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  • Author or Editor: Don L. Boyer x
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Don L. Boyer
and
Lijun Tao

Abstract

This study is an experimental investigation of impulsively started, linearly stratified flow over long ridges of triangular and cosine-squared cross sections. The experiments employ a rigid-lid boundary confining the fluid from above. Emphasis is given to the temporal development, as well as to the fully established character of the wake. Both qualitative and quantitative measures of the characteristics of the flow are introduced and thew are studied for a wide range of the system parameters, which include the internal Froude number, F i Reynolds number, Re ridge height to width ratio, G> d and ridge height to fluid depth ratio, G h

The governing equations for the laboratory experiments are related to those of the atmosphere. Flow regime diagrams based on the mode of the vertical wave structure are presented and the relationship of the observations to linear theory discussed. For the lowest vertical wave mode the constant phase lines of the Ice waves have a vertical slope implying total reflection of the waves by the rigid lid. At higher wave modes the constant phase lines slope upstream, indicating that some of the upward propagating momentum flux is absorbed by the lid. Experiments are presented for certain ranges of dimensionless parameter space which show the lee wave field breaking into turbulence and in certain cases subsequently relarminarizing.

Observables in the tee wave pattern are introduced, including a normalized wavelength, λ/D, and wave amplitude, a/hdimensioniess downstream distance to which significant oscillatory motion in the wake occurs, L/D, and a normalized vertical location of the principal rotor, z r /h, and measurements of these for a range of system parameters are presented.

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Don L. Boyer
and
Rui-Rong Chen

Abstract

An analysis concerning the modelling of large-scale atmospheric motions past orographic features in a linearly stratified rotating laboratory experiment is conducted; it is concluded that for an ƒ-plane model the similarity criteria include. matching the Rossby, Burger and Ekman numbers as well as a mountain height function normalized on the fluid depth. The fluid depth to topographic width parameter is not of zeroth order importance, and this allows for the use of distorted laboratory topographies; i.e., the laboratory model can employ exaggerated vertical to horizontal length scale ratios.

Experiments are conducted for the westerly f-plane flow over a model of the Rocky Mountains for a range of parameters appropriate for the atmosphere. Horizontal streamline patterns at various depths and at ranges of the system parameters are presented and analyzed quantitatively. The experiments demonstrate ~ qualitative agreement concerning the ridge over the mountain, the downstream trough to the ease of the mountains, and the general orientation of the ridge and trough. The experiments also show that relative to an observer moving with the mean wind, a closed cyclonic eddy is found to the southeast of the central portion of the mountains. This cyclonic disturbance is located and farther to the northeast as the Rossby number is increased. Furthemore, a stationary anticyclonic vortex is located along the crest just to the north of the mountain center, the location of this anticyclone is relatively insensitive to the Rossby number.

Experiments are also presented for vertical cross-section motions in the southern. central and northern regions of the model. These demonstrate the nature of the wake pattern, especially the vertical motion field Finally, experiments concerning the eastward advection of a trough with a cut-off low over the central portions of the model am presented. These show a weak northeasterly drift of the cut-off low upstream of the mountains and a sharp veer to the south on passing the mountain crest; the core of the cut-off low accelerates in passing over the mountains and then decelerates in the lee. These laboratory results are qualitatively similar to atmospheric observations of the advection of cut-off low across the Rocky Mountains.

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Don L. Boyer
and
Rui-Rong Chen

Abstract

The dimensionless governing equations and boundary conditions for the atmosphere are compared with those for a linearly stratified rotating dishpan laboratory experiment; by doing so a set of similarity criteria are determined. Model experiments on the effects of Greenland, the Rocky Mountains and Tibet on a uniform shear zonal flow in the Northern Hemisphere are presented. The laboratory model qualitatively simulates such semipermanent features of the Northern Hemisphere circulation as the Aleutian and Icelandic lows, the ridgesand troughs in the vicinity of the Rocky Mountains and Tibet and the shedding of the "southwest eddy" in the lee of Tibet.

Although the background flow is steady, the large-scale disturbances caused by the mountains are unsteady and have an inherent periodicity equal to the time required for a fluid parcel to make a single 'circuit around the dishpan (globe). The strengths of the Aleutian and Icelandic lows, for example, oscillate with this Iiod; the lows are out of phase in the sense that when one is weak the other is strong and vice versa. A number of other correlations between various regions of the flow field are also noted.

