Large-scale Topography Research Articles

Energy Transmission by Barotropic Rossby Waves across Large-Scale Topography

Abstract An analytical study investigates the energy transmission by free, barotropic, linear Rossby waves across a large scale bottom topography when topographic and beta-effects have the same order of magnitude. In open ocean regions which are not characterized by a flat bottom, the change in topography is assumed to occur in only one direction and is described by exponential depth profiles. The solution expressed as a plane-wave streamfunction has a nondivergent energy flux vector. An energy transmission coefficient, which represents the fraction of energy that an incident wave can carry across the topography, is defined as the ratio of the upslope component of the energy flux vector in the transmission region to the one in the incident region. The evaluation of the energy transmission coefficient for a ridge similar to the Mid-Atlantic Ridge shows that the low-frequency waves (of period larger than one week) are strongly reflected and that the ridge acts like a barrier which separates the ocean in two...

Numerical Studies of Major and Minor Stratospheric Warmings Caused by Orographic Forcing

A primitive equation spectral model using spherical harmonics is formulated to study dynamic interactions between the troposphere and stratosphere in association with sudden stratospheric warmings. Using sigma coordinates for five tropospheric layers and log-pressure coordinates for 26 stratospheric and mesospheric layers, separate model equations for each system are combined to form single matrix governing equations. The gradual introduction of large scale topography to balanced initial states representative of observed mean winter conditions in the Northern Hemisphere is used for the generation of planetary waves during 40-day time integrations. Results of these integrations indicate that stratospheric warmings can be simulated by this orographic forcing and that mean momentum flux divergence due to zonal mean motion appears to be an essential mechanism of these simulated sudden warmings. It was found that the strength of the polar night jet can be a determining factor whether a warming becomes 'major' or 'minor'.

On the geometrical conditions for recording X-ray topographs of large crystal slices

Geometrical aspects of large-scale transmission X-ray topography with Kα1 radiation have been considered. It is shown that the height of a crystal slice which can be effectively recorded is given as Heff = 2L(Δλ/λ)1/2, where L is the source-to-sample distance and Δλ is the wavelength difference for the Kα doublet. A geometrical scheme termed the moving-slit simulation has been developed to examine the influence of various experimental parameters such as the widths of source and slits, the involved distances and the adjustment of the sample. The usefulness of this scheme is demonstrated experimentally.

Quasi-geostrophic turbulence in a rotating homogeneous fluid

Abstract Quasi-two-dimensional turbulence is generated in a rotating cylinder by an homogeneous distribution of bottom sources and sinks which oscillate in time. Typical Reynolds numbers reach 2000 and inertial time-scales are about an order of magnitude smaller than the Ekman spin-up time. This allows the genuine inertial effects of the flow field such as the gross spectral evolution of the energy containing eddies to be emphasized. The case of no topography is first discussed. When the fluid is steadily forced, it is observed that the energy containing scale is significantly larger than the forcing scale. Theoretical arguments for that behavior are presented. In the decaying stage the red cascade is observed and rates of interaction are measured. The forcing is next applied over various large scale topographies modelling the geophysical beta effect. The polar beta plane geometry preserves the above spectral characteristics but at the same time introduces anisotropy into the flow pattern. A broad westwar...

