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- Author or Editor: David A. Schecter x
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Abstract
The dynamical core of the Regional Atmospheric Modeling System has been tailored to simulate the infrasound of vortex motions and diabatic cloud processes in a convective storm. Earlier studies have shown that the customized model (c-RAMS) adequately simulates the infrasonic emissions of generic vortex oscillations. This paper provides evidence that c-RAMS accurately simulates the infrasound associated with parameterized phase transitions of cloud moisture. Specifically, analytical expressions are derived for the infrasonic emissions of evaporating water droplets in dry and humid environments. The dry analysis considers two single-moment parameterizations of the microphysics, which have distinguishable acoustic signatures. In general, the analytical results agree with the numerical output of the model. An appendix briefly demonstrates the ability of c-RAMS to accurately simulate the infrasound of the entropy and mass sources generated by an equilibrating cloud of icy hydrometeors.
Abstract
The dynamical core of the Regional Atmospheric Modeling System has been tailored to simulate the infrasound of vortex motions and diabatic cloud processes in a convective storm. Earlier studies have shown that the customized model (c-RAMS) adequately simulates the infrasonic emissions of generic vortex oscillations. This paper provides evidence that c-RAMS accurately simulates the infrasound associated with parameterized phase transitions of cloud moisture. Specifically, analytical expressions are derived for the infrasonic emissions of evaporating water droplets in dry and humid environments. The dry analysis considers two single-moment parameterizations of the microphysics, which have distinguishable acoustic signatures. In general, the analytical results agree with the numerical output of the model. An appendix briefly demonstrates the ability of c-RAMS to accurately simulate the infrasound of the entropy and mass sources generated by an equilibrating cloud of icy hydrometeors.
Abstract
A method is outlined for quantitatively assessing the impact of inertia–gravity wave radiation on the multimechanistic instability modes of a columnar stratified vortex that resembles an intense tropical cyclone. The method begins by decomposing the velocity field into one part that is formally associated with sources inside the vortex and another part that is attributed to radiation. The relative importance of radiation is assessed by comparing the rates at which the two partial velocity fields act to amplify the perturbation of an arbitrary tracer field—such as potential vorticity—inside the vortex. Further insight is gained by decomposing the formal vortex contribution to the amplification rate into subparts that are primarily associated with distinct vortex Rossby waves and critical-layer perturbations.
Abstract
A method is outlined for quantitatively assessing the impact of inertia–gravity wave radiation on the multimechanistic instability modes of a columnar stratified vortex that resembles an intense tropical cyclone. The method begins by decomposing the velocity field into one part that is formally associated with sources inside the vortex and another part that is attributed to radiation. The relative importance of radiation is assessed by comparing the rates at which the two partial velocity fields act to amplify the perturbation of an arbitrary tracer field—such as potential vorticity—inside the vortex. Further insight is gained by decomposing the formal vortex contribution to the amplification rate into subparts that are primarily associated with distinct vortex Rossby waves and critical-layer perturbations.
Abstract
A cloud-resolving model is used to examine the virtually shear-free evolution of incipient tropical cyclones initialized with different degrees of misalignment between the lower- and middle-tropospheric centers of rotation. Increasing the initial displacement of rotational centers (the tilt) from a negligible value to several hundred kilometers extends the time scale of hurricane formation from 1 to 10 days. Hindered amplification of the maximum tangential velocity υ m at the surface of a strongly perturbed system is related to an extended duration of misalignment resulting from incomplete early decay and subsequent transient growth of the tilt magnitude. The prolonged misalignment coincides with a prolonged period of asymmetric convection peaked far from the surface center of the vortex. A Sawyer–Eliassen model is used to analyze the disparity between azimuthal velocity tendencies of selected pre–tropical storm vortices with low and high degrees of misalignment. Although no single factor completely explains the difference of intensification rates, greater misalignment is linked to weaker positive azimuthal velocity forcing near υ m by the component of the mean secondary circulation attributed to heating by microphysical cloud processes. Of note regarding the dynamics, enhanced tilt only modestly affects the growth rate of kinetic energy outside the core of the surface vortex while severely hindering intensification of υ m within the core for at least several days. The processes controlling the evolution of the misalignment associated with inefficient development are examined in detail for a selected simulation. It is found that adiabatic mechanisms are capable of driving the transient amplification of tilt, whereas diabatic processes are essential to ultimate alignment of the tropical cyclone.
