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David D. Houghton

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David D. Houghton

Abstract

A quasi-Lagrangian formulation for an inviscid barotropic fluid is presented and shown to afford a convenient basis for analysis of certain ageostrophic jet flows. Material fines serve as the references to delineate the north-south variations in the fluid, and Eulerian representation is used in the east west direction. Several desirable features are shown for the use of material lines in this manner. First, by orienting the lines approximately parallel to the jet axis, flows with finite horizontal curvature may be represented simply. This is illustrated by a development with east-west variations represented by only two harmonies and a mean. The solution of this semi-spectra model agrees closely with a non-spectral numerical solution for over three days. Second, the Lagrangian movement of the material lines gives an indication of non-linear adjustment motions and demonstrates a mixing in the fluid.

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David D. Houghton

Abstract

An incompressible, stratified, hydrostatic, inviscid fluid model is used to demonstrate non-linear effects in the interaction of finite amplitude gravity waves. A statically stable density stratification is approximated by the superposition of ten homogeneous fluid layers with a very deep layer on top. Computations are made using a two-step Lax-Wendroff finite difference system. The solutions reveal intrawave distortions comparable to those predicted by analytical studies of one- and two-layer fluid models. During the interaction of two waves, the solutions show overall changes in wave speed which are of the same magnitude as the variations in wave speed that cause the intrawave distortions. These changes are related differently to the horizontal fluid velocities in the waves.

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David D. Houghton

Abstract

Calculations are made with the NCAR six-layer general circulation model to determine the time evolution of errors initially confined to a region 4000 km in diameter superimposed upon real global data. Three experiments are made to distinguish between the effects of an error located initially on the northern or southern sides of the jet stream or in the tropical area. Results show that the largest error centers generally evolve in the jet stream; however, the propagation rate is much less than advection effects would suggest. Coverage of the Northern Hemisphere is accomplished as much by propagation across the north pole and via the tropical belt as it is via the jet stream. It is not complete even after seven days. As a whole the tropics are more sensitive than the middle latitudes to initial errors. Cross-equatorial effects are most pronounced at and just east of the initial longitude of the error.

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David D. Houghton
and
Robert M. Chervin

Abstract

Vertically-averaged meridional transports of westerly momentum are analyzed in sampled ensembles of January simulations of an NCAR GCM and an equivalent ensemble of five years of observational January data according to a simple time-domain decomposition. Ensemble averages and standard deviations are compared in terms of both zonally-averaged and grid-point presentations for the steady and transient flux components highlighting the relative characteristics of the fundamental time-domain elements. Results from 5 and 2.5° horizontal resolution versions of the model demonstrate the impact of truncation error on model simulations of these flux statistics.

Comparing grid point measures constitutes a more stringent model performance evaluation since regional differences between observed and simulated transports often are found to he considerably larger than zonally-averaged differences. Such regional considerations also reveal substantial differences between model and observations in the location and orientation of transport maxima and minima. Typically the transient flux component is smaller in the model simulations than in the observations although there are some regional exceptions. The steady flux component, however, is generally larger in the model simulations (particularly the 2.5° version) than in the observations and is affected more than the transient component by resolution changes. Analysis of the estimated standard deviations of the flux components shows that the model's inherent variability is typically at least a factor of two lower than the observed interannual variability with substantial regional differences.

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Walter L. Jones
and
David D. Houghton

Abstract

A numerical model of internal gravity waves allows momentum transport by the waves to interact with the mean flow. Momentum deposited at a critical level develops a “shelf” in the mean flow. Mean flow acceleration Doppler-shifts the wave frequency, allowing more penetration of wave energy than expected from linear theory.

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Walter L. Jones
and
David D. Houghton

Abstract

A simple numerical model is used to demonstrate that momentum exchange between wave and mean flow can substantially modify the process of “breaking” of internal gravity waves at great height. The momentum exchange results in appreciable transfer of energy from wave to mean flow.