A numerical model of the Martian polar cap winds

An investigation of the Martian polar cap winds and their response to a variety of factors is carried out by a series of numerical experiments based on a zonally symmetric primitive equation model. These factors are the seasonal thermal forcing, mass exchange between polar caps and atmosphere, large-scale topography, and polar cap size. The thermal forcing sets up a circulation whose surface winds adjust to achieve angular momentum balance, with low-latitude easterlies and high-latitude westerlies. The maximum westerlies occur roughly where the horizontal temperature gradients are largest. This pattern changes when cap and atmosphere exchange mass. Corriolis forces acting on the net outflow or inflow produce easterlies at the surface during spring (outflow) and westerlies during winter (inflow). Topography appears to have a small effect, but cap size does play a role, the circulation intensity increasing with cap size. Peak surface winds occur when outflow or inflow is a maximum and are 20 m sec −1 during spring and 30 m sec −1 during winter for the northern hemisphere. The model results show that surface winds near the edge of a retreating polar cap are substantially enhanced, a result which is consistent with the Viking observations of local dust storm activity near the edge of the south polar cap during spring. The results also indicate that the surficial wind indicators near the south pole are formed during spring and those near the north pole during winter. The implication is that the high-latitude dune fields in the northern hemisphere are formed at a time when the terrain is being covered with frost. It is therefore suggested that the saltating particles are “snowflakes” which have formed by the mechanism proposed by Pollack et al. The model results for the winter simulation, which have formed by the mechanism transport by large-scale eddies, compare favorably with general circulation model (GCM) calculations. This suggests that the eddy transports may be less important than those associated with the net mass flow, and that 2-D climate modeling may be more succesful for Mars than Earth.

Echo character of off-platform carbonates

A comprehensive study of the nature and distribution of echo types recorded on short-ping (2–5 msec), high-frequency (3.5 kHz) echograms combined with analyses of 100 bottom samples from the off-platform carbonate environment of Northwest Providence Channel (NWPC), Bahamas, has defined qualitative relationships between echo types, sedimentary processes and sediment types. Seven distinct echo types have been observed and their areal distributions mapped. These seven types fall into three larger groups: (I) Distinct; (II) Indistinct; and (III) Hyperbolic. A strong correlation of echo character with sediment type exists for the one distinct (IB) and two indistinct (IIA and IIB) echo types observed. Muddy, pelagic sediments occur in regions characterized by distinct, sharp, continuous, parallel, sub-bottom reflectors (Type IB). Pelagic sediments interbedded with coarse-sediment gravity-flow deposits or slope breccia are characterized by indistinct, discontinuous, intermittent sub-bottom reflectors (Type IIA). Coarse current-winnowed sands or thick biolithoclastic turbidites found near the surface correlate with regions characterized by an indistinct, prolonged echo with no sub-bottom reflectors (Type IIB). Hyperbolic echoes were observed from both erosional and constructional features. The most common echo type (IIIA) in NWPC consists of large, irregular, overlapping hyperbolae with varying vertex elevations above the sea floor which originate from areas of large-scale submarine erosional topography. This echo type is much more common on the slope south of Little Bahama Bank than north of Great Bahama Bank, indicating different sedimentary processes operative along these two deep carbonate bank margins. In contrast, echoes consisting of discrete single hyperbolae with approximately equal vertex elevations above the sea floor (Type IIIG) correlate with in-situ constructional deep-water bioherms (lithoherms). This study demonstrates the usefulness of 3.5-kHz profiles in mapping near-surface off-platform carbonate sediments. The relationships between echo types and sedimentary processes and/or sediment types described here in a region with good bottom-sample control may serve as a model to facilitate rapid mapping of off-platform carbonates in areas where there is less control.

Spin-up of a stratified ocean, with topography

Abstract This paper examines the linear response of a motionless stratified ocean in the presence of large-scale topography aligned north-south to a wind stress turned on at time zero. An unstratified ocean is first examined. Its response differs from the barotropic response discussed by Anderson and Gill (1975, Spin-up of a stratified ocean, with application to upwelling. Deep-Sea Research , 22 , 583–596) in two major respects: a higher frequency ‘basin mode’ dominates much of the solution, and thinning boundary layers can occur to the east of the topography. The position of the topography strongly affects the properties of the solution. When stratification is included, both barotropic and baroclinic responses are possible. The baroclinic response is affected very little by topography, and essentially the baroclinic spin-up resembles the flat-bottom case. The barotropic response, however, is very different from the unstratified case. Initially the response resembles the unstratified case—in particular the high frequency basin modes are unaffected by stratification—but the slow westward propagation of the baroclinic planetary wave leaves in its wake what is essentially a steady, flat-bottom Sverdrup solution for both barotropic and baroclinic modes. This can be interpreted as there being no motion at great depth, which removes the topographic effect.

