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- Author or Editor: D. R. Smith x
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Abstract
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Abstract
An anisotropic generalization of the Gent–McWilliams (GM) parameterization is presented for eddy-induced tracer transport and diffusion in ocean models, and it is implemented in an ocean general circulation model using a functional formalism to derive the spatial discretization. This complements the anisotropic viscosity parameterization recently developed by Smith and McWilliams. The anisotropic GM operator is potentially useful in both coarse- and high-resolution ocean models, and in this study the focus is on its application in high-resolution eddying solutions, for which it provides an adiabatic alternative to the more commonly used biharmonic horizontal diffusion operators. It is shown that realistically high levels of eddy energy can be simulated using harmonic anisotropic diffusion and friction operators. Isotropic forms can also be used, but these tend either to overly damp the solution when a large diffusion coefficient is used or to introduce unacceptable levels of numerical noise when a small coefficient is used. A series of numerical simulations of the North Atlantic Ocean are conducted at 0.2° resolution using anisotropic viscosity, anisotropic GM, and biharmonic mixing operators to investigate the effects of the anisotropic forms and to isolate changes in the solutions specifically associated with anisotropic GM. A high-resolution 0.1° simulation is then conducted using both anisotropic forms, and the results are compared with a similar run using biharmonic mixing. Modest improvements are seen in the mean wind-driven circulation with the anisotropic forms, but the largest effects are due to the anisotropic GM parameterization, which eliminates the spurious diapycnal diffusion inherent in horizontal tracer diffusion. This leads to significant improvements in the model thermohaline circulation, including the meridional heat transport, meridional overturning circulation, and deep-water formation and convection in the Labrador Sea.
Abstract
An anisotropic generalization of the Gent–McWilliams (GM) parameterization is presented for eddy-induced tracer transport and diffusion in ocean models, and it is implemented in an ocean general circulation model using a functional formalism to derive the spatial discretization. This complements the anisotropic viscosity parameterization recently developed by Smith and McWilliams. The anisotropic GM operator is potentially useful in both coarse- and high-resolution ocean models, and in this study the focus is on its application in high-resolution eddying solutions, for which it provides an adiabatic alternative to the more commonly used biharmonic horizontal diffusion operators. It is shown that realistically high levels of eddy energy can be simulated using harmonic anisotropic diffusion and friction operators. Isotropic forms can also be used, but these tend either to overly damp the solution when a large diffusion coefficient is used or to introduce unacceptable levels of numerical noise when a small coefficient is used. A series of numerical simulations of the North Atlantic Ocean are conducted at 0.2° resolution using anisotropic viscosity, anisotropic GM, and biharmonic mixing operators to investigate the effects of the anisotropic forms and to isolate changes in the solutions specifically associated with anisotropic GM. A high-resolution 0.1° simulation is then conducted using both anisotropic forms, and the results are compared with a similar run using biharmonic mixing. Modest improvements are seen in the mean wind-driven circulation with the anisotropic forms, but the largest effects are due to the anisotropic GM parameterization, which eliminates the spurious diapycnal diffusion inherent in horizontal tracer diffusion. This leads to significant improvements in the model thermohaline circulation, including the meridional heat transport, meridional overturning circulation, and deep-water formation and convection in the Labrador Sea.
Abstract
The decay of mesoscale eddies can be attributed to either frictional dissipation of kinetic energy through viscous effects or through dispersive spreading of the different constituent Rossby wave components at their own characteristic wave speeds. Several previous investigations of eddy decay have examined the role of variable friction in the spindown process. In addition to frictional results, these studies have shown that nonlinear advective processes can stabilize the vortex against dispersive effects. The quantification of this relation between nonlinear stabilization and beta dispersion is the primary focus of this paper.
Results are obtained using a finite difference “equivalent barotropic” numerical model with a fixed biharmonic friction formulation. Variable parameters in the study are vortex size and strength. Initial conditions are in the form of a Gaussian height field in gradient balance. Nonfrictional vortex decay is parameterized in terms of lateral spreading. This spreading is determined by the rate of increase of the second radial moment weighted by potential energy density. Estimates are made for the time required for this length to double in magnitude. Moments based on other weightings are also investigated.
Abstract
The decay of mesoscale eddies can be attributed to either frictional dissipation of kinetic energy through viscous effects or through dispersive spreading of the different constituent Rossby wave components at their own characteristic wave speeds. Several previous investigations of eddy decay have examined the role of variable friction in the spindown process. In addition to frictional results, these studies have shown that nonlinear advective processes can stabilize the vortex against dispersive effects. The quantification of this relation between nonlinear stabilization and beta dispersion is the primary focus of this paper.
