Search Results
You are looking at 1 - 10 of 27 items for
- Author or Editor: T. J. Simons x
- Refine by Access: All Content x
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
Numerical computations of water circulations, combined with observed temperature distribution, permit estimates of heat transports in Lake Ontario throughout the 1972 International Field Year. Weekly estimates of heat budgets and surface fluxes are available from ship surveys and meteorological observations, thus allowing for validation of model results. Whereas the hydrodynamic model employs a grid mesh of 5 km, the present verification study is based on 21 lake zones, with the mean depth contour separating shore zones from mid-lake areas.
Agreement between simulations and observations ranges from satisfactory to poor, as a function of both time and space. The model is found to perform best along the north shore and in the fall season. Analysis of the results points at low model resolution and gaps in the observational network as the major causes of failure to reproduce south shore heat transports. Persistent overestimates of heat advection by vertical circulations are ascribed to the present structure of the hydrodynamic model.
A brief discussion is included of the present heat transport calculations in the context of estimating vertical mixing properties for different lake zones.
Abstract
Numerical computations of water circulations, combined with observed temperature distribution, permit estimates of heat transports in Lake Ontario throughout the 1972 International Field Year. Weekly estimates of heat budgets and surface fluxes are available from ship surveys and meteorological observations, thus allowing for validation of model results. Whereas the hydrodynamic model employs a grid mesh of 5 km, the present verification study is based on 21 lake zones, with the mean depth contour separating shore zones from mid-lake areas.
Agreement between simulations and observations ranges from satisfactory to poor, as a function of both time and space. The model is found to perform best along the north shore and in the fall season. Analysis of the results points at low model resolution and gaps in the observational network as the major causes of failure to reproduce south shore heat transports. Persistent overestimates of heat advection by vertical circulations are ascribed to the present structure of the hydrodynamic model.
A brief discussion is included of the present heat transport calculations in the context of estimating vertical mixing properties for different lake zones.
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
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
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.
Abstract
Data from the 1972 International Field Year on Lake Ontario are used to test the performance of three-dimensional hydrodynamic models of large lakes. This study is a sequel to a previous report concerning the quasi-homogeneous model simulation of an episode associated with tropical storm Agnes during the latter part of June 1972. The present paper is concerned with the stratified model simulation of the circulation of Lake Ontario during and after a storm on 9 August 1972. In addition, this paper discusses the temperature predictions and beat content changes for both episodes.
With regard to time scales greater than the inertial period, the water levels and currents computed under stratified conditions agree with observations to the same extent as under homogeneous conditions. Stratification appears to exert an appreciable effect on the, circulation, but it is difficult to separate baroclinic and barotropic effects because of their interactions. For intermediate time scales, the quality of temperature predictions appears acceptable with reference to advective heat transports. Longer term mechanisms of thermocline formation, maintenance and erosion are not considered here in view of the proposed use of this model in conjunction with an operational monitoring program. Short-term dynamical effects with the associated phenomena of internal waves are not well simulated in the present experiments because of the choice of model parameters.
Abstract
Data from the 1972 International Field Year on Lake Ontario are used to test the performance of three-dimensional hydrodynamic models of large lakes. This study is a sequel to a previous report concerning the quasi-homogeneous model simulation of an episode associated with tropical storm Agnes during the latter part of June 1972. The present paper is concerned with the stratified model simulation of the circulation of Lake Ontario during and after a storm on 9 August 1972. In addition, this paper discusses the temperature predictions and beat content changes for both episodes.
With regard to time scales greater than the inertial period, the water levels and currents computed under stratified conditions agree with observations to the same extent as under homogeneous conditions. Stratification appears to exert an appreciable effect on the, circulation, but it is difficult to separate baroclinic and barotropic effects because of their interactions. For intermediate time scales, the quality of temperature predictions appears acceptable with reference to advective heat transports. Longer term mechanisms of thermocline formation, maintenance and erosion are not considered here in view of the proposed use of this model in conjunction with an operational monitoring program. Short-term dynamical effects with the associated phenomena of internal waves are not well simulated in the present experiments because of the choice of model parameters.
Abstract
No abstract available.
Abstract
No abstract available.
Abstract
No abstract available.
Abstract
No abstract available.
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
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.
