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1.
《大气与海洋》2013,51(4):405-422
Abstract To develop an understanding of the complex internal tidal phenomena observed near and inside Dixon Entrance, an idealized numerical model was developed for the area, which explores the influence of various topographic features on the scattering of internal tides. The model uses a non‐linear, two‐layered, frictionless finite difference formulation of the shallow water equations and is forced by a barotropic wave over simplified topography. It was found that the main bathymetric features responsible for the generation of semi‐diurnal internal tides are the steep continental slope together with the orientation of Dixon Entrance. The prevalent baroclinic wave pattern, which is similar to the one found by Buchwald (1971), suggests that the western end of Dixon Entrance can be considered as an internal tide generation region for the open ocean. Use of the simple model allows easy identification of the generated waves. When the model is run with a non‐flat channel it reproduces features observed inside Dixon Entrance. 相似文献
2.
Abstract In this study, a 24‐h high‐resolution numerical prediction of a prefrontal squall line associated with the 14 July 1987 Montreal flood is employed to investigate the origin and role of mesoscale gravity waves in the development of the squall system. The 24‐h integration using an improved mesoscale version of the Canadian regional finite‐element model is first validated against available observations; then non‐observable features are diagnosed to reveal the relationship between deep convection and gravity wave events. It is shown that the model reproduces well many aspects of the squall line, such as the propagation and organization of the convective system, as well as its associated precipitation. It is found that gravity waves are first excited near Lake Erie, following the initiation of early convective activity. Then, these waves propagate eastward and northeastward at speeds of 20 and 35 m s‐1, respectively. As the waves propagate downstream, deep convection radiates rapidly behind the wave trough axis, forming a long line of squall convection. Because the squall line moves with the gravity waves in a “phase‐locked” manner, deep convection has a significant influence on the structure and amplitude of the gravity waves. The sensitivity of the wave‐squall prediction to various parameters in convective parameterization is also examined. 相似文献
3.
Denis Gilbert 《大气与海洋》2013,51(1):99-122
Abstract According to linear inviscid theory, the reflection of internal waves off a uniformly sloping bottom should lead to greatly enhanced energy density, and a cross‐isobath alignment of motions, near the critical frequency ωc for which the wave ray slope equals the bottom slope. Current‐meter data from the continental rise and slope off Nova Scotia are used to test this hypothesis. Near‐bottom energy enhancement at ωc was found to be significant at the 95% level for 8 out of 30 tests performed, whereas cross‐isobath alignment of motions near ωc was significant at the 95% level for 10 out of 15 tests performed. Some aspects of the observations that appear to be consistent with non‐linear reflection theory are briefly discussed. 相似文献
4.
John H. E. Clark 《大气与海洋》2013,51(4):321-332
Abstract An explanation is sought for the rapid dispersal of Rossby‐wave energy over large distances such as is often observed at mid‐latitudes. In the proposed mechanism, a pair of Rossby waves is allowed to resonantly interact with an ultra‐long wavelength primary wave fixed in position by orographic or thermal forcing. The group velocity associated with the slow spatial modulation of the pair is shown to strongly depend upon the amplitude of the primary and can achieve meteorologically‐significant values for modest amplitudes. These velocities are maintained even for slight deviations from exact resonance. The implication then is that the mechanism might be relevant in the real atmosphere provided a mechanism to lock the waves into a condition of near resonance for a few days is found. 相似文献
5.
The dispersion relation is derived for long coastal trapped waves of sub‐inertial frequency that propagate along a single‐step continental shelf in a two‐layer fluid. When the internal (Rossby) deformation radius is smaller than the shelf width, we show that the dispersion relation can be factored exactly, giving two possible modes: i. an internal Kelvin wave modified by topography; ii. a continental shelf wave modified by the stratification. A detailed discussion of the eigen‐functions associated with each of these modes is presented. Then the shelf wave dispersion relation is plotted for parameters applicable to the Oregon‐Washington coast. Theoretical values for the periods and wavelengths predicted from these plots are shown to agree favorably with observed values for this region. 相似文献
6.
