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- Author or Editor: T. J. SIMONS x
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Abstract
Data from the 1972 International Field Year on Lake Ontario have been used to test the performance of three-dimensional hydrodynamical models of large lakes. The models vary with regard to computational details, but their common purpose is to predict water levels, currents and temperatures in the Great Lakes on the basis of prescribed atmospheric conditions. The period of observations dealt with in the present paper includes the passage of tropical storm Agnes during the latter part of June 1972.
Data from the meteorological buoy network on Lake Ontario are combined with routine observations at first-order synoptic stations around the lake to obtain hourly values of wind-stress and pressure fields. Initial temperature distributions as a function of depth and horizontal coordinates are derived from quasi-synoptic ship cruises. Verification of model results is based on hourly values of water level data from stations on the perimeter of the lake and currents and temperatures measured by the buoy network at four levels below the water surface. To determine the predictability of different time scales, both the data records and the model output have been treated by digital filters with sharp cutoffs at physically significant frequencies.
For periods of weak stratification the model is found to be most sensitive to parameters related to the vertical flux of momentum. Satisfactory simulations of observed water levels and currents require wind-stress coefficients considerably larger than those obtained from direct flux measurements. Short-term variations of vertical current profiles at individual stations can be modeled adequately by recourse to classical dynamic stability theories. Whereas inertial oscillations are governed largely by the magnitude of the vertical diffusion of momentum, the internal fluxes of momentum can be varied by an order of magnitude without changing the character of the solutions for time scales of a day or more.
Abstract
Data from the 1972 International Field Year on Lake Ontario have been used to test the performance of three-dimensional hydrodynamical models of large lakes. The models vary with regard to computational details, but their common purpose is to predict water levels, currents and temperatures in the Great Lakes on the basis of prescribed atmospheric conditions. The period of observations dealt with in the present paper includes the passage of tropical storm Agnes during the latter part of June 1972.
Data from the meteorological buoy network on Lake Ontario are combined with routine observations at first-order synoptic stations around the lake to obtain hourly values of wind-stress and pressure fields. Initial temperature distributions as a function of depth and horizontal coordinates are derived from quasi-synoptic ship cruises. Verification of model results is based on hourly values of water level data from stations on the perimeter of the lake and currents and temperatures measured by the buoy network at four levels below the water surface. To determine the predictability of different time scales, both the data records and the model output have been treated by digital filters with sharp cutoffs at physically significant frequencies.
For periods of weak stratification the model is found to be most sensitive to parameters related to the vertical flux of momentum. Satisfactory simulations of observed water levels and currents require wind-stress coefficients considerably larger than those obtained from direct flux measurements. Short-term variations of vertical current profiles at individual stations can be modeled adequately by recourse to classical dynamic stability theories. Whereas inertial oscillations are governed largely by the magnitude of the vertical diffusion of momentum, the internal fluxes of momentum can be varied by an order of magnitude without changing the character of the solutions for time scales of a day or more.
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Abstract
The development of atmospheric cyclones is studied from the viewpoint of the instability of large-scale wave perturbations superimposed on a zonal current. The stability properties of the observed mean January flow are investigated and the linear results are extended to include the effects of nonlinear processes on the growth of a cyclone-scale wave. An hemispheric model is employed in this investigation and solutions are obtained by spectral techniques.
It is found that the observed atmospheric zonal current is highly unstable in a hydrodynamic sense. The instability is of a baroclinic character with barotropic stabilizing effects. The nonlinear computations show that the growth of the most unstable waves is brought to a halt when the perturbation kinetic energy reaches a level consistent with atmospheric observations. The barotropic energy exchanges are found to play a major role in this process by feeding a large amount of kinetic energy into the zonal flow when the baroclinic energy conversions reach a maximum. The damping effect of the nonlinear processes on the growth of the unstable wave is found to be slightly reduced when the horizontal resolution of the model is increased in either zonal or latitudinal directions.
Abstract
The development of atmospheric cyclones is studied from the viewpoint of the instability of large-scale wave perturbations superimposed on a zonal current. The stability properties of the observed mean January flow are investigated and the linear results are extended to include the effects of nonlinear processes on the growth of a cyclone-scale wave. An hemispheric model is employed in this investigation and solutions are obtained by spectral techniques.
It is found that the observed atmospheric zonal current is highly unstable in a hydrodynamic sense. The instability is of a baroclinic character with barotropic stabilizing effects. The nonlinear computations show that the growth of the most unstable waves is brought to a halt when the perturbation kinetic energy reaches a level consistent with atmospheric observations. The barotropic energy exchanges are found to play a major role in this process by feeding a large amount of kinetic energy into the zonal flow when the baroclinic energy conversions reach a maximum. The damping effect of the nonlinear processes on the growth of the unstable wave is found to be slightly reduced when the horizontal resolution of the model is increased in either zonal or latitudinal directions.
Abstract
Model calculations and current meter observations are analyzed in the spectral domain and in the time domain to investigate effects of topographic waves on the response of nearshore currents to wind. The spectral response is computed for a shelf forced by a progressive atmospheric wave, and effects of friction and alongshore depth variations are considered. Comparisons are made with results for standing atmospheric waves and with the response of closed basins forced by winds uniform in space and periodic in time. It is found that coastline curvature is rather unimportant for the scales under consideration, and that the alongshore wind component represents the crucial forcing. Spectral model results are then compared with current meter spectra to show the resonant topographic wave character of the response of currents to wind.
Time series of observed and computed nearshore currents are compared, and the alongshore momentum balances are considered for models with and without topographic wave effect. It is found that simple models may produce results which seem comparable to those obtained from more complete models, but it is concluded that such simple models are basically erroneous in concept.
