Baroclinic oscillations of tidal frequency at ocean weather station P

Abstract

We analyse time series records of isopleth depths derived from two extended sequences of hourly and bi‐hourly sampled profiles taken at Ocean Weather Station P during the summers of 1961 and 1969. Vertical displacements to 240‐m depth are mainly of semidiurnal frequency with r.m.s. amplitudes of 1–4 m. Displacements at diurnal and near‐inertial frequencies are typically less than a metre and have little statistical significance. Within the semidiurnal band, motions appear to be predominantly at the principal solar (S₂) rather than the principal lunar (M₂) semidiurnal frequency. The phase of the M₂ baroclinic tide is roughly equal to that of the M₂ barotropic tide (as extrapolated from coastal and seamount observations); phases of other constituents differ appreciably from barotropic values.     

A tidal model of the northeast pacific

Abstract

The development of a tidal model for the west coast of Canada is described. The model is intermediate in resolution between coarse‐gridded global models and fine‐gridded local models; it provides a good representation of the main shelf regions and also includes a substantial area of the neighbouring ocean. The physical processes relevant to tides in both deep and shallow water are included. Calculations have been carried out for the M₂ and K₁ constituents and the model results were compared with extensive tide‐gauge observations and empirically based charts. For M₂, the agreement between model results and observations is generally excellent, but for K₁, which contains more small‐scale variability, the model results are not quite so good. The variability in K₁ is associated with tidally generated continental shelf waves. Examination of the computed currents and energy fluxes suggests that shelf‐wave components are present in the model solution but, for the Vancouver Island shelf, their propagation is not reproduced accurately. This may be due to deficiencies in the model and/or to the influences of stratification and mean currents, which are neglected here. The model predicts that shelf‐wave components should also occur in diurnal tides on the Alaskan shelf.

The significance of the tide‐generating potential and advection are also examined and further work proposed.     

Secular Trends in Tidal Parameters along the Coast of Japan

The secular trends for local tidal datums, tide ranges, and tidal harmonic constituents at 13 tidal gauge stations located along the coast of Japan were analyzed in this study. The general trends for mean higher high water (MHHW), mean high water (MHW), diurnal tide range (DTR), mean tide range (MTR), and relative mean sea level (RMSL) were positive, and for mean lower low water (MLLW) and mean low water (MLW) the trends were negative. The variation patterns were largest at Kushiro and Ofunato stations, both located in the open waters of the North Pacific Ocean. The tidal datums and tide ranges remained fairly stable at the Hakodate station, and an opposite trend was observed at the Maisaka station. The analysis of tidal harmonic constituents revealed a less pronounced pattern. The most distinct trend was observed for the M₂ amplitude, which appears to be negative at a majority of the stations. The largest decrease in the M₂ amplitude was observed at Kushimoto and Mera stations, also located in the open waters of the North Pacific Ocean. The negative trend is attributed to decreasing water depths at these stations, owing to small-scale local processes.     

