Showing papers by "Geophysical Fluid Dynamics Laboratory published in 1971"

A Dynamical Model of the Stratospheric Sudden Warming

Abstract: The dynamics of the stratosphere sudden warming phenomenon is discussed in terms of the interaction of vertically propagating planetary waves with zonal winds. If global-scale disturbances are generated in the troposphere, they propagate upward into the stratosphere, where the waves act to decelerate the polar night jet through the induction of a meridional circulation. Thus, the distortion and the break-down of the polar vortex occur. If the disturbance is intense and persists, the westerly jet may eventually disappear and an easterly wind may replace it. Then “critical layer interaction” takes place. Further intensification of the easterly wind and rapid warming of the polar air are expected to occur as well as weakening of the disturbance. The model is verified by numerical integrations of the adiabatic-geostrophic potential vorticity equation. Computed results possess features similar to those observed in sudden warming phenomena.

Simulation of climate by a global general circulation model

Abstract: The primitive equations of motion in spherical coordinates are integrated with respect to time on global grids with mean horizontal resolutions of 500 and 250 km There are nine levels in the models from 80 m to 28 km above the ground The models have realistic continents with smoothed topography and an ocean surface with February water temperatures prescribed The insolation is for a Northern Hemisphere winter In addition to wind, temperature, pressure, and water vapor, the models simulate precipitation, evaporation, soil moisture, snow depth, and runoff The models were run long enough beyond a state of quasi-equilibrium for meaningful statistics to be obtained Time means of meteorological and hydrological quantities computed by the models compare favorably with observed climatic means For example, the thermal structure of the model atmosphere is very similar to that of the actual atmosphere except in the Northern Hemisphere stratosphere; and the simulated distributions of the major arid reg

Parameterized processes in the surface boundary layer of an atmospheric circulation model

Abstract: A general circulation model is modified successively by adding five new features in the lower boundary layer; namely, (i) a different roughness parameter over land and sea, (ii) a Monin-Obukhov type treatment of the turbulent transfer process in the constant-flux layer, (iii) a Richardson number dependent parameterization for Ekman layer process, (iv) a diurnal variation of insolation, and (v) heat conduction into the soil. To assess the effect of each, the experiments are repeated by gradually increasing the complexity of the model. The circulation model has relatively low horizontal grid resolution and 9 vertical levels, and it is applied to a winter case for 14 day predictions. The results indicate that the sophistication of the boundary layer physics does not produce a particularly large effect on the synoptic scale prediction until about 7 days. Its effect may become large after 10 days. One noteworthy result is that the effect of the diurnal variation of insolation is not great for the free atmosphere but an influence is effectively transferred in the vertical, if the Richardson number dependent parameterization for the Ekman layer process is incorporated. The entire study is a preliminary test, the purpose of which is to determine the relative magnitude of each effect. A detailed comparison with observed data was not attempted.

The Observed Annual Cycle in the Meridional Transport of Atmospheric Energy

Abstract: The annual cycles in the atmospheric storage and in the meridional transport of energy are discussed. The calculations are based on a five-year sample from more than 500 radiosonde stations mainly located in the Northern Hemisphere. All statistics represent values integrated vertically between the earth's surface and 75 mb and horizontally along a latitude circle. Several new and interesting features of the eddy and mean transports of potential energy, sensible heat, latent heat and kinetic energy become apparent by the breakdown according to calendar month. In December through February more than half of the sensible heat is transported poleward by the standing eddies. The transient eddy heat flux does not peak in winter but in April and November. The strong annual cycle in the tropical Hadley circulation does not contribute to the poleward transfer of energy for the year as a whole.

Baroclinic annulus waves

Abstract: The thermally driven motion of water contained in a rotating annulus of square cross-section and having a free surface is investigated by numerical integration of the three-dimensional non-linear Navier–Stokes equations. The nature of steady wave flow is examined in detail and a comparison made with the corresponding axisymmetric solution in parameter space.The steady wave solution proves to be consistent kinematically, dynamically and energetically with Lorenz's hypothesis that the wave can be attributed to the baroclinic instability mechanism. The deviaboricThe deviation from the zonal mean. wave possesses some of the characteristics of the theoretical Eady wave and it is possible to define the complete deviaboric wave structure by means of two-dimensional quasi-phase, amplitude diagrams. These diagrams may also typify the nature of certain solutions to the non-separable baroclinic instability problem.The wave motion is almost completely independent of the side boundary layers which make little contribution to the characteristics and energetics of the deviakoric flow. These side layers are approximately axisymmetric and appear qualitatively indistinguishable from their counterparts in the axisymmetric solution. However, significant Ekman layer features appear in the deviatoric wave structure.Away from the boundaries the dynamical balance of terms is hydrostatic and quasi-geostrophic with changes of vertical vorticity influenced by stretching and viscous diffusion. Heat conduction is completely unimportant except in the side boundary layers.The angular momentum transport by the deviatoric motion is largest at the free surface and is mainly against the angular momentum gradient. A strong outward deviatoric flux of momentum is found in the Ekman layer.The dissipation of deviatoric kinetic energy occurs in the Ekman layer and jet whilst most of the dissipation of the mean kinetic energy occurs in the boundary layer of the inner wall.The large differences between the axisymmetric and zonal mean states indicate that linear baroclinic instability analysis of the axisymmetric state is not strictly relevant to an understanding of the wave formation. The character of the wave suggests that the mean environment with which the deviatoric wave interacts is the wave-present zonal mean state. Only a non-linear finite amplitude baroclinic instability analysis (as yet undeveloped) could possibly explain the wave formation.

