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Abstract
Deep moist convection generates turbulence in the clear air above and around developing clouds, penetrating convective updrafts and mature thunderstorms. This turbulence can be due to shearing instabilities caused by strong flow deformations near the cloud top, and also to breaking gravity waves generated by cloud–environment interactions. Turbulence above and around deep convection is an important safety issue for aviation, and improved understanding of the conditions that lead to out-of-cloud turbulence formation may result in better turbulence avoidance guidelines or forecasting capabilities. In this study, a series of high-resolution two- and three-dimensional model simulations of a severe thunderstorm are conducted to examine the sensitivity of above-cloud turbulence to a variety of background flow conditions—in particular, the above-cloud wind shear and static stability. Shortly after the initial convective overshoot, the above-cloud turbulence and mixing are caused by local instabilities in the vicinity of the cloud interfacial boundary. At later times, when the convection is more mature, gravity wave breaking farther aloft dominates the turbulence generation. This wave breaking is caused by critical-level interactions, where the height of the critical level is controlled by the above-cloud wind shear. The strength of the above-cloud wind shear has a strong influence on the occurrence and intensity of above-cloud turbulence, with intermediate shears generating more extensive regions of turbulence, and strong shear conditions producing the most intense turbulence. Also, more stable above-cloud environments are less prone to turbulence than less stable situations. Among other things, these results highlight deficiencies in current turbulence avoidance guidelines in use by the aviation industry.
Abstract
Deep moist convection generates turbulence in the clear air above and around developing clouds, penetrating convective updrafts and mature thunderstorms. This turbulence can be due to shearing instabilities caused by strong flow deformations near the cloud top, and also to breaking gravity waves generated by cloud–environment interactions. Turbulence above and around deep convection is an important safety issue for aviation, and improved understanding of the conditions that lead to out-of-cloud turbulence formation may result in better turbulence avoidance guidelines or forecasting capabilities. In this study, a series of high-resolution two- and three-dimensional model simulations of a severe thunderstorm are conducted to examine the sensitivity of above-cloud turbulence to a variety of background flow conditions—in particular, the above-cloud wind shear and static stability. Shortly after the initial convective overshoot, the above-cloud turbulence and mixing are caused by local instabilities in the vicinity of the cloud interfacial boundary. At later times, when the convection is more mature, gravity wave breaking farther aloft dominates the turbulence generation. This wave breaking is caused by critical-level interactions, where the height of the critical level is controlled by the above-cloud wind shear. The strength of the above-cloud wind shear has a strong influence on the occurrence and intensity of above-cloud turbulence, with intermediate shears generating more extensive regions of turbulence, and strong shear conditions producing the most intense turbulence. Also, more stable above-cloud environments are less prone to turbulence than less stable situations. Among other things, these results highlight deficiencies in current turbulence avoidance guidelines in use by the aviation industry.
Abstract
This study examines the characteristics of convective momentum transport (CMT) and gravity wave momentum transport (GWMT) in two-dimensional cloud-system-resolving model simulations, including the relationships between the two transports. A linear group velocity criterion is shown to objectively separate CMT and GWMT. The GWMT contribution is mostly consistent with upward-propagating gravity waves and is present in the troposphere and the stratosphere. The CMT contribution forms a large part of the residual (nonupward-propagating contribution) and dominates the fluxes in the troposphere. Additional analysis of the vertical sensible heat flux supports the physical interpretation of the two contributions, further isolating the effects of unstable convection from vertically propagating gravity waves.
The role of transient and nonconservative (friction and diabatic heating) processes in generating momentum flux and their dependence on changes in convective organization was assessed using a pseudomomentum budget analysis. Nonconservative effects were found to dominate the transports; the GWMT contribution involved a diabatic source region in the troposphere and a dissipative sink region in the stratosphere. The CMT contribution was consistent with transport between the boundary layer and free troposphere via tilted convection. Transient buoyancy–vorticity correlations highlighted wave sources in the region of convective outflow and the boundary layer. These sources were akin to the previously described “mechanical oscillator” mechanism. Fluxes associated with this upper-level source were most sensitive to convective organization, highlighting the mechanism by which changes in organization are communicated to GWMT. The results elucidate important interactions between CMT and GWMT, adding further weight to suggestions that the two transports should be linked in parameterizations.
