowing it to be better resolved. The stronger circulation results from an overprediction of the vertical mass transport produced by the convection at the leading edge of the system, due to the inability of the coarse-resolution simulations to properly represent nonhydrostatic effects.
Abstract
The representation of convective processes within mesoscale models with horizontal grid sizes smaller than 20 km has become a major concern for the simulation of mesoscale weather systems. In this paper, the authors investigate the effects of grid resolution on convective processes using a nonhydrostatic cloud model to help clarify the capabilities and limitations of using explicit physics to resolve convection in mesoscale models. By varying the horizontal grid interval between 1 and 12 km, the degradation in model response as the resolution is decreased is documented and the processes that are not properly represented with the coarser resolutions are identified.
Results from quasi-three-dimensional squall-line simulations for midlatitude-type environments suggest that resolutions of 4 km are sufficient to reproduce much of the mesoscale structure and evolution of the squall-line-type convective systems produced in 1-km simulations. The evolution at coarser resolutions is characteristically slower, with the resultant mature mesoscale circulation becoming stronger than those produced in the 1-km case. It is found that the slower evolution in the coarse-resolution simulations is largely a result of the delayed strengthening of the convective cold pool, which is crucial to the evolution of a mature, upshear-tilted convective system. The relative success in producing realistic circulation patterns at later times for these cases occurs because the cold pool does eventually force the system to grow upscale, allowing it to be better resolved. The stronger circulation results from an overprediction of the vertical mass transport produced by the convection at the leading edge of the system, due to the inability of the coarse-resolution simulations to properly represent nonhydrostatic effects.
Abstract
A three-dimensional nonhydrostatic cloud model is used to study the evolution of supercell thunderstorms, with emphasis on the low-level mesocyclone, interacting with preexisting boundaries. The impacts of low-level environmental shear, storm motion relative to boundary orientation, and boundary strength are assessed. In the low-level shear experiments, significant low-level rotation is consistently observed earlier, tends to be stronger, and is longer lived in storms interacting with a boundary than in storms initiated in a homogeneous environment. Low-level rotation is weaker in storms crossing the boundary and moving into the colder air. In contrast, all storms moving along or into the warm air ahead of the boundary develop significant low-level rotation. Increasing the temperature gradient and shear across the boundary has little impact on the low-level mesocyclone evolution. Storms interacting with a boundary characterized by only horizontal shear produce weaker mesocyclones than those created when a temperature gradient also exists across the boundary.
It will be shown that the mechanisms generating the low-level mesocyclone appear to be different for storms interacting with boundaries than those initiated in a homogeneous environment. Consistent with previous studies, storms initiated in a homogeneous environment derive their low-level rotation from tilting of streamwise horizontal vorticity generated along the storm’s forward flank region. In contrast, for storms interacting with a boundary, a significant fraction of the air composing the low-level mesocyclone originates at low levels from the cool air side of the boundary. These parcels contain significant streamwise vorticity, which is tilted and stretched by the storms updraft. Vertical vorticity along the preexisting boundary may also have contributed to mesocyclogenesis. The forward-flank region appears to play a minor role in generating low-level rotation when a preexisting boundary is present.
Abstract
A three-dimensional nonhydrostatic cloud model is used to study the evolution of supercell thunderstorms, with emphasis on the low-level mesocyclone, interacting with preexisting boundaries. The impacts of low-level environmental shear, storm motion relative to boundary orientation, and boundary strength are assessed. In the low-level shear experiments, significant low-level rotation is consistently observed earlier, tends to be stronger, and is longer lived in storms interacting with a boundary than in storms initiated in a homogeneous environment. Low-level rotation is weaker in storms crossing the boundary and moving into the colder air. In contrast, all storms moving along or into the warm air ahead of the boundary develop significant low-level rotation. Increasing the temperature gradient and shear across the boundary has little impact on the low-level mesocyclone evolution. Storms interacting with a boundary characterized by only horizontal shear produce weaker mesocyclones than those created when a temperature gradient also exists across the boundary.
It will be shown that the mechanisms generating the low-level mesocyclone appear to be different for storms interacting with boundaries than those initiated in a homogeneous environment. Consistent with previous studies, storms initiated in a homogeneous environment derive their low-level rotation from tilting of streamwise horizontal vorticity generated along the storm’s forward flank region. In contrast, for storms interacting with a boundary, a significant fraction of the air composing the low-level mesocyclone originates at low levels from the cool air side of the boundary. These parcels contain significant streamwise vorticity, which is tilted and stretched by the storms updraft. Vertical vorticity along the preexisting boundary may also have contributed to mesocyclogenesis. The forward-flank region appears to play a minor role in generating low-level rotation when a preexisting boundary is present.