Abstract
Model calculations and current meter observations are analyzed in the spectral domain and in the time domain to investigate effects of topographic waves on the response of nearshore currents to wind. The spectral response is computed for a shelf forced by a progressive atmospheric wave, and effects of friction and alongshore depth variations are considered. Comparisons are made with results for standing atmospheric waves and with the response of closed basins forced by winds uniform in space and periodic in time. It is found that coastline curvature is rather unimportant for the scales under consideration, and that the alongshore wind component represents the crucial forcing. Spectral model results are then compared with current meter spectra to show the resonant topographic wave character of the response of currents to wind.
Time series of observed and computed nearshore currents are compared, and the alongshore momentum balances are considered for models with and without topographic wave effect. It is found that simple models may produce results which seem comparable to those obtained from more complete models, but it is concluded that such simple models are basically erroneous in concept.
Abstract
A critical evaluation is presented of the conventional view that long-term mean circulation of homogeneous basins may be identified with the quasi-steady response of linear models to atmospheric forcing. Based on climatological wind records for the Great Lakes region, it is shown that the mean circulation for unstratified seasons is dominated by rectified effects of nonlinear topographic wave interactions. It is also shown that the dependence of the rectified flow on the frequency of periodic forcing is similar to the linear topographic response to wind with resonance characteristics for certain frequencies determined by topography and friction.
Abstract
A critical evaluation is presented of the conventional view that long-term mean circulation of homogeneous basins may be identified with the quasi-steady response of linear models to atmospheric forcing. Based on climatological wind records for the Great Lakes region, it is shown that the mean circulation for unstratified seasons is dominated by rectified effects of nonlinear topographic wave interactions. It is also shown that the dependence of the rectified flow on the frequency of periodic forcing is similar to the linear topographic response to wind with resonance characteristics for certain frequencies determined by topography and friction.
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Abstract
Observations of winds and currents along the northshore of Lake Ontario are analyzed to evaluate effects of topographic wave propagation on wind-driven currents. Lagged cross-correlations and spectral transfer functions between winds and currents are found to be consistent with the mechanism of resonant topographic-wave response in the presence of bottom friction. Transfer function models in the time domain are shown to explain 70 to 80 percent of the variance of observed currants.
Abstract
Observations of winds and currents along the northshore of Lake Ontario are analyzed to evaluate effects of topographic wave propagation on wind-driven currents. Lagged cross-correlations and spectral transfer functions between winds and currents are found to be consistent with the mechanism of resonant topographic-wave response in the presence of bottom friction. Transfer function models in the time domain are shown to explain 70 to 80 percent of the variance of observed currants.
Abstract
The baroclinic instability problem is formulated as an initial value problem to evaluate the effects of the initial configuration of the wave perturbation. The vertical shape of the initial perturbation is found to be as important as its wavelength in determining the energy conversions during the early stages of its development. The general character of the solution of the initial value problem is compared with normal mode studies of baroclinic instability. It is concluded that the initial value formulation bridges the gap between hydrodynamic stability theory and synoptic studies of cyclone development in the atmosphere.
Abstract
The baroclinic instability problem is formulated as an initial value problem to evaluate the effects of the initial configuration of the wave perturbation. The vertical shape of the initial perturbation is found to be as important as its wavelength in determining the energy conversions during the early stages of its development. The general character of the solution of the initial value problem is compared with normal mode studies of baroclinic instability. It is concluded that the initial value formulation bridges the gap between hydrodynamic stability theory and synoptic studies of cyclone development in the atmosphere.
Abstract
Current meter observations in Lake Ontario covering the 140-day period from 4 November 1982 to 23 March 1983 are used to evaluate the performance of circulation models for different time scales. The measurements were taken in a single cross-section of the lake with sufficiently high resolution to verify conservation of total water transport through the section. The results indicate that a typical linear hydrodynamic model can reproduce short- and medium-term circulations induced by wind variations, but that nonlinear effects must be included to simulate seasonal-mean current patterns.
Abstract
Current meter observations in Lake Ontario covering the 140-day period from 4 November 1982 to 23 March 1983 are used to evaluate the performance of circulation models for different time scales. The measurements were taken in a single cross-section of the lake with sufficiently high resolution to verify conservation of total water transport through the section. The results indicate that a typical linear hydrodynamic model can reproduce short- and medium-term circulations induced by wind variations, but that nonlinear effects must be included to simulate seasonal-mean current patterns.
Abstract
The dynamics of downwelling fronts observed along the steep and elongated southern shore of Lake Ontario are investigated by considering the nonlinear response to surface forcing of one-layer and two-layer fluids on a rotating, semi-infinite plane. Analytical and numerical solutions for idealized situations exhibit typical characteristics of the observed fronts such as offshore propagation and periodic recurrence with near-inertial periods. A numerical simulation of an actual downwelling episode in Lake Ontario shows that this type of model reproduces the observed behavior of the thermocline as well as the associated oscillatory currents. It is concluded that the fronts are to be visualized as internal surges associated with the oscillatory rather than the quasi-geostrophic response of a lake to wind.
Abstract
The dynamics of downwelling fronts observed along the steep and elongated southern shore of Lake Ontario are investigated by considering the nonlinear response to surface forcing of one-layer and two-layer fluids on a rotating, semi-infinite plane. Analytical and numerical solutions for idealized situations exhibit typical characteristics of the observed fronts such as offshore propagation and periodic recurrence with near-inertial periods. A numerical simulation of an actual downwelling episode in Lake Ontario shows that this type of model reproduces the observed behavior of the thermocline as well as the associated oscillatory currents. It is concluded that the fronts are to be visualized as internal surges associated with the oscillatory rather than the quasi-geostrophic response of a lake to wind.