Dissipative effects on inertial flows over a sill

A systematic investigation of the effects of various parametrizations of dissipation, e.g. quadratic and linear frictional drag, harmonic lateral viscosity, and harmonic lateral diffusion on inertial flow over a sill and possible hydraulic control is presented. Rotation effects are ignored and the geometry is assumed to vary only slowly with downstream distance so that the flow may be considered one-dimensional. Results are given both for a single-active layer and for two-active layers with a rigid lid.If the parametrization is only a function of the dependent variables and not of their spatial derivatives, then it may be possible to hydraulically control the flow. A general expression is derived for the possible control point and the two gradients there, which are functions of the slope and possibly of flow rate. Specific energy is irreversibly removed from the flow and non-controlled as well as controlled flows can exhibit significant asymmetry in fluid depth over a sill. The upstream specific energy, and hence depth of the lower layer, of the controlled flow is greater than for an ideal fluid. Frictional effects modify the behaviour of long gravity waves, such that they are dispersive and damped with time. The system will only exhibit hydraulic control if these effects are small.For a viscous single layer of fluid, the gradient in surface elevation is always uniquely defined, so classically defined hydraulic control, as such, cannot exist. However, for values of non-dimensional lateral eddy viscosity coefficient, , where q is the flow rate, there is a narrow band of specific energies centred around that for the control solution in an ideal fluid, E_crit, for which the surface elevation, h is very asymmetric over the sill; the solutions resemble the inviscid, hydraulically controlled solutions. Outside this range, either the fluid depth tends to zero, or the surface elevation is almost uniform over the sill. A ‘control’-type solution exists which has the conjugate values of the inviscid equation up- and downstream of the sill, where the gradient in fluid depth, and hence the viscous term, is zero. For larger values of A_M, the band of specific energies is much wider, and the upstream specific energy of the ‘control’-type solution is much lower than that for an inviscid fluid. Long gravity waves are dispersive and damped with time. There is a short-wave cut-off, k² > h/(4A_M²), above which waves are stationary in the flow. Longer waves, k² h/(4A_M²), are critical if , as for an ideal fluid. If these waves can propagate significant distances, then any observed asymmetry in h will be due to inertial and not to viscous effects. The behaviour of unidirectional, two-layer flow is similar. The governing equation for viscous, two-layer exchange flow is singular, and typically excludes the ‘control’-type solutions found for unidirectional flows.Establishing the existence and behaviour of steady inertial flows in the presence of lateral diffusion between layers is more difficult. It significantly alters the single-layer solutions once the non-dimensional coefficient A_H is large, i.e. . The flow rate may become zero on the downslope as all the fluid diffuses into the inert, infinitely deep, overlaying layer. The fluid depth is maintained by reverse flow from downstream. In this case, the depth of the active layer tends to zero downstream for all values of specific energy. For two-layer flow, both unidirectional and exchange, the governing equation is such that the lower-layer flow rate and interfacial height return to their upstream values.Motivation for the study is provided by the increasingly fine spatial resolution achievable in large-scale numerical models of the ocean general circulation, and the question of whether they are capable of simulating some form of hydraulic control. Application to modelling oceanic flows over a sill is discussed.     

The partition of internal tidal motions in knight inlet,British Columbia

Abstract

A reanalysis of 4 current‐meter records taken from Knight Inlet, B.C., is presented. The data indicate that the internal tide in Knight Inlet at M₂ frequency undergoes a substantial reflection near a sharp bend in the channel. This result conflicts directly with a result obtained recently by Blackford and presented in this issue.     

Tidal circulation and buoyancy effects in the St. Lawrence Estuary

Abstract

The action of tides on density‐driven circulation, internal gravity waves, and mixing was investigated in the St. Lawrence Estuary between Rimouski and Québec City. Time‐varying fields of water level, currents and density were computed under typical summer conditions using a three‐dimensional hydrostatic coastal ocean model that incorporates a second order turbulence closure submodel. These results are compared with current meter records and other observations. The model and the observations reveal buoyancy effects produced by tidal forcing. The semi‐diurnal tide raises the isopycnals over the sills at the head of the Laurentian Trough and English Bank, producing internal tides radiating seaward. Relatively dense intermediate waters rise from below 75‐m depth to the near surface over the sills, setting up gravity currents on the inner slopes. Internal hydraulic controls develop over the outer sills; during flood, surface flow separation occurs at the entrances of the Saguenay Fjord and the upper estuary west of Ilet Rouge Bank. Early during ebb flow (restratification), the surface layer deepens to encompass the tops of the sills. As the ebb current intensifies, the model predicts the formation of seaward internal jumps over the outer sills, which were confirmed from acoustic reflection observations. As the internal Froude number increases further, flow separation migrates up to sill height. As a result of these transitions, internal bores emanate from the head region one to two hours before low water. We find that the mixing of oceanic and surface waters near the sills is driven by the vertical shear produced during ebb in the channel south of Ilet Rouge, the shear produced in the bottom gravity flood currents, and, to a lesser extent, the processes over the sills.     

Current Observations of Internal Tides and Parametric Subharmonic Instability in Luzon Strait