Removal of the model of Tibet does not greatly affect ihe qualitative nature of the general flow pattern. For example, the Aleutian and Icelandic lows remain as distinct entities, although their strengths and locations are altered somewhat, and the general character of the flow in the vicinity of the Rocky Mountains remains essentially unchanged. On the other hand, removal of the Rocky Mountains eliminates the Aleutian and Icelandic lows as separate entities. The joint effect ofthe Rocky Mountains and Tibet is to deflect the streamlines in the higherlatitudes toward the north, thus causing deeper Aleutian and Icelandic lows and locations of these features which are more similar to observations in the atmosphere than if either of these features is absent. The experiments show in a quite straightforward fashion the important effects mountains have on the formation of some of the semipermanent features of the Northern Hemisphere.

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Rui-Rong Chen
,
Don L. Boyer
, and
Lijun Tao

Abstract

Laboratory experiments concerned with surface cooling, were conducted to simulate the surface wind patterns and the free atmosphere general circulation in the vicinity of Antarctica. The principal dynamical similarity parameter is shown to be RO T /S, where Ro T is the thermal Rossby number and S the Burger number. At parameter values appropriate to the atmosphere, the physical model experiments led to a surface drainage flow and an accompanying polar cyclone that had characteristics similar to atmospheric observations. The simulated polar cyclone contained two low centers and planetary wavelike structures. The interior streamlines near the cyclone centers tended to follow the continental height contours. In the laboratory simulations, eastward-propagating wave troughs were periodically generated in the vicinity of 110°E and developed maximum strengths in the vicinity of the Ross Ice Shelf at 160°W before dissipating by 70°W. Jets in the polar cyclone were observed over regions of the model Antarctic continent having large slope. The boundary drainage flows spread out from the interior of the continent and concentrated in several valleys leading to the oceans. On approaching the model ocean, the drainage flows tended to move around the edge of the continent in an anticlockwise pattern (i.e., an anticyclone) with anticyclonic spiral tongues spreading to the surrounding ocean regions. Experiments conducted by varying Ro T and S, while fixing Ro T /S, demonstrated that the strength and areal extent of the polar cyclone do not vary greatly in what is in effect a change of season. The results demonstrated that the thermal forcing of the Antarctic continent and the unique nature of the Antarctic orography are important features in determining the principal characteristics of the continental surface winds and the general circulation of the high-latitude Southern Hemisphere atmosphere.

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Rui-Rong Chen
,
Neil S. Berman
,
Don L. Boyer
, and
Harindra J. S. Fernando

Abstract

Laboratory experiments were conducted to simulate the diurnal heating-cooling cycle in the vicinity of a ridge of constant cross section. In the model the fluid is a water solution stratified with salt to simulate the background stratification of the atmosphere. The flow is driven by recirculating water of a controlled temperature beneath the model; the model surface temperature is thus varied in a specified way to simulate the surface heating by solar insolation during the daytime hours and surface cooling by radiation during the nighttime.

The pertinent similarity parameters are shown to be G c , for daytime convective flow and G d for nocturnal flow; here G c = H b /H c , G d = H b /H d , where H b , is the mountain height, H c the neutral buoyancy height of free convection. and H d the characteristic thickness of the nighttime drainage flow. The model demonstrates some of the principal features of thermally driven mountain circulations, including daytime upslope winds and nocturnal downslope drainage flows. The spatial and temporal structures of these motion fields are delineated, with the following being among the most important observations: (i) during the daytime, the upslope convective flow in the vicinity of the mountain tends to suppress convective turbulence over the horizontal plains; (ii) during the early evening, horizontal jets, with the principal one directed toward the mountain, develop above the mountain surface, and vortices in the vertical cross section develop both above and below the jets, following the collapse of the convective motion over the mountain; and (iii) in the evening, a downslope drainage flow is initiated following the establishment of a vertical vortex on the mountain slope and under the jet.

Quantitative experimental observations are made, which demonstrate the variation of various flow observables with the pertinent similarity parameters. These results are applied to the atmosphere following similarity relations between the physical model and the atmosphere. The predicted characteristic speeds and length scales of the daytime upslope flow and the nocturnal drainage flow for typical atmospheric parameters are in reasonable agreement with limited field observations.

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