Results are obtained using a finite difference “equivalent barotropic” numerical model with a fixed biharmonic friction formulation. Variable parameters in the study are vortex size and strength. Initial conditions are in the form of a Gaussian height field in gradient balance. Nonfrictional vortex decay is parameterized in terms of lateral spreading. This spreading is determined by the rate of increase of the second radial moment weighted by potential energy density. Estimates are made for the time required for this length to double in magnitude. Moments based on other weightings are also investigated.
Abstract
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Abstract
This paper presents the results of a field expedition mounted in late September/early October 1979 to investigate the structure and origin of the “morning glory” of the Gulf of Carpentaria in northern Australia. The morning glory is a line wind squall, accompanied by a pressure jump, and often by a long roll-cloud or series of such clouds. It frequently occurs in the early morning, especially in October, in the Gulf area.
A light aircraft, fitted with a temperature and humidity probe, was flown in two glories to determine their thermodynamic structure, and wind fields wore obtained principally by tracking pilot balloons using the double theodolite method. Data also were obtained from a network of surface stations, recording wind velocity and pressure, installed at locations across Cape York Peninsula, which is believed to be the area of genesis.
The morning glory is identified as an internal undular bore propagating on the nocturnal and/or maritime inversion. Its origin appears to lie frequently in the interaction of a deeply penetrating sea breeze front with a developing nocturnal inversion, but there is evidence also that on occasion it may result from the effect of a katabatic flow. The factors which appear to make the Gulf region particularly favorable for the common occurrence of this phenomenon are discussed.
Abstract
This paper presents the results of a field expedition mounted in late September/early October 1979 to investigate the structure and origin of the “morning glory” of the Gulf of Carpentaria in northern Australia. The morning glory is a line wind squall, accompanied by a pressure jump, and often by a long roll-cloud or series of such clouds. It frequently occurs in the early morning, especially in October, in the Gulf area.
A light aircraft, fitted with a temperature and humidity probe, was flown in two glories to determine their thermodynamic structure, and wind fields wore obtained principally by tracking pilot balloons using the double theodolite method. Data also were obtained from a network of surface stations, recording wind velocity and pressure, installed at locations across Cape York Peninsula, which is believed to be the area of genesis.
The morning glory is identified as an internal undular bore propagating on the nocturnal and/or maritime inversion. Its origin appears to lie frequently in the interaction of a deeply penetrating sea breeze front with a developing nocturnal inversion, but there is evidence also that on occasion it may result from the effect of a katabatic flow. The factors which appear to make the Gulf region particularly favorable for the common occurrence of this phenomenon are discussed.
Abstract
A numerical solution to the three-dimensional advection-diffusion equation is developed and applied to the dispersion of power plant stack contaminants throughout the boundary layer. The method employs Lagrangian marker particles undergoing variable random-walk displacements to simulate a gradient-transfer process. The size of the particle displacements is directly related to the magnitude of the vertical and horizontal diffusivities which can be any functions of space and time. Using recent atmospheric turbulence data, empirical expressions for the eddy diffusivities are derived for the entire boundary layer in terms of common meteorologic parameters. Reasonable agreement is found between the numerical predictions and actual fly-ash data collected in the vicinity of a 500 MW coal-fired power station.
The random-walk technique has a number of distinct advantages over both finite-difference and particle-in-a-cell methods. It is mathematically simple, computationally fast, and requires only modest amounts of computer memory. The accuracy of the method is evaluated by comparison with a series solution of the two-dimensional diffusion equation appropriate to the surface layer.
Abstract
A numerical solution to the three-dimensional advection-diffusion equation is developed and applied to the dispersion of power plant stack contaminants throughout the boundary layer. The method employs Lagrangian marker particles undergoing variable random-walk displacements to simulate a gradient-transfer process. The size of the particle displacements is directly related to the magnitude of the vertical and horizontal diffusivities which can be any functions of space and time. Using recent atmospheric turbulence data, empirical expressions for the eddy diffusivities are derived for the entire boundary layer in terms of common meteorologic parameters. Reasonable agreement is found between the numerical predictions and actual fly-ash data collected in the vicinity of a 500 MW coal-fired power station.
The random-walk technique has a number of distinct advantages over both finite-difference and particle-in-a-cell methods. It is mathematically simple, computationally fast, and requires only modest amounts of computer memory. The accuracy of the method is evaluated by comparison with a series solution of the two-dimensional diffusion equation appropriate to the surface layer.
Abstract
No Abstract Available.
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No Abstract Available.
Abstract
This paper addresses shelf-scale simulation with dominant open-water boundary conditions obtained by inversion of interior data. Important, established operational data streams are located along the shore of the study area, in areas influenced strongly by the local geometry. Failure to properly resolve the modeled near field surrounding these data results in their incorrect interpretation, causing invalid inversions and erroneous field estimates far across the shelf. Specifically, improving the model fit to the unresolved data leads to skill degeneration farther offshore and generally unacceptable field estimates remarkably far from shore. Proper near-field resolution leads to valid interpretation and inversion of the same data, with high inverse skill apparent across the shelf. The resolution required is within reach of today's technology.
