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- Author or Editor: Craig D. Smith x
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Abstract
Interactions between a turbulent boundary layer and nonlinear mountain waves are explored using a large-eddy simulation model. Simulations of a self-induced critical layer, which develop a stagnation layer and a strong leeside surface jet, are considered. Over time, wave breaking in the stagnation region forces strong turbulence that influences the formation and structure of downstream leeside rotors. Shear production is an important source of turbulence in the stagnation zone and along the interface between the stagnation zone and surface jet, as well as along the rotor edges. Buoyancy perturbations act as a source of turbulence in the stagnation zone but are shown to inhibit turbulence generation on the edges of the stagnation zone.
Surface heating is shown to have a strong influence on the strength of downslope winds and the formation of leeside rotors. In cases with no heating, a series of rotor circulations develops, capped by a region of increased winds. Weak heating disrupts this system and limits rotor formation at the base of the downslope jet. Strong heating has a much larger impact through a deepening of the upstream boundary layer and an overall decrease in the downslope winds. Rotors in this case are nonexistent. In contrast to the cases with surface warming, negative surface fluxes generate stronger downslope winds and intensified rotors due to turbulent interactions with an elevated stratified jet capping the rotors. Overall, the results suggest that for nonlinear wave systems over mountains higher than the boundary layer, strong downslope winds and rotors are favored in late afternoon and evening when surface cooling enhances the stability of the low-level air.
Abstract
Interactions between a turbulent boundary layer and nonlinear mountain waves are explored using a large-eddy simulation model. Simulations of a self-induced critical layer, which develop a stagnation layer and a strong leeside surface jet, are considered. Over time, wave breaking in the stagnation region forces strong turbulence that influences the formation and structure of downstream leeside rotors. Shear production is an important source of turbulence in the stagnation zone and along the interface between the stagnation zone and surface jet, as well as along the rotor edges. Buoyancy perturbations act as a source of turbulence in the stagnation zone but are shown to inhibit turbulence generation on the edges of the stagnation zone.
Surface heating is shown to have a strong influence on the strength of downslope winds and the formation of leeside rotors. In cases with no heating, a series of rotor circulations develops, capped by a region of increased winds. Weak heating disrupts this system and limits rotor formation at the base of the downslope jet. Strong heating has a much larger impact through a deepening of the upstream boundary layer and an overall decrease in the downslope winds. Rotors in this case are nonexistent. In contrast to the cases with surface warming, negative surface fluxes generate stronger downslope winds and intensified rotors due to turbulent interactions with an elevated stratified jet capping the rotors. Overall, the results suggest that for nonlinear wave systems over mountains higher than the boundary layer, strong downslope winds and rotors are favored in late afternoon and evening when surface cooling enhances the stability of the low-level air.
Abstract
Simulations are presented focusing on the role of temperature inversions in controlling the formation and strength of downslope wind storms. Three mechanisms are examined depending on the relative height of the inversion with respect to the mountain and the stability of vertically propagating mountain waves. For low-level inversions, flows are generated that closely resemble a reduced gravity shallow water hydraulic response with a large vertical displacement of the inversion on the lee side of the mountain. For higher-level inversion cases, simulated flows more closely followed a stratified hydraulic behavior with the inversion acting as a rigid reflective lid. In the third mechanism, downslope winds were forced by a self-induced critical layer located below the inversion height. The presence of the inversion in this case had little effect on the resulting downslope winds.
Observations made on the Falkland Islands show that downslope windstorms may preferentially occur in early morning even without synoptic-scale changes in atmospheric structure. Most windstorms on the Falkland Islands generally have a short jet length; rare, longer jet length storms typically occur in conjunction with a strong low-level inversion. Idealized numerical experiments tend to produce a similar response depending on the presence of strong low-level inversion and surface cooling. Results suggest that surface heating can have significant control on the flow response by reducing the low-level inversion strength, or by changing the stratification and wind velocity below the inversion, thereby preventing a strong downslope windstorm.
Abstract
Simulations are presented focusing on the role of temperature inversions in controlling the formation and strength of downslope wind storms. Three mechanisms are examined depending on the relative height of the inversion with respect to the mountain and the stability of vertically propagating mountain waves. For low-level inversions, flows are generated that closely resemble a reduced gravity shallow water hydraulic response with a large vertical displacement of the inversion on the lee side of the mountain. For higher-level inversion cases, simulated flows more closely followed a stratified hydraulic behavior with the inversion acting as a rigid reflective lid. In the third mechanism, downslope winds were forced by a self-induced critical layer located below the inversion height. The presence of the inversion in this case had little effect on the resulting downslope winds.
Observations made on the Falkland Islands show that downslope windstorms may preferentially occur in early morning even without synoptic-scale changes in atmospheric structure. Most windstorms on the Falkland Islands generally have a short jet length; rare, longer jet length storms typically occur in conjunction with a strong low-level inversion. Idealized numerical experiments tend to produce a similar response depending on the presence of strong low-level inversion and surface cooling. Results suggest that surface heating can have significant control on the flow response by reducing the low-level inversion strength, or by changing the stratification and wind velocity below the inversion, thereby preventing a strong downslope windstorm.