Acoustic Doppler Current Profiler (ADCP) measurements of the velocity structure in the thermocline in Luzon Strait are presented. The statistics for current, vertical variation of the inertia-gravity waveband, parametric subharmonic instability (PSI), and current shear are analyzed. It was found that 1) barotropic flow primarily consists of a nearly circular mixed tide. Diurnal tides are strongest and show smooth variance with a fortnightly spring-neap cycle, indicative of the astronomical tide-generating force. However, the semi-diurnal band power exhibits a high-frequency oscillation as a result of non-linear interactions. The high-frequency band power with high values during the spring tide oscillates with the tidal cycle. Near-inertial wave motions showing random variance may be caused by changes in the wind forcing at the sea surface or by random forcing. 2) Baroclinic velocities exhibit strong shear structure. The observed large changes in the amplitude of the baroclinic velocity and the limited vertical extent of the high-velocity cores may be interpreted as internal wave beams that pass through the observed water column. Semi-diurnal tides are dominant in the baroclinic velocity. Kinetic energy spectra also revealed that additional peaks were centred at sum-tidal-inertial interaction frequencies (such as M₂ ?+?f) and difference-interaction frequencies (such as M₂ ???f). The spectral exponent of the baroclinic velocity is ω^?α (1?<?α?<?3). 3) Strong non-linear interactions among internal waves exist, and the semi-diurnal (M₂) component plays a key role in these interactions. Bicoherence analysis showed that M₂/2 waves were non-linearly coupled with the dominant M ₂ internal tide. 4) The polarization relations were used to diagnose observational internal tidal motions. Diurnal waves propagate to the east-northeast, and the semi-diurnal–diurnal waves propagate westward. In the case of diurnal tides, the minor to major axis ratio is different from the expected value of f/ω_{K 1} because of the deviation of inclinations, whereas, for semi-diurnal tides, it is close to the expected value of f/ω _M2 at depths from 30 to 150 m.

RÉSUMÉ?[Traduit par la rédaction] Nous présentons les mesures de la structure des vitesses dans la thermocline faites par profileur de courant à effet Doppler (ADCP) dans le détroit de Luçon. Nous analysons les statistiques sur le courant, la variation verticale de la gamme d'ondes d'inertie-gravité, l'instabilité subharmonique paramétrique (PSI) et le cisaillement du courant. Il ressort que 1) l’écoulement barotrope consiste principalement en une marée mixte presque circulaire. Les marées diurnes sont les plus fortes et présentent une variance régulière dans un cycle vives-eaux mortes-eaux de deux semaines qui révèle la nature astronomique de la force qui produit les marées. Cependant, la puissance de bande semi-diurne affiche une oscillation de haute fréquence causée par des interactions non linéaires. La puissance de bande de haute fréquence avec des valeurs élevées durant les vives-eaux oscille avec le cycle des marées. Les mouvements des ondes quasi inertielles affichant une variance aléatoire peuvent être dus à des variations dans le forçage par le vent à la surface de la mer ou à un forçage aléatoire. 2) Les vitesses baroclines exhibent une structure fortement cisaillée. Les changements marqués observés dans l'amplitude des vitesses baroclines et l'extension verticale limitée des c?urs de vitesse élevée peuvent être interprétés comme des faisceaux d'ondes internes qui traversent la colonne d'eau observée. Les marées semi-diurnes sont dominantes dans les vitesses baroclines. Les spectres d’énergie cinétique ont aussi révélé que des crêtes additionnelles étaient centrées aux fréquences d'interaction cumulative marée-inertie (comme à M₂ ?+?f) et aux fréquences d'interaction différentielle (comme à M₂ ???f). L'exposant spectral de la vitesse barocline est ω^?α (1?<?α?<?3). 3) Il existe de fortes interactions non linéaires entre les ondes internes et la semi-diurne (M₂) joue un rôle clé dans ces interactions. L'analyse de bicohérence a montré que les ondes M₂/2 étaient non-linéairement couplées avec la marée interne dominante M₂ . 4) Nous avons utilisé les relations de polarisation pour diagnostiquer les mouvements de marée internes déduits des observations. Les ondes diurnes se propagent vers l'est-nord-est et les ondes semi-diurnes se propagent vers l'ouest. Dans le cas des marées diurnes, le rapport de l'axe secondaire à l'axe principal diffère de la valeur attendue (f/ω_{K 1} ) à cause de l’écart des inclinaisons alors que pour les marées semi-diurnes, il est proche de la valeur attendue (f/ω _M2 ) à des profondeurs de 30 à 150 m.     

Oceanic tides over the Newfoundland and Scotian Shelves from TOPEX/POSEIDON altimetry

Abstract

A modified response analysis in the form of an orthotide parametrization is used to derive major semi‐diurnal and diurnal tidal constituents over the Newfoundland and Scotian Shelves from TOPEX/POSEIDON altimeter data. The orthotide formulation simultaneously solves for eight major semi‐diurnal and diurnal constituents, and has an accuracy of better than ～1.5 cm for each constituent. Tidal elevations are derived from the altimeter data on the ascending and descending satellite ground tracks, and interpolated using the method of statistical interpolation. The semi‐diurnal constituents (M₂, S₂ and N₂) compare favourably with available in‐situ observations and a numerical model. The diurnals (K₁ and O₁) are not as accurate as the semi‐diurnals, especially in the nearshore environment. The paper demonstrates the ability of TOPEX/POSEIDON altimetry to provide accurate tidal data over a continental shelf.     

