A simple approach to adjust tidal forcing in fjord models

To model currents in a fjord accurate tidal forcing is of extreme importance. Due to complex topography with narrow and shallow straits, the tides in the innermost parts of a fjord are both shifted in phase and altered in amplitude compared to the tides in the open water outside the fjord. Commonly, coastal tide information extracted from global or regional models is used on the boundary of the fjord model. Since tides vary over short distances in shallower waters close to the coast, the global and regional tidal forcings are usually too coarse to achieve sufficiently accurate tides in fjords. We present a straightforward method to remedy this problem by simply adjusting the tides to fit the observed tides at the entrance of the fjord. To evaluate the method, we present results from the Oslofjord, Norway. A model for the fjord is first run using raw tidal forcing on its open boundary. By comparing modelled and observed time series of water level at a tidal gauge station close to the open boundary of the model, a factor for the amplitude and a shift in phase are computed. The amplitude factor and the phase shift are then applied to produce adjusted tidal forcing at the open boundary. Next, we rerun the fjord model using the adjusted tidal forcing. The results from the two runs are then compared to independent observations inside the fjord in terms of amplitude and phases of the various tidal components, the total tidal water level, and the depth integrated tidal currents. The results show improvements in the modelled tides in both the outer, and more importantly, the inner parts of the fjord.     

Relative importance of wind and buoyancy forcing for interdecadal regime shifts in the Pacific Ocean

Interdecadal variations in the Northern Hemisphere and the North Pacific have been documented in many studies[1 4]. The connection between the subtropical North Pacific and the tropics is regarded as the most important process triggering and maintaining t…     

Modelling of waves in the Irish Sea: effects of oceanic wave and wind forcing

This study uses a series of scenarios of wave (boundary) and wind (local) forcing to examine the sensitivity and to quantify the effects associated with nesting ProWAM and POLCOMS models for downscaling predictions of waves in the Irish Sea. The model results show that the response of the modelling system to the wave and wind forcing during the downscaling varies widely depending on wind conditions. Generally, the wave forcing has a greater effect on overall wave prediction in most of the Irish Sea, except for the eastern Irish Sea/Liverpool Bay. The study also suggests detailed look-up tables at specific locations to quantify the impacts of the different forcing scenarios over the Irish Sea, which can be readily extended to the location on any other sites.     

The influence of wind forcing on across-shelf transport in the Florida Keys

Acoustic Doppler current profiles and current meter data are combined with wind observations to describe the transport of water leaving Florida Bay and moving onto the inner shelf on the Atlantic side of the Florida Keys. A 275-day study in the Long Key Channel reveals strong tidal exchanges, but the average ebb tide volume leaving Florida Bay is 19% greater than the average flood tide volume entering the bay. The long-term net outflow averages 472 m³ s⁻¹. Two studies in shelf waters describe the response to wind forcing during spring and summer months in 2004 and during fall and winter months in 2004–2005. During the spring–summer study, southeasterly winds have a distinct shoreward component, and a two-layer pattern appears. Surface layers move shoreward while near-bottom layers move seaward. During the winter study, the resultant wind direction is parallel to the Keys and to the local isobaths. The entire water column moves in a nearly downwind direction, and across-shelf transport is relatively small. During the summer wet season, Florida Bay water should be warmer, fresher, and thus less dense than Atlantic shelf waters. Ebbing bay water should move onto the shelf as a buoyant plume and be held close to the Keys by southeasterly winds. During the winter dry season, colder and saltier Florida Bay water should leave the tidal channels with relatively high density and be concentrated in the near-bottom layers. But little across-shelf flow occurs with northeasterly winds. The study suggests that seasonally changing wind forcing and hydrographic conditions serve to insulate the reef tract from the impact of low-quality bay water.     

Numerical modelling of stratified tidal flow over a fjord sill

Field observations of tidally driven stratified flow in the sill area of Knight Inlet (British Columbia) revealed a very complicated structure, which includes solitary waves, upstream bifurcation, hydraulic jump and mixing processes. Recent observations suggest that the flow instabilities on the plunging pycnocline at the lee side of the sill may contribute to solitary wave generation through a subharmonic interaction. The present study reports on a series of numerical experiments of stratified tidal flow in Knight Inlet performed with the help of a fine resolution fully non-linear non-hydrostatic numerical model. The model reproduces all important stages of the baroclinic tidal dynamics observed in Knight Inlet. Results demonstrate that solitary waves are generated apart from the area of hydrodynamic instability. Accelerating tidal flux forms a baroclinic hydraulic jump just above the top of the sill, whereas the bifurcations and zones of shear instabilities are formed downstream of the sill. The first baroclinic mode having the largest velocity escapes from the generation area and propagates upstream, disintegrating further into a packet of solitary waves reviling the classical “non-subharmonic” mechanism of generation. The remaining part of the disturbance (slow baroclinic modes) is arrested by tidal flow and carried away to the lee side of the obstacle, where shear instability, billows and mixing processes are developed. Some sensitivity runs were performed for different value of tidal velocity.     

