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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
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 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
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
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
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
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
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.
Abstract
This study develops conceptual models of how a land–water interface affects the strength and structure of squall lines. Two-dimensional numerical simulations using the Advanced Regional Prediction System model are employed. Five sets of simulations are performed, each testing eight wind shear profiles of varying strength and depth. The first set of simulations contains a squall line but no surface or radiation physics. The second and third sets do not contain a squall line but include surface and radiation physics with a land surface on the right and a water surface on the left of the domain. The land is either warmer or cooler than the sea surface. These three simulations provide a control for later simulations. Finally, the remaining two simulation sets examine squall-line interaction with a relatively cool or warm land surface. The simulations document the thermodynamic and shear characteristics of squall lines interacting with the coastline. Results show that the inclusion of a land surface did not sufficiently affect the thermodynamic properties ahead of the squall line to change its overall structure. Investigation of ambient shear ahead of the squall line revealed that the addition of either warm or cool land reduced the strength of the net circulation in the inflow layer as measured by ambient shear. The amount of reduction in shear was found to be directly proportional to the depth and strength of the original shear layer. For stronger and deeper shears, the reduction in shear is sufficiently great that the buoyancy gradient circulation at the leading edge of the cold pool is no longer in balance with the shear circulation leading to changes in squall-line updraft structure. The authors hypothesize two ways by which a squall line might respond to passing from water to land. The weaker and more shallow the ambient shear, the greater likelihood that the squall-line structure remains unaffected by this transition. Conversely, the stronger and deeper the shear, the greater likelihood that the squall line changes updraft structure from upright/downshear to upshear tilted.
Abstract
This study develops conceptual models of how a land–water interface affects the strength and structure of squall lines. Two-dimensional numerical simulations using the Advanced Regional Prediction System model are employed. Five sets of simulations are performed, each testing eight wind shear profiles of varying strength and depth. The first set of simulations contains a squall line but no surface or radiation physics. The second and third sets do not contain a squall line but include surface and radiation physics with a land surface on the right and a water surface on the left of the domain. The land is either warmer or cooler than the sea surface. These three simulations provide a control for later simulations. Finally, the remaining two simulation sets examine squall-line interaction with a relatively cool or warm land surface. The simulations document the thermodynamic and shear characteristics of squall lines interacting with the coastline. Results show that the inclusion of a land surface did not sufficiently affect the thermodynamic properties ahead of the squall line to change its overall structure. Investigation of ambient shear ahead of the squall line revealed that the addition of either warm or cool land reduced the strength of the net circulation in the inflow layer as measured by ambient shear. The amount of reduction in shear was found to be directly proportional to the depth and strength of the original shear layer. For stronger and deeper shears, the reduction in shear is sufficiently great that the buoyancy gradient circulation at the leading edge of the cold pool is no longer in balance with the shear circulation leading to changes in squall-line updraft structure. The authors hypothesize two ways by which a squall line might respond to passing from water to land. The weaker and more shallow the ambient shear, the greater likelihood that the squall-line structure remains unaffected by this transition. Conversely, the stronger and deeper the shear, the greater likelihood that the squall line changes updraft structure from upright/downshear to upshear tilted.