Momentum flux variations at lower thermospheric levels as seen from mid-latitude meteor wind observations

Summary Computations of momentum flux were made for the meteor wind data collected at Atlanta (34°N), during the period August 1974–March 1978. Substantial transport of momentum due to diurnal tide during summer and semidiurnal tide during winter was observed over the station. Monthly mean momentum flux variations of diurnal and semidiurnal tides exhibited oscillations with periods of about six and four months respectively. The height variations of annual mean momentum flux due to diurnal and semidiurnal tides revealed poleward flux throughout the height region 82–98 km and severe damping of momentum flux due to diurnal tide between the heights 90 and 92 km. The results of the present study relating to the northern hemisphere mid-latitude station are compared with those of Adelaide (35°S), the southern hemisphere mid-latitude station.

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Zusammenfassung Es wurden Impulsflußberechnungen für die in Atlanta (34°N) von August 1974 bis März 1978 gesammelten Meteor-Winddaten gemacht. Über der Station wurde ein beträchtlicher Impulstransport, der durch Tagesgezeiten während des Sommers und Halbtagesgezeiten während des Winter verursacht wurde, beobachtet. Die monatlichen, mittleren Impulsflußvariationen der Tages- und Halbtagesgezeiten zeigten Schwankungen mit sechsbzw, viermonatlichen Perioden. Die Höhenschwankungen des jährlichen, mittleren Impulsflusses infolge der Tages- und Halbtagesgezeiten zeigten einen polwärts gerichteten Flß innerhalb der Höhenregion von 82–98 km und eine heftige Dämpfung des Impulsflusses infolge der Tagesgezeiten zwischen 90 und 92 m Höhe. Die Ergebnisse dieser Untersuchung, die sich auf eine Station der mittleren Breiten in der nördlichen Hemisphäre bezieht, werden mit jenen von Adelaide (35°S), einer Station der mittleren Breiten in der südlichen Hemisphäre verglichen.

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With 3 Figures     

On Measurements of the Tide at Churchill,Hudson Bay

Since the late 1990s the semi-diurnal tide at Churchill, on the western shore of Hudson Bay, has been decreasing in amplitude, with M₂ amplitudes falling from approximately 154?cm in 1998 to 146?cm in 2012 and 142?cm in 2014. There has been a corresponding small increase in phase lag. Mean low water, decreasing throughout most of the twentieth century, has levelled off. Although the tidal changes could reflect merely a malfunctioning tide gauge, the fact that there are no other measurements in the region and the possibility that the tide is revealing important environmental changes calls for serious investigation. Satellite altimeter measurements of the tide in Hudson Bay are complicated by the seasonal ice cover; at most locations less than 40% of satellite passes return valid ocean heights and even those can be impacted by errors from sea ice. Because the combined TOPEX/Poseidon, Jason-1, and Jason-2 time series is more than 23 years long, it is now possible to obtain sufficient data at crossover locations near Churchill to search for tidal changes. The satellites sense no changes in M₂ that are comparable to the changes seen at the Churchill gauge. The changes appear to be localized to the harbour, or to the Churchill River, or to the gauge itself.     

Oceanographic observations on the Scotian Shelf during CASP

Abstract

Winter‐time (1985–86) observations of sea‐level, surface waves, currents, bottom pressure, and water properties were made on the Scotian Shelf as part of the Canadian Atlantic Storms Program (CASP). The purpose of the field program and the locations, instrument systems, and sampling schemes are described.

Statistics on the observed currents are presented, and monthly estimates of the longshore transport of the Nova Scotian Current are given for four months spanning the winter season (December to March). The 1985–86 transports are compared with previous estimates of the baroclinic transport made using the dynamic method.

Cross‐shelf temperature and salinity distributions corresponding to the beginning and end of the field program are described and compared with the climatological means.

The autospectra of the observed currents exhibit energy concentrations at frequencies of 0.2–0.5, 1.0, 1.4 and 2.0 cycles per day (cpd). Coastal‐trapped wave motions account for significant portions of the energy in the two lowest frequency bands, forced by wind stress and the diurnal tide, respectively. Inertial oscillations generated by wind stress events predominate at 1.4 cpd, and the semidiurnal tide, at 2 cpd. These motions are described separately, and references to more detailed discussions are given.     