Abstract
This study examines the characteristics of convective momentum transport (CMT) and gravity wave momentum transport (GWMT) in two-dimensional cloud-system-resolving model simulations, including the relationships between the two transports. A linear group velocity criterion is shown to objectively separate CMT and GWMT. The GWMT contribution is mostly consistent with upward-propagating gravity waves and is present in the troposphere and the stratosphere. The CMT contribution forms a large part of the residual (nonupward-propagating contribution) and dominates the fluxes in the troposphere. Additional analysis of the vertical sensible heat flux supports the physical interpretation of the two contributions, further isolating the effects of unstable convection from vertically propagating gravity waves.
The role of transient and nonconservative (friction and diabatic heating) processes in generating momentum flux and their dependence on changes in convective organization was assessed using a pseudomomentum budget analysis. Nonconservative effects were found to dominate the transports; the GWMT contribution involved a diabatic source region in the troposphere and a dissipative sink region in the stratosphere. The CMT contribution was consistent with transport between the boundary layer and free troposphere via tilted convection. Transient buoyancy–vorticity correlations highlighted wave sources in the region of convective outflow and the boundary layer. These sources were akin to the previously described “mechanical oscillator” mechanism. Fluxes associated with this upper-level source were most sensitive to convective organization, highlighting the mechanism by which changes in organization are communicated to GWMT. The results elucidate important interactions between CMT and GWMT, adding further weight to suggestions that the two transports should be linked in parameterizations.
Abstract
This study examines how variations in relatively simple terrain geometries influence orographic precipitation and its spatial patterns of sensitivity to small changes in upstream conditions. An idealized three-dimensional model is used to simulate a moist flow impinging upon three alpine-scale terrain shapes: a straight ridge, a concave ridge, and a convex ridge. A variety of simulations are conducted to investigate the sensitivity of precipitation patterns to ridge length and upstream thermodynamic and wind conditions for an impinging flow with a nondimensional mountain height of approximately unity. It is found that for the straight and convex ridges, the flow response is mostly linear for the conditions examined here and passes over the obstacles with little lateral deflection. The concave ridge, however, exhibits strengthened flow deceleration, wave breaking in the lee, and flow confluence between the ridge arms. The concave ridge also generates substantially more precipitation than the other two ridge geometries via an established precipitation-enhancing funneling mechanism near the ridge vertex. However, for some concave ridge configurations the results feature dual-precipitation maxima, which is an important difference from previous findings. Finally, results from a simple ensemble of simulations elucidate the sensitivity of precipitation patterns to small variations in upstream conditions and how these vary for the different terrain geometries.
Abstract
This study examines how variations in relatively simple terrain geometries influence orographic precipitation and its spatial patterns of sensitivity to small changes in upstream conditions. An idealized three-dimensional model is used to simulate a moist flow impinging upon three alpine-scale terrain shapes: a straight ridge, a concave ridge, and a convex ridge. A variety of simulations are conducted to investigate the sensitivity of precipitation patterns to ridge length and upstream thermodynamic and wind conditions for an impinging flow with a nondimensional mountain height of approximately unity. It is found that for the straight and convex ridges, the flow response is mostly linear for the conditions examined here and passes over the obstacles with little lateral deflection. The concave ridge, however, exhibits strengthened flow deceleration, wave breaking in the lee, and flow confluence between the ridge arms. The concave ridge also generates substantially more precipitation than the other two ridge geometries via an established precipitation-enhancing funneling mechanism near the ridge vertex. However, for some concave ridge configurations the results feature dual-precipitation maxima, which is an important difference from previous findings. Finally, results from a simple ensemble of simulations elucidate the sensitivity of precipitation patterns to small variations in upstream conditions and how these vary for the different terrain geometries.
Abstract
This study examines how variations to the nondimensional mountain height Ĥ and the horizontal aspect ratio β of a straight ridge and a concave ridge influence orographic precipitation. An idealized three-dimensional model is used to simulate a moist flow impinging upon these two ridges with Ĥ = 0.66–2.0 and β = 1.0–8.0. The concave ridge generates substantially more precipitation than the straight ridge via an established precipitation-enhancing funneling mechanism near the ridge vertex when the flow is unblocked. Based on previous work, it was hypothesized that when the approaching flow becomes blocked, the strength of the precipitation enhancement by the concave ridge relative to the straight ridge becomes negligible. This study reveals that, if Ĥ is sufficiently large to induce flow reversal on the windward slope, then a secondary circulation develops that is strengthened by the concave ridge with a subsequent enhancement of precipitation. It is also shown that the competing effects of the ridge length and width render the strength of the precipitation enhancement largely insensitive to β. A flow regime diagram for the straight ridge and the concave ridge is also constructed to illustrate the sensitivity of the critical Ĥ value for flow regime transition to changes in the terrain geometry; variations to the low-level relative humidity are also explored.