《大气与海洋》2013,51(4):391-404
Abstract An experimental study was conducted to investigate the transition of two‐layer stratified flow from the slope of bottom topography to a horizontal channel. Three experiments, with a reduced gravity of g’ = 1.64, 6.47 and 18.0 cm s?2, were performed. Particle image velocimetry and planar laser‐induced fluorescence were used to obtain the measurements of velocity and concentration fields. The flow rate, obtained from the measured velocity field, increases significantly toward the toe of the topography by almost 40% from that at the sill crest due to the interfacial wave activities. In the horizontal channel, however, the flow rate only increases marginally. Estimates of the composite Froude number indicate that the supercritical flow on the slope of the topography goes through the transition to the subcritical flow in the horizontal channel. The transition is mainly due to the increase in the lower‐layer thickness because of increasing interfacial friction caused by the breaking of interfacial waves, and no internal hydraulic jumps are observed. The measured mean concentration field showed the formation of an intermediate layer of medium density, which increased its thickness with g’ and helped to suppress turbulence. Spectral analysis of the density interfacial fluctuations indicated that the interfacial waves that developed on the slope of the topography broke up downstream of the toe into smaller amplitude waves at larger frequencies. The waves at several channel cross‐sections were also examined. 相似文献
7.
Abstract In this paper, an overview of storm waves associated with intense weather systems affecting the east and west coasts of Canada is presented. The paper presents the wave climatology of the east and west coasts in terms of the 100‐year significant and maximum wave heights and further analyses the directional distribution of wave heights at selected locations in the Canadian east and west coasts offshore. The paper also analyses wave hazards associated with storm waves in the Beaufort Sea as well as the Canadian Great Lakes region. A section on ocean wave modelling provides a brief history of the development of ocean surface wave models and its present status. The paper further considers the impact of climate change scenarios on wave hazards and finally examines mitigation measures in terms of wave products available from operational wave models and related wave climatology. 相似文献
8.
Peter C. Smith 《大气与海洋》2013,51(1):181-209
Abstract Analysis of current, temperature and salinity records in the nearshore region of the Scotian Shelf during the Canadian Atlantic Storms Program (CASP), reveals that the inertial wave field is highly intermittent, with comparable amplitudes in the surface and deep layers. Clockwise current energy in the surface layer is concentrated at a frequency slightly below inertial, consistent with Doppler shifting by the strong mean current and/or straining by the mean flow shear, whereas the spectral peak in deep water is at the local inertial frequency. Clockwise coherence is high (γ2 ≥ 0.8) horizontally over the scale of the array (60 km × 120 km) and in the vertical, with upward phase propagation rates of 0.15–0.50 × 10?12 ms?1, inversely proportional to the local value of the Brunt Väisälä frequency. Clockwise current energy decreases in the onshore direction and appears to be completely inhibited on the 60‐m isobath. A case study of the response to the CASP IOP 14 storm indicates that the inertial waves may be generated by a strong wind shift propagating onshore at a speed of 10 ms?1. On the eastern side of the array (Liscomb line), clockwise current oscillations propagate onshore in the surface layer at a rate (8.1 ± 0.9 m s?1) comparable with the speed of the atmospheric front, while waves in the pycnocline move offshore at a lower (internal wave) speed (1.8 m s?1). Furthermore the temperature and salinity fluctuations are in (out) of phase with longshore current in the deep (surface) layer. However, on the western side of the array (Halifax line), the inertial waves are more complex. A sharp steepening of phase lines at the coast indicates that the phase speed of clockwise current oscillations is considerably reduced and the evidence for offshore propagation of internal waves is less clear. The discrepancies between observations on the two lines suggest that the internal wave field is three‐dimensional. Results of simple mixed‐layer models indicate that the inertial response near the surface is sensitive to the accurate definition of the local wind field, but not to certain model physics, such as the form of the decay term. The observations also show some qualitative similarities with models for two‐dimensional response to a moving front (e.g. Kundu, 1986), but the actual forcing terms are more complicated, based on IOP 14 wind measurements. 相似文献
9.