On the mean and tidal currents in Hudson strait

Abstract

The vertical structures of the mean and tidal flows in Hudson Strait are described from moored current‐meter data collected during an 8‐week period in August to October of 1982. The residual flow in the strongly stratified waters off Quebec is directed along the Strait to the southeast, is highly baroclinic and is concentrated near shore (within an offshore length scale of approximately an internal Rossby radius). Maximum mean speeds of 0.3 m s^?1 were observed near‐surface (30 m). In the weakly stratified waters on the northern side of the Strait along Baffin Island the mean flow is northwestward. The maximum speeds are 0.1 m s^?1 near‐surface (30 m) and the current amplitudes decrease to 0.05 m s^?1 at 100 m. The mean southeastward transport is estimated to be 0.93 ±0.23 × 10⁶ m³ s^?1 with a northwestward transport of 0.82 ± 0.24 × 10⁶ m³ s^?1. Over most of the Strait the across‐channel residual currents are directed towards the Quebec shore with velocities ranging from 0.02 to 0.1 ms^?1. Current variability is dominated by the tides, the M₂ being the major tidal constituent. In the vicinity of the mooring the M₂ tide is primarily barotropic, progressive in nature, and has along‐channel current amplitudes varying across the Strait from 0.20 to 0.45 m s^?1. Observed differences in tidal sea‐level elevations across the Strait can be accounted for by the cross‐channel variations characteristic of Kelvin waves.     

Internal tides in the coastal zone of the Sea of Japan in autumn

The results are presented of hydrological studies in the coastal zone of the Sea of Japan in autumn in different years. It is revealed that the typical density stratification of water is formed there in autumn. The amplitudes are estimated of regular (with the periodicity of tidal harmonic M₂) vertical displacements of isotherms in the intermediate layer and maximum values of flow velocity in the surface and bottom layers. It is demonstrated that temperature perturbations are induced at the shelf edge and propagate to the coastal zone with the velocity that is close to the velocity of the first mode of internal gravity waves with the frequency of the tidal harmonic M₂.     

Tidal currents and mixing at the INSTANT mooring locations

Tides affect transport and mixing in the Indonesian Seas, impacting the throughflow and the return flow of the global thermohaline circulation. In a previous study, barotropic and baroclinic tides were simulated for the Indonesian Seas at 5 km resolution in order to characterize the tides of the region and to identify and quantify locations of tidal mixing. Baroclinic tidal velocities exceeded barotropic velocities except in shallow regions and their variability was on smaller scales. Model results agreed reasonably with observations and are consistent with the resolution. However, only four mooring locations were available for comparison. The new International Nusantara Stratification (INSTANT) data set enables a more comprehensive comparison. With the exception of Lombok Strait, the model replicated the observed INSTANT velocity spectra, falling within the 90% confidence limits of the observed spectra, both in regions of high and low baroclinic tidal activity for the band of frequencies from 0.02 cph to 0.33 cph (periods of 50–3 h, respectively), which includes the major semidiurnal and diurnal tides and several of their first harmonics. The model overestimated the semidiurnal baroclinic tides in the narrow Lombok Strait, which is not well resolved in the model. Comparisons of vertical profiles of the major axes of the tidal ellipses at the mooring sites generally reproduced the vertical pattern, although there were exceptions, such as Lombok and Ombai Straits. Rms differences between the model estimates and hourly observations for the major axes of the tidal ellipses were typically 1–8 cm s⁻¹ in regions of high tidal activity, 1–5 cm s⁻¹ in regions of low tidal activity, and 1–20 cm s⁻¹ for the semidiurnal tides in Lombok and Ombai Straits. Rms errors of 1–6 cm s⁻¹ are typical in regions of moderate baroclinic tidal activity at this model resolution (5 km). Many of the larger rms differences result from vertical discrepancies in the depths of the internal tidal beams. The local nature of the internal tides generation and beam propagation results in large differences from small vertical shifts in the beams or generation due to topographic differences between the model topography and the actual topography. In addition, the moorings experienced severe blowdown. The blowdown adds uncertainty to the depths of the instruments and introduces errors in the observational tidal analysis in magnitude of the tidal constituents, both of which contribute to rms differences. Tidal mixing was found to occur in intense local regions with strong internal tidal shear. The local regions of mixing were typically along the bottom in steep slopes and over sills. In conclusion, the tidal model was found to reproduce the kinetic energy distribution and transfer of energy from tides to other frequencies in the Indonesian Seas and to roughly replicate the observed structure and magnitude of the tidal currents. Improvements in the tidal simulations in reproducing observations are expected with increased resolution.     