Abstract
A cloud-resolving model is used to examine the virtually shear-free evolution of incipient tropical cyclones initialized with different degrees of misalignment between the lower- and middle-tropospheric centers of rotation. Increasing the initial displacement of rotational centers (the tilt) from a negligible value to several hundred kilometers extends the time scale of hurricane formation from 1 to 10 days. Hindered amplification of the maximum tangential velocity υ m at the surface of a strongly perturbed system is related to an extended duration of misalignment resulting from incomplete early decay and subsequent transient growth of the tilt magnitude. The prolonged misalignment coincides with a prolonged period of asymmetric convection peaked far from the surface center of the vortex. A Sawyer–Eliassen model is used to analyze the disparity between azimuthal velocity tendencies of selected pre–tropical storm vortices with low and high degrees of misalignment. Although no single factor completely explains the difference of intensification rates, greater misalignment is linked to weaker positive azimuthal velocity forcing near υ m by the component of the mean secondary circulation attributed to heating by microphysical cloud processes. Of note regarding the dynamics, enhanced tilt only modestly affects the growth rate of kinetic energy outside the core of the surface vortex while severely hindering intensification of υ m within the core for at least several days. The processes controlling the evolution of the misalignment associated with inefficient development are examined in detail for a selected simulation. It is found that adiabatic mechanisms are capable of driving the transient amplification of tilt, whereas diabatic processes are essential to ultimate alignment of the tropical cyclone.
Abstract
This article presents a new theory for the rate at which a quasigeostrophic vortex realigns, under conservative dynamics, after being tilted by an episode of external vertical shear. The initial tilt is viewed as the excitation of a three-dimensional “vortex Rossby mode.” This mode, that is, the tilt, decays exponentially with time during its early evolution. The decay rate γ is proportional to the potential vorticity gradient at a critical radius, where the fluid rotation is resonant with the mode. The decay rate γ also depends on the internal Rossby deformation radius l R , which is proportional to the stratification strength of the atmospheric or oceanic layer containing the vortex. The change of γ with l R is sensitive to the form of the vortex. For the case of a “Rankine-with-skirt” vortex, the magnitude of γ increases (initially) with increasing l R . On the other hand, for the case of a “Gaussian” vortex, the magnitude of γ decreases with increasing l R . The relevance of this theory to tropical cyclogenesis is discussed.
Abstract
This article presents a new theory for the rate at which a quasigeostrophic vortex realigns, under conservative dynamics, after being tilted by an episode of external vertical shear. The initial tilt is viewed as the excitation of a three-dimensional “vortex Rossby mode.” This mode, that is, the tilt, decays exponentially with time during its early evolution. The decay rate γ is proportional to the potential vorticity gradient at a critical radius, where the fluid rotation is resonant with the mode. The decay rate γ also depends on the internal Rossby deformation radius l R , which is proportional to the stratification strength of the atmospheric or oceanic layer containing the vortex. The change of γ with l R is sensitive to the form of the vortex. For the case of a “Rankine-with-skirt” vortex, the magnitude of γ increases (initially) with increasing l R . On the other hand, for the case of a “Gaussian” vortex, the magnitude of γ decreases with increasing l R . The relevance of this theory to tropical cyclogenesis is discussed.
Abstract
Intense atmospheric vortices such as tropical cyclones experience various asymmetric instabilities during their life cycles. This study investigates how vortex properties and ambient conditions determine the relative importance of different mechanisms that can simultaneously influence the growth of an asymmetric perturbation. The focus is on three-dimensional disturbances of barotropic vortices with nonmonotonic radial distributions of potential vorticity. The primary modes of instability are examined for Rossby numbers between 10 and 100 and Froude numbers in the broad neighborhood of unity. This parameter regime is deemed appropriate for tropical cyclone perturbations with vertical length scales ranging from the depth of the vortex to moderately smaller scales. At relatively small Froude numbers, the main cause of instability inferred from analysis typically involves the interaction of vortex Rossby waves with each other and/or critical-layer potential vorticity perturbations. As the Froude number increases from its lower bound, the main cause of instability transitions to inertia–gravity wave radiation. In some cases, the transition occurs abruptly at a critical point where a mode whose growth is driven almost entirely by radiation suddenly becomes dominant. In other cases, the transition is gradual and less direct as the fastest-growing mode continuously changes its structure. Examination of the angular pseudomomentum budget helps quantify the impact of radiation. The radiation-driven instabilities examined herein are shown to be quite fast and potentially relevant to real-world tropical cyclones. Their sensitivities to parameterized moisture and outer vorticity skirts are briefly addressed.