Abstract The frequency and directional wave‐modelling capability of the Ocean Data Gathering Program (ODGP) deep water spectral wave model is assessed through comparison with WAVEC data gathered at Hibernia. Both qualitative and quantitative analyses indicate better agreement with observations during storms and with the wind‐driven component of the wave spectra. There is statistically poor modelling of the swell. A coherence analysis on derived wave vectors indicates that the ODGP model does not simulate geophysical variability with time‐scales less than about 30 h for overall spectral energy and less than 24 h for wave energy of frequency greater than 0.6 rad s?1 (0.095 Hz). The signals associated with swell waves are incoherent at nearly all time‐scales. 相似文献
10.
R. H. J. Grimshaw 《Dynamics of Atmospheres and Oceans》1985,9(2):85-120
We consider the three-dimensional reflection and diffraction properties of internal waves in a continuously stratified rotating fluid which are incident on the junction of a vertical slit and a half-space. This geometry is a model for submarine canyons on continental slopes in the ocean, where various physical phenomena embodying reflection and diffraction effects have been observed. Three types of incident wave are considered: (1) Kelvin waves in the slit (canyon); (2) Kelvin waves on the slope; and (3) plane internal waves incident from the half-space (ocean). These are scattered into Kelvin and Poincaré waves in the slit, a Kelvin wave on the slope and Poincaré waves in the half-space. Most of the discussion is centered around case (1). Various properties of the wave field are calculated for ranges of the parameters c/cot θ, γα and ƒ/ω where cot θ is the topographic slope, c is the internal wave ray slope, α is the canyon half-width, γ is the down-slope wave-number, ƒ is the Coriolis parameter and ω is the wave frequency. Analytical results are obtained for small γα and some approximate results for larger values of γα. The results show that significant wave trapping may occur in oceanic situations, and that submarine canyons may act as source regions for internal Kelvin waves on the continental slope. 相似文献
11.
《大气与海洋》2013,51(4):415-427
Abstract An Mw = 7.2 earthquake occurred on 15 June 2005 (utc) seaward of northern California off the west coast of North America. Based on the earthquake location and source parameters, the West Coast and Alaska Tsunami Warning Center issued a tsunami warning for the region extending from the California‐Mexico border to northern Vancouver Island, British Columbia (the first tsunami warning for this region since the 1994 Mw = 8.2 Shikotan earthquake). Six tide gauges on the west coast recorded tsunami waves from this event, with a maximum trough‐to‐crest wave height of 27.7 cm observed at Crescent City, California. Waves of 2.5 to 6.5 cm were measured at the five other sites: Port Orford (Oregon), North Spit and Arena Cove (California), and Tofino and Bamfield (British Columbia). The open‐ocean Deep‐ocean Assessment and Reporting of Tsunami (DART) buoys, 46404 and 46405, recorded tsunami waves of 0.5 and 1.5 cm, respectively, closely matching wave heights derived from numerical models. Incoming tsunami wave energy was mainly at periods of 10 to 40 min. The observed tsunami wave field is interpreted in terms of edge (trapped) and leaky (non‐trapped) waves and a “trapping coefficient” is introduced to estimate the relative contribution of these two wave types. Due to the high (3000 m) water depth in the source area, approximately two‐thirds of the total tsunami energy went to leaky wave modes and only one‐third to edge wave modes. The improved response to and preparedness for the 2005 California tsunami compared to the 1994 Shikotan tsunami is attributable, in part, to the operational capability provided by the open‐ocean bottom‐pressure recorder (DART) system, higher quality coastal tide gauges, and the effective use of numerical models to simulate real‐time tsunamis. 相似文献
12.
Abstract Three arrays of current‐meter moorings were deployed under landfast sea ice in southeast Hudson Bay for eight weeks in spring 1986. Spectral analysis shows low‐frequency signals with periods of 3 to 11 days. These signals are interpreted as being due to coastal‐trapped waves propagating cyclonically in Hudson Bay; their theoretical dispersion relations and corresponding modal structures are presented for winter stratification and are compared with observations. At a period of 3 days both the modified external Kelvin wave and higher mode continental shelf waves may be important in describing the observed low‐frequency variability, whereas at a period of 10 days the Kelvin wave appears to be the dominant mode. The generation mechanisms for these coastal trapped waves are also investigated. Two sources have been studied: the longshore atmospheric pressure gradient and the average atmospheric pressure over the ice cover in Hudson Bay. Coherence and phase analyses performed with time series of longshore current and atmospheric forcing data reveal that both the average atmospheric pressure and the longshore atmospheric pressure gradient are important in explaining the observed low‐frequency variability, without indicating which one is the most important. 相似文献
13.