An observational study of the influence of large-scale mountains on air flow and lee cyclogenesis

It was observed that large-scale topography induced significant orographic modifications in upper airflows. Only 14–26% of upper cold lows were recorded to cross major mountain systems, and the remainder of the cold lows passed them as cold troughs. In the course of prolonged upslope motions over a cordillera, upper-level contours were found to fan out laterally due to the generation of orographic mass-divergence and increased friction, etc. A weak upstream trough was also identified in the upslope side of mountains.

Baroclinic waves in a container with sloping end walls

The effects of sloping upper and lower boundaries on baroclinic waves in a differentially heated rotating fluid annulus have been studied experimentally. Measurements have been made of the internal temperature structure, the transition from axisymmetric to non-axisymmetric flow, the wave numbers and flow types, the drift rates and the heat transfer. The sloping boundaries influence the baroclinic waves in two distinct ways: if they are parallel the energy releasing mechanism is directly affected, while if they are not parallel effects similar to the latitudinal variation of coriolis parameter in a planetary atmosphere (the (3-effect) are introduced. In order to identify essentially non-linear effects the results on the transition from axisymmetric to non-axisymmetric flow, the wave numbers and the drift rates have been compared with an appropriately extended version (by Hide) of Eady’s linear perturbation theory of baroclinic instability. In the case of the transition from axisymmetric to non-axisymmetric flow, when account was taken of concomitant changes in the internal temperature structure, surprisingly good agreement with linear theory was obtained. Within the regular wave regime, the azimuthal wave numbers agreed quite well when the theoretically predicted value was 1, 2 or 3 but for higher predicted wave numbers the observed wave numbers were smaller. In the case of oppositely sloping boundaries the observed drift rates relative to the mean flow were generally 30 % less than that of the unstable waves of the linear theory. Another departure from the linear theory also occurred when the boundaries sloped in opposite senses, namely the occurrence at sufficiently rapid rotation rates of two separate wave trains of limited radial extent and differing wave numbers and drift speeds; one dominated the flow near the inner cylinder and the other dominated the flow near the outer cylinder. This result suggests that the variation of coriolis parameter with latitude can determine the radial extent of the baroclinic waves. Apart from its intrinsic interest as a fluid-mechanical study this work bears on the role played by the (3-effect and large-scale topography in natural systems such as the atmosphere and oceans. It also shows that at the higher rotation rates used the sloping eeopotentials inevitably present in previous laboratory experiments on baroclinic waves have important effects, and in particular explains the failure of previous theories to account for the position of part of the transition from axisymmetric to non-axisymmetric flow.

Planetary scaleの大気運動に及ぼす大規模山岳の影響

Planetary scaleの大気運動に及ぼす大規模山岳の影響

A comparison of Martian albedo features with topography

Martian albedo features photographed from the Earth are compared with the topography observed by Mariner 9. We present three Lambert azimuthal equal-area charts prepared to facilitate the comparison. No correlation between large-scale albedos and topography can be generalized, but several different kinds of relationships seem to be indicated in particular regions. Examples of these are discussed.

Analysis of Large-Scale Martian Topography Variations--I: Data Preparation from Earth-Based Radar, Earth-Based CO2 Spectroscopy, and Mariners 6 and 7 CO2 Spectroscopy

Using direct radar ranging of surface heights on Mars and spectrophotometric observations of absorptions produced by carbon dioxide molecules in the Martian atmosphere, data have been obtained on Martian topographical variations at spatial resolutions ranging from about 100 to 1000 km. These data have been studied and analyzed. As a result, a surface height contour map has been produced which clearly reveals a structural complex of blocks and basins whose distribution enhances the magnitude of low-degree surface harmonics. It is emphasized that Mars possesses unexpectedly pronounced topography which can have important geophysical consequences.