Abstract
A large eddy simulation (LES) model and the Advanced Regional Prediction System (ARPS) model, which does not resolve turbulent eddies, are used to study the effect of a slope angle decrease on the structure of katabatic slope flows. For a simple, uniform angle slope, simulations from both models produce turbulence kinetic energy and momentum budgets that are in good overall agreement. Simulations of a compound angle slope are compared to a uniform angle slope to demonstrate how a changing slope angle can strongly affect the strength of katabatic flows. Both ARPS and the LES model show that slopes with a steep upper slope followed by a shallower lower slope (concave shape) generate a rapid acceleration on the upper slope followed by a transition to a slower evolving structure characterized by an elevated jet over the lower slope. In contrast, the case with uniform slope (having the same total height change) yields a more uniform flow profile with stronger winds at the slope bottom. Higher average slope in the uniform slope angle case generates greater gravitational potential energy, which is converted to kinetic energy at the bottom of the slope. Analysis of the total energy budget of slope flows indicates a consistent structure where potential energy generated at the top of the slope is transported downslope and converted into kinetic energy near the slope base.
Abstract
A large eddy simulation (LES) model and the Advanced Regional Prediction System (ARPS) model, which does not resolve turbulent eddies, are used to study the effect of a slope angle decrease on the structure of katabatic slope flows. For a simple, uniform angle slope, simulations from both models produce turbulence kinetic energy and momentum budgets that are in good overall agreement. Simulations of a compound angle slope are compared to a uniform angle slope to demonstrate how a changing slope angle can strongly affect the strength of katabatic flows. Both ARPS and the LES model show that slopes with a steep upper slope followed by a shallower lower slope (concave shape) generate a rapid acceleration on the upper slope followed by a transition to a slower evolving structure characterized by an elevated jet over the lower slope. In contrast, the case with uniform slope (having the same total height change) yields a more uniform flow profile with stronger winds at the slope bottom. Higher average slope in the uniform slope angle case generates greater gravitational potential energy, which is converted to kinetic energy at the bottom of the slope. Analysis of the total energy budget of slope flows indicates a consistent structure where potential energy generated at the top of the slope is transported downslope and converted into kinetic energy near the slope base.
Severe thunderstorms are a common occurrence in summer on the Canadian prairies, with a large number originating along the Alberta, Canada, foothills, just east of the Rocky Mountains. Most of these storms move eastward to affect the Edmonton–Calgary corridor, one of the most densely populated and fastest-growing regions in Canada. Previous studies in the United States, Europe, and Canada have stressed the importance of mesoscale features in thunderstorm development. However, such processes cannot be adequately resolved using operational observation networks in many parts of Canada. Current conceptual models for severe storm outbreaks in Alberta were developed almost 20 years ago and do not focus explicitly on mesoscale boundaries that are now known to be important for thunderstorm development.
The Understanding Severe Thunderstorms and Alber ta Boundary Layers Experiment (UNSTABLE) is a field and modeling study aiming to improve our understanding of the processes associated with the initiation of severe thunderstorms, to refine associated conceptual models, and to assess the ability of convectivescale NWP models to simulate relevant physical processes. As part of UNSTABLE in 2008, Environment Canada and university scientists conducted a pilot field experiment over the Alberta foothills to investigate mesoscale processes associated with the development of severe thunderstorms. Networks of fixed and mobile surface and upper-air instrumentation provided observations of the atmospheric boundary layer at a level of detail never before seen in this region. Preliminary results include the most complete documentation of a dryline in Canada and an analysis of variability in boundary layer evolution across adjacent forest and crop vegetation areas. Convective-scale NWP simulations suggest that although additional information on convective mode may be provided, there is limited benefit overall to downscaling to smaller grid spacing without assimilation of mesoscale observations.
Severe thunderstorms are a common occurrence in summer on the Canadian prairies, with a large number originating along the Alberta, Canada, foothills, just east of the Rocky Mountains. Most of these storms move eastward to affect the Edmonton–Calgary corridor, one of the most densely populated and fastest-growing regions in Canada. Previous studies in the United States, Europe, and Canada have stressed the importance of mesoscale features in thunderstorm development. However, such processes cannot be adequately resolved using operational observation networks in many parts of Canada. Current conceptual models for severe storm outbreaks in Alberta were developed almost 20 years ago and do not focus explicitly on mesoscale boundaries that are now known to be important for thunderstorm development.
The Understanding Severe Thunderstorms and Alber ta Boundary Layers Experiment (UNSTABLE) is a field and modeling study aiming to improve our understanding of the processes associated with the initiation of severe thunderstorms, to refine associated conceptual models, and to assess the ability of convectivescale NWP models to simulate relevant physical processes. As part of UNSTABLE in 2008, Environment Canada and university scientists conducted a pilot field experiment over the Alberta foothills to investigate mesoscale processes associated with the development of severe thunderstorms. Networks of fixed and mobile surface and upper-air instrumentation provided observations of the atmospheric boundary layer at a level of detail never before seen in this region. Preliminary results include the most complete documentation of a dryline in Canada and an analysis of variability in boundary layer evolution across adjacent forest and crop vegetation areas. Convective-scale NWP simulations suggest that although additional information on convective mode may be provided, there is limited benefit overall to downscaling to smaller grid spacing without assimilation of mesoscale observations.