Abstract A new approach to fetch‐limited wave studies is taken in this paper. Using data from five towers arranged along a line from the eastern shore of Lake St Clair, the differential growth between towers is explored as a function of local wave age. It is argued that this method avoids the usual fetch‐limited pitfall of inhomogeneity over long fetches and, in particular, the changes in wind speed downfetch of an abrupt roughness change. It is found that the growth rate decreases uniformly downfetch as the waves approach full development. This differential method leads to a smooth transition from rapidly growing short fetch waves to the asymptotic invariant state of full development. When the variation in wind speed after an abrupt (land to water) roughness change is taken into account, the idea of a universal fetch‐limited growth curve is called into question. 相似文献
14.
Abstract The propagation of baroclinic Kelvin and Rossby waves in a fairly coarse‐resolution numerical reduced‐gravity ocean model is investigated using simple geostrophic adjustment experiments in a box‐like domain. Numerical experiments using three different horizontal resolutions (4° × 5°,2° × 2.5° and l° × 1.25°) with properly scaled eddy viscosity coefficients show that the phase speed of the model Kelvin waves is almost exactly proportional to the grid resolution, but is virtually independent of the model viscosity. These results are consistent with the findings of Hsieh et al. (1983) and Wajsowicz and Gill (1986). It is also shown that the two relevant parameters that govern the propagation and decay of these waves, namely the grid‐resolution parameter Δ = Δx/a (where Δx is the grid size and a is the baroclinic Rossby radius, viz. a = C/f, with C being the phase speed of inviscid internal gravity waves in a continuum) and the viscosity parameterΔ = Amλ/2πfa3 (where Am is the eddy viscosity coefficient and λ is the alongshore wavelength) can be replaced with Δ only. This is because in Munk (1950)‐type models, the viscosity parameter Δ scales with Δ3. For Δ3 >1, the Kelvin wave phase speed is cK ΔC/Δ and the alongshore decay length scale is of the order of the perimeter of the basin, viz., 0(104) km. In contrast to the case for Kelvin waves, the phase speed of the model Rossby waves is not that much different from its value in a continuum and depends only weakly on the model resolution. This is in good agreement with the theoretical results of Wajsowicz (1986). On the other hand, the model Rossby waves are severely damped, within a distance of the order of a wavelength, by the large eddy viscosity of the model. We therefore extrapolate that for a proper simulanon of Kelvin and Rossby waves in this type of numerical ocean model, we need a grid size smaller than 1° × 1°, and a higher‐order turbulent closure scheme that will reduce the eddy viscosity coefficient. 相似文献
15.
Abstract As part of an extensive survey of the temperature and currents of Lake Erie, a vertical automatic profiling system (EVAPS) was deployed for three days in August 1980. This system consisted of acoustic current meters, temperature sensors and a pressure gauge. The buoyant sensor package was winched from the bottom up and down through the water column. The system collected essentially continuous vertical temperature and velocity profiles. From these profiles, the barotropic component of the flow was extracted; it shows the presence of longitudinal seiche and lunar tidal motions and demonstrates favourable agreement with a numerical model. An experimental momentum balance allows the identification of the important terms (Coriolis force, pressure gradient and local acceleration) and the crude estimation of the numerical value of the wind drag coefficient. The baroclinic part of the flow was decomposed into the three lowest order empirically‐computed internal wave modes. The first and second modes could account for most of the variation of the profiles. The temporal variation of the second mode was characteristic of pure inertial waves while that of the first mode was characteristic of inertial‐gravitational waves. 相似文献
16.