Abstract
Intense atmospheric vortices such as tropical cyclones experience various asymmetric instabilities during their life cycles. This study investigates how vortex properties and ambient conditions determine the relative importance of different mechanisms that can simultaneously influence the growth of an asymmetric perturbation. The focus is on three-dimensional disturbances of barotropic vortices with nonmonotonic radial distributions of potential vorticity. The primary modes of instability are examined for Rossby numbers between 10 and 100 and Froude numbers in the broad neighborhood of unity. This parameter regime is deemed appropriate for tropical cyclone perturbations with vertical length scales ranging from the depth of the vortex to moderately smaller scales. At relatively small Froude numbers, the main cause of instability inferred from analysis typically involves the interaction of vortex Rossby waves with each other and/or critical-layer potential vorticity perturbations. As the Froude number increases from its lower bound, the main cause of instability transitions to inertia–gravity wave radiation. In some cases, the transition occurs abruptly at a critical point where a mode whose growth is driven almost entirely by radiation suddenly becomes dominant. In other cases, the transition is gradual and less direct as the fastest-growing mode continuously changes its structure. Examination of the angular pseudomomentum budget helps quantify the impact of radiation. The radiation-driven instabilities examined herein are shown to be quite fast and potentially relevant to real-world tropical cyclones. Their sensitivities to parameterized moisture and outer vorticity skirts are briefly addressed.
Abstract
This paper addresses the physics and numerical simulation of the adiabatic generation of infrasound by tornadoes. Classical analytical results regarding the production of infrasound by vortex Rossby waves and by corotating “suction vortices” are reviewed. Conditions are derived for which critical layers damp vortex Rossby waves that would otherwise grow and continually produce acoustic radiation. These conditions are similar to those that theoretically suppress gravity wave radiation from larger mesoscale cyclones, such as hurricanes. To gain perspective, the Regional Atmospheric Modeling System (RAMS) is used to simulate the infrasound that radiates from a single-cell thunderstorm in a shear-free environment. In this simulation, the dominant infrasound in the 0.1–10-Hz frequency band appears to radiate from the vicinity of the melting level, where diabatic processes involving hail are active. It is shown that the 3D Rossby waves of a tornado-like vortex (simulated with RAMS) can generate stronger infrasound if the maximum wind speed of the vortex exceeds a modest threshold. Technical issues regarding the numerical simulation of tornado infrasound are also addressed. Most importantly, it is shown that simulating tornado infrasound likely requires a spatial resolution that is an order of magnitude finer than the current practical limit (10-m grid spacing) for modeling thunderstorms.
Abstract
This paper addresses the physics and numerical simulation of the adiabatic generation of infrasound by tornadoes. Classical analytical results regarding the production of infrasound by vortex Rossby waves and by corotating “suction vortices” are reviewed. Conditions are derived for which critical layers damp vortex Rossby waves that would otherwise grow and continually produce acoustic radiation. These conditions are similar to those that theoretically suppress gravity wave radiation from larger mesoscale cyclones, such as hurricanes. To gain perspective, the Regional Atmospheric Modeling System (RAMS) is used to simulate the infrasound that radiates from a single-cell thunderstorm in a shear-free environment. In this simulation, the dominant infrasound in the 0.1–10-Hz frequency band appears to radiate from the vicinity of the melting level, where diabatic processes involving hail are active. It is shown that the 3D Rossby waves of a tornado-like vortex (simulated with RAMS) can generate stronger infrasound if the maximum wind speed of the vortex exceeds a modest threshold. Technical issues regarding the numerical simulation of tornado infrasound are also addressed. Most importantly, it is shown that simulating tornado infrasound likely requires a spatial resolution that is an order of magnitude finer than the current practical limit (10-m grid spacing) for modeling thunderstorms.