Abstract
This study examines how variations to the nondimensional mountain height Ĥ and the horizontal aspect ratio β of a straight ridge and a concave ridge influence orographic precipitation. An idealized three-dimensional model is used to simulate a moist flow impinging upon these two ridges with Ĥ = 0.66–2.0 and β = 1.0–8.0. The concave ridge generates substantially more precipitation than the straight ridge via an established precipitation-enhancing funneling mechanism near the ridge vertex when the flow is unblocked. Based on previous work, it was hypothesized that when the approaching flow becomes blocked, the strength of the precipitation enhancement by the concave ridge relative to the straight ridge becomes negligible. This study reveals that, if Ĥ is sufficiently large to induce flow reversal on the windward slope, then a secondary circulation develops that is strengthened by the concave ridge with a subsequent enhancement of precipitation. It is also shown that the competing effects of the ridge length and width render the strength of the precipitation enhancement largely insensitive to β. A flow regime diagram for the straight ridge and the concave ridge is also constructed to illustrate the sensitivity of the critical Ĥ value for flow regime transition to changes in the terrain geometry; variations to the low-level relative humidity are also explored.
Abstract
Over the past decade, numerous numerical modeling studies have shown that deep convective clouds can produce gravity waves that induce a significant vertical flux of horizontal momentum. Such studies used models with horizontal grid spacings of O(1 km) and produced strong gravity waves with horizontal wavelengths greater than about 20 km. This paper is an examination of how simulated gravity waves and their momentum flux are sensitive to model resolution. It is shown that increases in horizontal resolution produce more power in waves with shorter horizontal wavelengths. This change in the gravity waves’ spectra influences their vertical propagation. In some cases, gravity waves that were vertically propagating in coarse simulations become vertically trapped in fine simulations, which strongly influences the vertical flux of horizontal momentum.
Abstract
Over the past decade, numerous numerical modeling studies have shown that deep convective clouds can produce gravity waves that induce a significant vertical flux of horizontal momentum. Such studies used models with horizontal grid spacings of O(1 km) and produced strong gravity waves with horizontal wavelengths greater than about 20 km. This paper is an examination of how simulated gravity waves and their momentum flux are sensitive to model resolution. It is shown that increases in horizontal resolution produce more power in waves with shorter horizontal wavelengths. This change in the gravity waves’ spectra influences their vertical propagation. In some cases, gravity waves that were vertically propagating in coarse simulations become vertically trapped in fine simulations, which strongly influences the vertical flux of horizontal momentum.
Abstract
The generation of gravity waves by multiscale cloud systems evolving in an initially motionless and thermodynamically uniform environment is explored using a two-dimensional cloud-system-resolving model. The simulated convection has similar depth and intensity to observed tropical oceanic systems. The convection self-organizes into preferred horizontal and temporal scales involving weakly organized propagating cloud clusters. The multiscale systems generate a broad spectrum of gravity waves with horizontal scales that range from the cloud-system scale up to the cloud-cluster scale. The gravity waves with the largest horizontal scale play an important role in modifying layered tropospheric inflow and outflow to the cloud systems, which in turn influence the multiscale convective organization. Slower-moving short-scale gravity waves make the strongest individual contribution to the vertical flux of horizontal momentum and cause a robust peak in the momentum flux spectrum that corresponds to the lifetime and spatial scale of the individual cloud systems.