Abstract
This paper addresses shelf-scale simulation with dominant open-water boundary conditions obtained by inversion of interior data. Important, established operational data streams are located along the shore of the study area, in areas influenced strongly by the local geometry. Failure to properly resolve the modeled near field surrounding these data results in their incorrect interpretation, causing invalid inversions and erroneous field estimates far across the shelf. Specifically, improving the model fit to the unresolved data leads to skill degeneration farther offshore and generally unacceptable field estimates remarkably far from shore. Proper near-field resolution leads to valid interpretation and inversion of the same data, with high inverse skill apparent across the shelf. The resolution required is within reach of today's technology.
Abstract
The mountainous Caribbean island of Dominica was chosen as a natural laboratory for studying orographic convection in the tropics. Here, the authors focus on a prototypical case study, taken from the Dominica Experiment (DOMEX) field campaign in the spring of 2011. Airborne measurements and high-resolution numerical experiments are used to examine the mesoscale dynamics of moist airflow over Dominica and its relationship to convection, turbulence, and rainfall.
Upwind of the island, there is minimal lateral deflection or lifting of the flow, largely because of latent heat release in the overisland convection. Over the terrain, forced ascent leads to rapid development of moist convection, buoyancy-generated turbulence, and rainfall. Although this convection produces sporadic bursts of heavy rainfall, it does not appear to enhance the time-mean rainfall. Over the lee slopes, a dry plunging flow produces anisotropic shear-generated turbulence and strong low-level winds while quickly dissipating convection and rainfall. In the wake, low-level air is decelerated, both by turbulence in the plunging flow and by frictional drag over the island. Low-level wake air is also dried and warmed, primarily by turbulent vertical mixing and regional descent, both associated with the downslope flow. Rainfall and latent heating play only a secondary role in warming and drying the wake.
Abstract
The mountainous Caribbean island of Dominica was chosen as a natural laboratory for studying orographic convection in the tropics. Here, the authors focus on a prototypical case study, taken from the Dominica Experiment (DOMEX) field campaign in the spring of 2011. Airborne measurements and high-resolution numerical experiments are used to examine the mesoscale dynamics of moist airflow over Dominica and its relationship to convection, turbulence, and rainfall.
Upwind of the island, there is minimal lateral deflection or lifting of the flow, largely because of latent heat release in the overisland convection. Over the terrain, forced ascent leads to rapid development of moist convection, buoyancy-generated turbulence, and rainfall. Although this convection produces sporadic bursts of heavy rainfall, it does not appear to enhance the time-mean rainfall. Over the lee slopes, a dry plunging flow produces anisotropic shear-generated turbulence and strong low-level winds while quickly dissipating convection and rainfall. In the wake, low-level air is decelerated, both by turbulence in the plunging flow and by frictional drag over the island. Low-level wake air is also dried and warmed, primarily by turbulent vertical mixing and regional descent, both associated with the downslope flow. Rainfall and latent heating play only a secondary role in warming and drying the wake.
Abstract
The “natural laboratory” of mountainous Dominica (15°N) in the trade wind belt is used to study the physics of tropical orographic precipitation in its purest form, unforced by weather disturbances or by the diurnal cycle of solar heating. A cross-island line of rain gauges and 5-min radar scans from Guadeloupe reveal a large annual precipitation at high elevation (7 m yr−1) and a large orographic enhancement factor (2 to 8) caused primarily by repetitive convective triggering over the windward slope. The triggering is caused by terrain-forced lifting of the conditionally unstable trade wind cloud layer. Ambient humidity fluctuations associated with open-ocean convection may play a key role. The convection transports moisture upward and causes frequent brief showers on the hilltops. The drying ratio of the full air column from precipitation is less than 1% whereas the surface air dries by about 17% from the east coast to the mountain top. On the lee side, a plunging trade wind inversion and reduced instability destroys convective clouds and creates an oceanic rain shadow.
Abstract
The “natural laboratory” of mountainous Dominica (15°N) in the trade wind belt is used to study the physics of tropical orographic precipitation in its purest form, unforced by weather disturbances or by the diurnal cycle of solar heating. A cross-island line of rain gauges and 5-min radar scans from Guadeloupe reveal a large annual precipitation at high elevation (7 m yr−1) and a large orographic enhancement factor (2 to 8) caused primarily by repetitive convective triggering over the windward slope. The triggering is caused by terrain-forced lifting of the conditionally unstable trade wind cloud layer. Ambient humidity fluctuations associated with open-ocean convection may play a key role. The convection transports moisture upward and causes frequent brief showers on the hilltops. The drying ratio of the full air column from precipitation is less than 1% whereas the surface air dries by about 17% from the east coast to the mountain top. On the lee side, a plunging trade wind inversion and reduced instability destroys convective clouds and creates an oceanic rain shadow.