Abstract
Lateral stirring is a basic oceanographic phenomenon affecting the distribution of physical, chemical, and biological fields. Eddy stirring at scales on the order of 100 km (the mesoscale) is fairly well understood and explicitly represented in modern eddy-resolving numerical models of global ocean circulation. The same cannot be said for smaller-scale stirring processes. Here, the authors describe a major oceanographic field experiment aimed at observing and understanding the processes responsible for stirring at scales of 0.1–10 km. Stirring processes of varying intensity were studied in the Sargasso Sea eddy field approximately 250 km southeast of Cape Hatteras. Lateral variability of water-mass properties, the distribution of microscale turbulence, and the evolution of several patches of inert dye were studied with an array of shipboard, autonomous, and airborne instruments. Observations were made at two sites, characterized by weak and moderate background mesoscale straining, to contrast different regimes of lateral stirring. Analyses to date suggest that, in both cases, the lateral dispersion of natural and deliberately released tracers was O(1) m2 s–1 as found elsewhere, which is faster than might be expected from traditional shear dispersion by persistent mesoscale flow and linear internal waves. These findings point to the possible importance of kilometer-scale stirring by submesoscale eddies and nonlinear internal-wave processes or the need to modify the traditional shear-dispersion paradigm to include higher-order effects. A unique aspect of the Scalable Lateral Mixing and Coherent Turbulence (LatMix) field experiment is the combination of direct measurements of dye dispersion with the concurrent multiscale hydrographic and turbulence observations, enabling evaluation of the underlying mechanisms responsible for the observed dispersion at a new level.
Abstract
Lateral stirring is a basic oceanographic phenomenon affecting the distribution of physical, chemical, and biological fields. Eddy stirring at scales on the order of 100 km (the mesoscale) is fairly well understood and explicitly represented in modern eddy-resolving numerical models of global ocean circulation. The same cannot be said for smaller-scale stirring processes. Here, the authors describe a major oceanographic field experiment aimed at observing and understanding the processes responsible for stirring at scales of 0.1–10 km. Stirring processes of varying intensity were studied in the Sargasso Sea eddy field approximately 250 km southeast of Cape Hatteras. Lateral variability of water-mass properties, the distribution of microscale turbulence, and the evolution of several patches of inert dye were studied with an array of shipboard, autonomous, and airborne instruments. Observations were made at two sites, characterized by weak and moderate background mesoscale straining, to contrast different regimes of lateral stirring. Analyses to date suggest that, in both cases, the lateral dispersion of natural and deliberately released tracers was O(1) m2 s–1 as found elsewhere, which is faster than might be expected from traditional shear dispersion by persistent mesoscale flow and linear internal waves. These findings point to the possible importance of kilometer-scale stirring by submesoscale eddies and nonlinear internal-wave processes or the need to modify the traditional shear-dispersion paradigm to include higher-order effects. A unique aspect of the Scalable Lateral Mixing and Coherent Turbulence (LatMix) field experiment is the combination of direct measurements of dye dispersion with the concurrent multiscale hydrographic and turbulence observations, enabling evaluation of the underlying mechanisms responsible for the observed dispersion at a new level.
As part of the U.K. contribution to the international Surface Ocean-Lower Atmosphere Study, a series of three related projects—DOGEE, SEASAW, and HiWASE—undertook experimental studies of the processes controlling the physical exchange of gases and sea spray aerosol at the sea surface. The studies share a common goal: to reduce the high degree of uncertainty in current parameterization schemes. The wide variety of measurements made during the studies, which incorporated tracer and surfactant release experiments, included direct eddy correlation fluxes, detailed wave spectra, wind history, photographic retrievals of whitecap fraction, aerosolsize spectra and composition, surfactant concentration, and bubble populations in the ocean mixed layer. Measurements were made during three cruises in the northeast Atlantic on the RRS Discovery during 2006 and 2007; a fourth campaign has been making continuous measurements on the Norwegian weather ship Polarfront since September 2006. This paper provides an overview of the three projects and some of the highlights of the measurement campaigns.
As part of the U.K. contribution to the international Surface Ocean-Lower Atmosphere Study, a series of three related projects—DOGEE, SEASAW, and HiWASE—undertook experimental studies of the processes controlling the physical exchange of gases and sea spray aerosol at the sea surface. The studies share a common goal: to reduce the high degree of uncertainty in current parameterization schemes. The wide variety of measurements made during the studies, which incorporated tracer and surfactant release experiments, included direct eddy correlation fluxes, detailed wave spectra, wind history, photographic retrievals of whitecap fraction, aerosolsize spectra and composition, surfactant concentration, and bubble populations in the ocean mixed layer. Measurements were made during three cruises in the northeast Atlantic on the RRS Discovery during 2006 and 2007; a fourth campaign has been making continuous measurements on the Norwegian weather ship Polarfront since September 2006. This paper provides an overview of the three projects and some of the highlights of the measurement campaigns.
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