Abstract
The generation of gravity waves by multiscale cloud systems evolving in an initially motionless and thermodynamically uniform environment is explored using a two-dimensional cloud-system-resolving model. The simulated convection has similar depth and intensity to observed tropical oceanic systems. The convection self-organizes into preferred horizontal and temporal scales involving weakly organized propagating cloud clusters. The multiscale systems generate a broad spectrum of gravity waves with horizontal scales that range from the cloud-system scale up to the cloud-cluster scale. The gravity waves with the largest horizontal scale play an important role in modifying layered tropospheric inflow and outflow to the cloud systems, which in turn influence the multiscale convective organization. Slower-moving short-scale gravity waves make the strongest individual contribution to the vertical flux of horizontal momentum and cause a robust peak in the momentum flux spectrum that corresponds to the lifetime and spatial scale of the individual cloud systems.
Abstract
This study uses a two-dimensional cloud-resolving model to examine how convectively generated gravity waves modify the environment of an isolated convective cloud. The model is initialized with an idealized sounding, and the cloud is initiated by adding a locally buoyant perturbation. The modeled convection generates a spectrum of gravity waves with vertical wavelengths that are harmonics of the depth of the troposphere. It is shown that the first three wave modes significantly modify the cloud environment.
The modification of the cloud environment is quantified in terms of the convective available potential energy (CAPE) and convective inhibition (CIN). The first two wave modes travel fastest away from the cloud and are responsible for the changes in CAPE, whereas the third wave mode causes low-level lifting and hence a reduction in CIN. The maximum far-field perturbations in CAPE and CIN are approximately 15% and 33% of the initial background values, respectively. These results agree with previous studies of more organized convection, predicting the existence of a region surrounding the convective system that favors the development of new convection.
Abstract
This study uses a two-dimensional cloud-resolving model to examine how convectively generated gravity waves modify the environment of an isolated convective cloud. The model is initialized with an idealized sounding, and the cloud is initiated by adding a locally buoyant perturbation. The modeled convection generates a spectrum of gravity waves with vertical wavelengths that are harmonics of the depth of the troposphere. It is shown that the first three wave modes significantly modify the cloud environment.
The modification of the cloud environment is quantified in terms of the convective available potential energy (CAPE) and convective inhibition (CIN). The first two wave modes travel fastest away from the cloud and are responsible for the changes in CAPE, whereas the third wave mode causes low-level lifting and hence a reduction in CIN. The maximum far-field perturbations in CAPE and CIN are approximately 15% and 33% of the initial background values, respectively. These results agree with previous studies of more organized convection, predicting the existence of a region surrounding the convective system that favors the development of new convection.
Abstract
An idealized cloud-system-resolving model simulation is used to examine the coupling between a tropical cloud population and the mesoscale gravity waves that it generates. Spectral analyses of the cloud and gravity wave fields identify a clear signal of coupling between the clouds and a deep tropospheric gravity wave mode with a vertical wavelength that matches the depth of the convection, which is about two-thirds of the tropospheric depth. This vertical wavelength and the period of the waves, defined by a characteristic convective time scale, means that the horizontal wavelength is constrained through the dispersion relation. Indeed, the wave–convection coupling manifests at the appropriate wavelength, with the emergence of quasi-regular cloud-system spacing of order 100 km. It is shown that cloud systems at this spacing achieve a quasi-resonant state, at least for a few convective life cycles. Such regular spacing is a key component of cloud organization and is likely a contributor to the processes controlling the upscale growth of convective systems. Other gravity wave processes are also elucidated, including their apparent role in the maintenance of convective systems by providing a mechanism for renewed convective activity and system longevity.
Abstract
An idealized cloud-system-resolving model simulation is used to examine the coupling between a tropical cloud population and the mesoscale gravity waves that it generates. Spectral analyses of the cloud and gravity wave fields identify a clear signal of coupling between the clouds and a deep tropospheric gravity wave mode with a vertical wavelength that matches the depth of the convection, which is about two-thirds of the tropospheric depth. This vertical wavelength and the period of the waves, defined by a characteristic convective time scale, means that the horizontal wavelength is constrained through the dispersion relation. Indeed, the wave–convection coupling manifests at the appropriate wavelength, with the emergence of quasi-regular cloud-system spacing of order 100 km. It is shown that cloud systems at this spacing achieve a quasi-resonant state, at least for a few convective life cycles. Such regular spacing is a key component of cloud organization and is likely a contributor to the processes controlling the upscale growth of convective systems. Other gravity wave processes are also elucidated, including their apparent role in the maintenance of convective systems by providing a mechanism for renewed convective activity and system longevity.