On the effects of large-scale temperature advection in the Martian atmosphere

A theoretical model in which large-scale topography can generate steady winds is analyzed. The wind is determined by the radiative-equilibrium thermal structure, but gradient-level winds are affected to lowest order by the small temperature anomalies induced by large-scale temperature advection. The application of the model to the Martian general circulation is discussed and examples in which Martian parameters are used are presented and analyzed.

The distribution of CO 2 on Mars: A spectroscopic determination of surface topography

A map of the distribution of CO 2 over the surface of Mars has been prepared from spectrophotometric observations of the ν 1 + 2 ν 2 + 3 ν 3 band of CO 2 at 1.05μ. The map covers the equatorial of the planet between latitudes −20° and +40°, stretches from 240°W through the meridian to 160°W and has a nominal precision of ±1 mb. Emphasis is placed on the details of the experimental method and reduction procedure. The pressure map is interpreted in terms of large-scale surface topography (resolution 800–1500 km). The dominance of the second harmonic in zonal topography is clearly evident by the existence of two broad (4000 km), high (12 km) ridges running approximately north-south across the planet seperated by about 180° in longitude. With one minor exception, all bright areas are high, although low areas can be both dark and bright. Syrtis Major is associated with a strong slope but the signiificance of this is doubtful because of the presence of a similar slope in a light area. The spectroscopic map is compared with data from other sources. Good agreement is found with the infrared spectrometer results of Herr et al. (1970), and the radio occultation data, both from Mariners 6 and 7. Comparison with ground-based radar data show good agreemenr at most longitudes but there is a considerable discrepancy between 80°W and 160°W. The most probable cause is found to be a systematic calibration error, incurred in normalizing spectroscopc data from different nughts to a common standard. (The problem is a closure error which can be avoided in the future.) A second map, normalized against radar data, is presented; it does not agree as well with the Mariner results. The physical bases of the radar and spectroscopic methods are compared, and the possibility is discussed of using the discrepancies to provide further inforation. The most promising prospects are the oblateness of the planet, gravitational anomalies, and violent Maatian weather.

MAPS AND METEOROLOGY

The graphic or chart form of presentation is essential to the practice of meteorology, and many different types of maps, charts, and diagrams are used to depict the 3-dimensional characteristics of the atmosphere. This paper concerns base maps required for synoptic meteorology, that is, for the plotting of daily weather data so that they may be analysed and organized into patterns from which a weather forecast may be derived. For this purpose a new series of base maps has been designed for the Canadian Weather Service, in several different scales, and in polar stereographic projection. On the large-scale maps land-water relationships and topography are shown by tints of tan and blue, designed so that differences between tints are perceptible but unobtrusive. A section of a 1:5M. base map is included with this paper to illustrate the design characteristics. La présentation en forme graphique ou diagrammique est essentielle à la pratique de la météorologic et l'on se sert de beaucoup de genres de cartes, de graphiques et de diagrammes pour représenter les caractéristiques, à trois dimensions, de l'atmosphère. Ce document traite de graphiques-base, exigés pour la météorologie synoptique, c'est-à-dire, servant à l'établissement de données sur la température de tous les jours. Ces données servent à l'analyse et à l'arrangement des dessins, par moyen desquels on peut dériver les prévisions atmosphériques. Dans cette intention on a tracé une nouvelle série de cartes-base en plusieurs échelles différentes et en projection polaire-stéréographique pour le service météorologique canadien. Sur les cartes à grande échelle, les connexités eau-terre ainsi que la topographie sont données en nuances jaune et bleu et dessinées de manière à faire voir les différences entre les tintes sans être obstructives. Une section d'une carte-base 1:5M est jointe à ce document pour illustrer les caractéristiques du dessin.

The introduction of topographic and frictional effects in a baroclinic model

The effects of large-scale topography and surface friction on short-range numerical forecasts are investigated using a baroclinic two-parameter prediction model, modified to include forced vertical velocities. Trial forecasts showed that the damping effect of surface friction is reproducible, but only moderate success was achieved in accounting for orographic influences.