《大气与海洋》2013,51(3):184-203
Abstract Three widely used wave models, namely, the open ocean wave model (Cycle‐4.5, hereinafter referred to as WAM4.5) and the coastal models, Simulation of WAves Nearshore (Cycle III version 40.31, hereinafter referred to as SWAN) and the K‐model, are applied to Lake Erie to simulate waves at a spatial resolution of about 4 km. The results of a three‐week hindcast study are compared with buoy observations in terms of integrated parameters, one‐dimensional (1‐D) and two‐dimensional (2‐D) energy spectra, scatter plots and statistical analyses of the wave fields. The time development of the 1‐D spectra by the models matches the buoy measurements well. All the wave models tend to overpredict the wave heights and underpredict (particularly the K‐model) the peak period. SWAN performs best for the wave heights and WAM4.5 for the peak periods and is computationally less demanding, whereas the spatial resolution applied to Lake Erie seems to be too coarse for an adequate use of the K‐model. In general, WAM4.5 has advantages over coastal wave models in operational intermediate‐scale applications. 相似文献
17.
水汽空间分布对大气船舶重力波影响的数 值试验 总被引：4，自引：0，他引：4
利用中尺度数值模式ARPS模拟研究了水汽在山脉重力波和大气船波的产生和演变中的作用。研究发现水汽和非绝热效应对大气船波的影响与水汽的空间分布有关,大气船波的产生和演变对水汽的空间分布具有极端的敏感性,在一定条件下水汽的引入有可能减少大气船波的活动。对于3层模式结构的气流过山而言,如果初始的水汽分布在中层大气,则水汽和非绝热效应对大气船波的影响较小,而如果初始的水汽分布在中下层大气,则引入水汽后减少了大气船波的强度,但是如果初始的水汽分布在整个模式大气层,则水汽的引入减少了大气船波的活动。 相似文献
18.
Lawrence Cheng 《大气与海洋》2013,51(2):279-305
Abstract A simple, moist two‐dimensional numerical model is developed and used to study the mechanisms for the formation of hailstorm feeder clouds. By using high spatial resolution, the model is able to simulate low‐level storm outflows and reproduce the observed separation between hailstorm feeder clouds. The cold outflow emanating from the storm is induced by evaporation of precipitation. The simulations show that feeder clouds develop in response to the gravity waves generated by wind shear that is inherent in the ambient environment and/or created by the interaction between the low‐level inflow and the storm outflow. The subcloud moisture also plays an important role in the formation of a wave‐like line of feeder clouds. It contributes in the cloud development as well as in the enhancement of gravity waves. 相似文献
19.
Abstract The medium‐scale wave regime, consisting largely of zonal wavenumbers 5–7, frequently dominates the summer Southern Hemisphere tropospheric circulation. We perform a diagnostic study of this circulation as simulated by the Canadian Climate Centre (CCC) general circulation model (GCM). The analysis of Hövmöller diagrams, space‐time and zonal wavenumber spectra shows that the CCC GCM is able to simulate the observed medium‐scale wave regime. The zonally averaged meridional eddy heat and momentum transports and the associated baroclinic and barotropic energy conversions are also examined. The distributions of the transports on the vertical plane agree well with the observations. After comparison with the observed December‐January‐February 1979 distributions, some quantitative differences remain: the heat transport is too weak aloft and too large near the surface, whereas the momentum transport tends to be too weak. The baroclinic and barotropic conversions show a maximum in the medium‐scale waves. The time evolution of the Richardson number of the mean flow suggests that the medium‐scale wave is due to a baroclinic instability. 相似文献
20.
Abstract During the Labrador Ice Margin Experiments, LIMEX ‘87 in March 1987 and LIMEX ‘89 in March and April 1989, the Canada Centre for Remote Sensing (CCRS) CV‐580 aircraft collected synthetic aperture radar (SAR) image data over the marginal ice zone off the east coast of Newfoundland, Canada. One aspect of these experimental programs was the observation of ocean waves penetrating into the marginal ice zone (MIZ). Based upon directional wavenumber spectra derived from SAR image data, the wave attenuation rate is estimated using SAR image spectra and compared with predictions from a model developed by Liu and Mollo‐Christensen (1988). The wave and ice conditions were considerably different in LIMEX ‘87 and LIMEX ‘89. However, the model‐data comparisons are very good for all ice conditions observed. Both the model and the SAR‐derived wave attenuation rates show a characteristic roll‐over at high wavenumbers. A model for the eddy viscosity is proposed, using dimensional analysis, as a simple function of ice roughness and wave‐induced velocity. Eddy viscosities derived from SAR and wave buoy data for the wave attenuation rate show a trend that is consistent with the proposed model. 相似文献