Identification and elimination of an inflow boundary computational solution in limited area model integrations

Abstract

The linearized non‐divergent barotropic vorticity equation in one dimension is used for the study of a problem associated with the specification of lateral boundaries in limited area models. This problem presents itself in the form of a “pillow” that builds up near the inflow boundary of the model. Linear analysis shows that this pillow can easily be eliminated. Linear integrations carried out with a corrector seem to be reasonably accurate.

Similar integrations with the linearized shallow water equations in one dimension also produce a pillow and the same corrector gives improved results. Additional runs are performed in order to show that some commonly used nesting strategies do not control this computational problem in a satisfactory manner. It seems that these strategies could be improved with an appropriate corrector.     

Gas ventilation of the Saguenay fjord by an energetic tidal front

Abstract

Dissolved noble gas samples were taken during a pilot study in the Saguenay Fjord, Quebec, Canada, in order to determine the contribution of different air‐sea gas exchange mechanisms in an estuary and to assess the contribution of tidal fronts to the aeration of subsurface waters. The noble gases He, Ne, Ar, Kr, and Xe span a large range of molecular diffusivities and solubilities and hence constitute a useful probe of various gas exchange and bubble injection processes. Samples were taken at flood tide upstream and downstream of an energetic tidal front that is generated by a hydraulically controlled flow over a shallow sill at the entrance to the Fjord. The results are interpreted with the help of hydrographic measurements of density and currents along cross‐sill transects describing the physical forcing at the sill. High gas saturations downstream of the sill indicate the aeration of water within the frontal region. An inverse model is used to compare the contribution of bubble injection in the front to diffusion across the air‐sea interface. The large ratio of completely ‘trapped’ bubbles to diffusion suggests that bubbles injected by waves breaking in the front contribute significantly to air‐sea gas exchange with 76% for He, 79% for Ne, 56% for Ar, 47% for Kr, and 35% for Xe.

Water samples were analyzed for helium isotopes and tritium in order to explore the possibility of constraining ventilation time scales. The relationship between tritium and salinity revealed two end‐member waters: a freshwater component from the Saguenay River of 23.6 ± 0.5 TU, likely a residual of bomb‐produced tritium, and a seawater end‐member of approximately 1.5 TU originating in the subpolar Atlantic. An unexpected contribution of radiogenic 4He was detected in the deep waters of the St. Lawrence Estuary, likely a consequence of out‐gassing from old, uranium and thorium rich granitic terrain.     

A Comprehensive Modelling Approach for the Neutral Atmospheric Boundary Layer: Consistent Inflow Conditions, Wall Function and Turbulence Model

We report on a novel approach for the Reynolds-averaged Navier-Stokes (RANS) modelling of the neutral atmospheric boundary layer (ABL), using the standard k-ek-{\varepsilon} turbulence model. A new inlet condition for turbulent kinetic energy is analytically derived from the solution of the k-ek-{\varepsilon} model transport equations, resulting in a consistent set of fully developed inlet conditions for the neutral ABL. A modification of the standard k-ek-{\varepsilon} model is also employed to ensure consistency between the inlet conditions and the turbulence model. In particular, the turbulence model constant C _μ is generalized as a location-dependent parameter, and a source term is introduced in the transport equation for the turbulent dissipation rate. The application of the proposed methodology to cases involving obstacles in the flow is made possible through the implementation of an algorithm, which automatically switches the turbulence model formulation when going from the region where the ABL is undisturbed to the region directly affected by the building. Finally, the model is completed with a slightly modified version of the Richards and Hoxey rough-wall boundary condition. The methodology is implemented and tested in the commercial code Ansys Fluent 12.1. Results are presented for a neutral boundary layer over flat terrain and for the flow around a single building immersed in an ABL.     

On the influence of stratospheric conditions on forced tropospheric waves in a steady‐state primitive equation model

Abstract

The structure of a forced planetary wave is computed by means of a linearized steady‐state primitive equation model on a hemisphere. The vertical velocity in pressure coordinates is specified at the lower boundary to simulate orographie forcing. The vertical finite differences are on equally spaced pressure levels with a moderately high vertical resolution. The upper boundary condition dp/dt = 0 is applied at p =0 in the model. Numerical experiments show that the tropospheric structure of forced planetary waves is sensitive to the stratospheric background conditions in the model.