A study of the hydrology of eastern north america using atmospheric vapor flux data

Abstract: The atmospheric water vapor flux divergence and certain aspects of the water balance of Eastern North America are investigated, using data from the period May 1, 1958, to Apr. 30, 1963. Mean monthly values of evapotranspiration and storage change are computed as residuals, using measured values of vapor flux divergence, precipitation, and streamflow. Computations are performed for regions varying in size from 42 × 105 km2 to approximately 5 × 105 km2. The results for the smaller areas, which are the least reliable, are critically examined. Computed values of evapotranspiration and storage change are compared with the climatological estimates Thornthwaite Associates and Budyko. The Thomthwaite climatic water balance data appear to overestimate , the difference between precipitation and evapotranspiration, during winter and underestimate it during summer. Budyko's values of evapotranspiration generally show a slightly smaller seasonal variation and appear to lead the values obtained from the atmosp...

Ocean tracer distributions: Part I. A preliminary numerical experiment

Abstract: A numerical experiment has been carried out to determine the three-dimensional distributions of dissolved oxygen and radiocarbon as well as the fields of temperature, salinity, and velocity in an ocean driven by wind and thermohaline processes. The aim in this preliminary calculation was to test the usefulness of a tracer model for studying the origin of water masses in the sea and to provide a framework for looking further into the nature of the sources, age, and circulation of the abyssal ocean. Much of our knowledge about circulation patterns and mixing in the sea has come from examining the distribution of properties in simplified one and two dimensional models. In this investigation tracer studies are extended to three dimensions using a numerical model of the ocean circulation of the kind described by Bryan (1969). With a given set of parameters and boundary conditions governing the model ocean, a calculation is made to predict the distributions of four “tracers”, temperature, salinity, dissolved oxygen, and radiocarbon, as well as the structure of the circulation. Based upon this first experiment some important conclusions can be made regarding those features necessary in any model to satisfactorily account for observed distributions. Part I describes the physical model and its numerical analogue as well as the numerical techniques used in finding a steady state. The results of an experiment with tracer eddy diffusivities K = 1 cm 2 /sec (vertical) and A H = 5times10 7 cm 2 /sec (horizontal) are examined in some detail to understand the time scales involved for the deep sea as well as the shallower regions. The patterns of flow and the distributions of the four tracers are related to each other by determining the roles of diffusion, advection, and decay processes in the model. Comparisons are made between observed and predicted surface fluxes of heat and evaporation minus precipitation in order to ascertain the effect of the boundary conditions on the nature of the distributions. These results suggest the direction further studies should take in constructing realistic models of the ocean. Calculations with smaller diffusivities and other boundary conditions are underway and will be reported in Part II. DOI: 10.1111/j.2153-3490.1971.tb00583.x

The assimilation of past data in dynamical analysis. II

Abstract: Systems for continuous data assimilation are presented and discussed. The balanced barotropic equation is used as an example of a dynamical (evolution) equation. In the first method, geopotential height data are assimilated into the model's computation, as the data become available, without any modification. It turns out that this leads to intolerable growth of error, suggesting the necessity of filtering the input information or alleviating discrepancies between the newly injected data and the prediction fields. In the second method, an optimal assimilation is attempted by applying adequate weights to the input data, which minimize statistically the mean-square difference between the estimate and the true solution. The analysis result is acceptable. But the magnitude of the error reduction is comparable to that of the previous synthesis analysis discussed in Part I of this paper. In the assimilation process we employed, the quality of the analysis seems to be determined by the characteristics of the measurements, i.e., density, distribution and magnitude of error, and also by the rate of inherited error growth in the dynamical prediction. DOI: 10.1111/j.2153-3490.1971.tb00578.x

Bénard convection in a rotating fluid

Abstract: The steady nonlinear regime of Benard convection in a uniformly rotating fluid is treated using a two-dimensional primitive-equation numerical model with rigid boundaries. Quantitative comparisons ...

Novel Correlations between Global Features of the Earth's Gravitational and Magnetic Fields: Further Statistical Considerations

Abstract: Novel Correlations between Global Features of the Earth's Gravitational and Magnetic Fields: Further Statistical Considerations

Viscosity of the earth's core

Abstract: ESTIMATES of the coefficient of kinematical viscosity v of the Earth's core that are given in the geophysical literature range from 10−3 cm2 s−1, the viscosity of molten iron at ordinary pressures, to an upper limit of 109 cm2 s−1 based on the observation that compressional waves traverse the core without suffering appreciable attenuation1. I shall argue that the existence of a statistically significant correlation between global features of the Earth's gravitational and magnetic fields2–4 strongly implies that the effective value of v (that is eddy plus molecular) does not exceed 106 cm2 s−1.

Magnetohydrodynamic Oscillations of Neutron Stars

Abstract: FLUID motions in neutron stars have been investigated theoretically several times1–3, but the problem of free magnetohydrodynamic oscillations has evidently not been considered previously, notwithstanding its possible importance in connexion with the interpretation of pulsar observations4–7, especially the modulation of pulse rates8,9. The arguments put forward in the letter indicate that the spectrum of these oscillations i would typically range in period from 0.5 to several powers of f ten times the rotation period of the star.