Abstract
Tropical convection is inherently multiscalar, involving complex fields of clouds and various regimes of convective organization ranging from small disorganized cumulus up to large organized convective clusters. In addition to being a crucial component of the atmospheric water cycle and the global heat budget, tropical convection induces vertical fluxes of horizontal momentum. There are two main contributions to the momentum transport. The first resides entirely in the troposphere and is due to ascent, descent, and organized circulations associated with precipitating convective systems. The second resides in the troposphere, stratosphere, and farther aloft and is caused by vertically propagating gravity waves. Both the convective momentum transport and the gravity wave momentum flux must be parameterized in general circulation models; yet in existing parameterizations, these two processes are treated independently. This paper examines the relationship between the convective momentum transport and convectively generated gravity wave momentum flux by utilizing idealized simulations of multiscale tropical convection in different wind shear conditions. The simulations produce convective systems with a variety of regimes of convective organization and therefore different convective momentum transport properties and gravity wave spectra. A number of important connections are identified, including a consistency in the sign of the momentum transports in the lower troposphere and stratosphere that is linked to the generation of gravity waves by tilted convective structures. These results elucidate important relationships between the convective momentum transport and the gravity wave momentum flux that will be useful for interlinking their parameterization in the future.
Abstract
Tropical convection is inherently multiscalar, involving complex fields of clouds and various regimes of convective organization ranging from small disorganized cumulus up to large organized convective clusters. In addition to being a crucial component of the atmospheric water cycle and the global heat budget, tropical convection induces vertical fluxes of horizontal momentum. There are two main contributions to the momentum transport. The first resides entirely in the troposphere and is due to ascent, descent, and organized circulations associated with precipitating convective systems. The second resides in the troposphere, stratosphere, and farther aloft and is caused by vertically propagating gravity waves. Both the convective momentum transport and the gravity wave momentum flux must be parameterized in general circulation models; yet in existing parameterizations, these two processes are treated independently. This paper examines the relationship between the convective momentum transport and convectively generated gravity wave momentum flux by utilizing idealized simulations of multiscale tropical convection in different wind shear conditions. The simulations produce convective systems with a variety of regimes of convective organization and therefore different convective momentum transport properties and gravity wave spectra. A number of important connections are identified, including a consistency in the sign of the momentum transports in the lower troposphere and stratosphere that is linked to the generation of gravity waves by tilted convective structures. These results elucidate important relationships between the convective momentum transport and the gravity wave momentum flux that will be useful for interlinking their parameterization in the future.
Abstract
This study explores the generation of turbulence in the upper outflow regions of simulated idealized mesoscale convective systems. The simulated storms are shown to generate parameterized turbulence that occurs significant distances (>100 km) from the main convective regions, in both the clear air surrounding the convection and low simulated reflectivity regions with cloud ice but negligible amounts of graupel and snow. The source of the turbulence is related to Kelvin–Helmholtz instabilities that occur in the shear zones above and below the storm-induced upper-level outflow jet that is centered near the tropopause; the model produces resolved-scale billows within regions of low gradient Richardson number. Short-scale gravity waves are also coincident with the regions of turbulence, become trapped within the jet core, and appear to be generated by the shear instability. Additional experiments with different initial upper-level wind shear show similar mechanisms to those simulations with no initial upper-level shear. These results help elucidate the dynamics of turbulence generation near convection, which has important implications for the aviation industry and the fundamental understanding of how convective clouds interact with their environment.
Abstract
This study explores the generation of turbulence in the upper outflow regions of simulated idealized mesoscale convective systems. The simulated storms are shown to generate parameterized turbulence that occurs significant distances (>100 km) from the main convective regions, in both the clear air surrounding the convection and low simulated reflectivity regions with cloud ice but negligible amounts of graupel and snow. The source of the turbulence is related to Kelvin–Helmholtz instabilities that occur in the shear zones above and below the storm-induced upper-level outflow jet that is centered near the tropopause; the model produces resolved-scale billows within regions of low gradient Richardson number. Short-scale gravity waves are also coincident with the regions of turbulence, become trapped within the jet core, and appear to be generated by the shear instability. Additional experiments with different initial upper-level wind shear show similar mechanisms to those simulations with no initial upper-level shear. These results help elucidate the dynamics of turbulence generation near convection, which has important implications for the aviation industry and the fundamental understanding of how convective clouds interact with their environment.