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David D. Houghton
,
John E. Kutzbach
,
Michael McClintock
, and
David Suchman

Abstract

Sea temperature anomalies which departed from the December climatic mean by approximately 2C off the coast of Newfoundland were inserted into the NCAR six-layer, 5° mesh, general circulation model of the atmosphere in order to test the model's response to small perturbations in sea surface temperature. The response of the model to the anomalies was analyzed with respect to pressure patterns, heat flux, and cyclone frequency, path and intensity. This response was compared with a statistical analysis of the response of the atmosphere to similar sea temperature anomalies based on approximately 80 years of observations as described by Ratcliffe and Murray.

Analyses of the anomaly experiments are preceded by an analysis of the basic (control) statistics for both model and atmosphere. The most pronounced discrepancies between the two were noted in cyclone statistics. A calculation with double horizontal resolution greatly improved the model features. Detailed comparison was complicated by the fact that the model failed to achieve statistical stationarity.

The extensive verification data of Ratcliffe and Murray proved valuable in distinguishing meaningful anomaly responses from those that could be attributed to the many limitations in the model, including a pronounced natural variability. Both warm and cold anomaly cases were tested. Best agreement with observed data was obtained for the case of the warm anomaly; this agreement was most evident during the middle portions of the integrations and then only in the North Atlantic sector. The response in the case with a cold anomaly was not as satisfactory although there were clear distinctions between the warm and cold anomaly cases.

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Bette L. Otto-Bliesner
,
Grant W. Branstator
, and
David D. Houghton

Abstract

A global, spectral, primitive equation model is developed to study the seasonal climatology of the large-scale features of the atmosphere. The model resolution is five equally-spaced sigma levels in the vertical and triangular truncation at wavenumber 10 in the horizontal. Included in the model are: orography; time-varying (but prescribed) sea-surface temperatures, snowcover, and solar declination angle; parameterizations for radiation, convection, condensation, diffusion, and surface transports; and a surface heat budget. The external seasonal forcing of the model atmosphere is composed of sinusoidal time variations in the incoming solar radiation and latitude of the snowline and more complicated variations in the albedo of the snow and the sea-surface temperatures. A five-year seasonal simulation has been analyzed. The model reasonably reproduces the general features of the observed atmospheric circulation, seasonal cycles, interannual variations and hemispheric differences. The success of this low-resolution model in simulating the large-scale features of the atmospheric seasonal cycle illustrates the usefulness of such models for climate studies in conjunction with high-resolution general circulation model simulations.

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Yun-Qi Ni
,
Bette L. Otto-Bliesner
, and
David D. Houghton

Abstract

An analysis is made of the effect of orography on the atmospheric energetics in a low-resolution general circulation model to determine the temporal and scale dependency of these effects. The numerical model is a global, spectral, primitive equation model of the atmosphere with five equally spaced sigma levels in the vertical and triangular truncation at wavenumber 10 in the horizontal. A one-year seasonal simulation of the general circulation without mountains is compared to the results from a five-year seasonal simulation of the general circulation with mountains. The statistical significance of the topographic effects is evaluated by comparing them to magnitudes of model interannual variability determined from the five-year control simulation.

A small, but important, portion of the changes due to topography are significant. At northern extratropical latitudes, the increases of eddy activities and baroclinic instability in summer resulting from incorporation of the effect of the mountains give rise to significantly increased eddy components of atmospheric energetics and the conversion from eddy available potential energy to eddy kinetic energy. These increases are generally present and significant at each wavenumber and for the overall stationary component. In winter, topography significantly increases the zonal kinetic energy and dissipation. Examination of the individual zonal spectral components for winter reveals that topography increases long-wave energies and their transfer, but with proportional decreases at medium waves, resulting in little change in the total eddy components. A similar compensation occurs between the stationary and transient components of the heat transport. Less pronounced topographic features at tropical and southern extratropical latitudes result in fewer significant changes due to topography.

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