Suspended sediment fluxes at an intertidal flat: The shifting influence of wave,wind, tidal,and freshwater forcing

Using in situ, continuous, high frequency (8–16 Hz) measurements of velocity, suspended sediment concentration (SSC), and salinity, we investigate the factors affecting near-bed sediment flux during and after a meteorological event (cold front) on an intertidal flat in central San Francisco Bay. Hydrodynamic forcing occurs over many frequency bands including wind wave, ocean swell, seiching (500–1000 s), tidal, and infra-tidal frequencies, and varies greatly over the time scale of hours and days. Sediment fluxes occur primarily due to variations in flow and SSC at three different scales: residual (tidally averaged), tidal, and seiching. During the meteorological event, sediment fluxes are dominated by increases in tidally averaged SSC and flow. Runoff and wind-induced circulation contribute to an order of magnitude increase in tidally averaged offshore flow, while waves and seiching motions from wind forcing cause an order of magnitude increase in tidally averaged SSC. Sediment fluxes during calm periods are dominated by asymmetries in SSC over a tidal cycle. Freshwater forcing produces sharp salinity fronts which trap sediment and sweep by the sensors over short (∼30 min) time scales, and occur primarily during the flood. The resulting flood dominance in SSC is magnified or reversed by variations in wind forcing between the flood and ebb. Long-term records show that more than half of wind events (sustained speeds of greater than 5 m/s) occur for 3 h or less, suggesting that asymmetric wind forcing over a tidal cycle commonly occurs. Seiching associated with wind and its variation produces onshore sediment transport. Overall, the changing hydrodynamic and meteorological forcing influence sediment flux at both short (minutes) and long (days) time scales.     

Numerical modelling of organic waste dispersion from fjord located fish farms

In this study, a three-dimensional particle tracking model coupled to a terrain following ocean model is used to investigate the dispersion and the deposition of fish farm particulate matter (uneaten food and fish faeces) on the seabed due to tidal currents. The particle tracking model uses the computed local flow field for advection of the particles and random movement to simulate the turbulent diffusion. Each particle is given a settling velocity which may be drawn from a probability distribution according to settling velocity measurements of faecal and feed pellets. The results show that the maximum concentration of organic waste for fast sinking particles is found under the fish cage and continue monotonically decreasing away from the cage area. The maximum can split into two maximum peaks located at both sides of the centre of the fish cage area in the current direction. This process depends on the sinking time (time needed for a particle to settle at the bottom), the tidal velocity and the fish cage size. If the sinking time is close to a multiple of the tidal period, the maximum concentration point will be under the fish cage irrespective of the tide strength. This is due to the nature of the tidal current first propagating the particles away and then bringing them back when the tide reverses. Increasing the cage size increases the likelihood for a maximum waste accumulation beneath the fish farm, and larger farms usually means larger biomasses which can make the local pollution even more severe. The model is validated by using an analytical model which uses an exact harmonic representation of the tidal current, and the results show an excellent agreement. This study shows that the coupled ocean and particle model can be used in more realistic applications to help estimating the local environmental impact due to fish farms.     

Evaluation of wave model performance in the South Atlantic Ocean: a study about physical parameterization and wind forcing calibration

Ocean Dynamics - This paper evaluates the performance of the spectral wave model WAVEWATCH III for the South Atlantic Ocean forced by wind inputs from the most recent reanalyses, NCEP/CFSR and...     

Response of large-scale coastal basins to wind forcing: influence of topography

Because wind is one of the main forcings in storm surge, we present an idealised process-based model to study the influence of topographic variations on the frequency response of large-scale coastal basins subject to time-periodic wind forcing. Coastal basins are represented by a semi-enclosed rectangular inner region forced by wind. It is connected to an outer region (represented as an infinitely long channel) without wind forcing, which allows waves to freely propagate outward. The model solves the three-dimensional linearised shallow water equations on the f plane, forced by a spatially uniform wind field that has an arbitrary angle with respect to the along-basin direction. Turbulence is represented using a spatially uniform vertical eddy viscosity, combined with a partial slip condition at the bed. The surface elevation amplitudes, and hence the vertical profiles of the velocity, are obtained using the finite element method (FEM), extended to account for the connection to the outer region. The results are then evaluated in terms of the elevation amplitude averaged over the basin’s landward end, as a function of the wind forcing frequency. In general, the results point out that adding topographic elements in the inner region (such as a topographic step, a linearly sloping bed or a parabolic cross-basin profile), causes the resonance peaks to shift in the frequency domain, through their effect on local wave speed. The Coriolis effect causes the resonance peaks associated with cross-basin modes (which without rotation only appear in the response to cross-basin wind) to emerge also in the response to along-basin wind and vice versa.     

Radar rainfall uncertainty modelling influenced by wind

Radar‐based estimates of rainfall are affected by many sources of uncertainties, which would propagate through the hydrological model when radar rainfall estimates are used as input or initial conditions. An elegant solution to quantify these uncertainties is to model the empirical relationship between radar measurements and rain gauge observations (as the ‘ground reference’). However, most current studies only use a fixed and uniform model to represent the uncertainty of radar rainfall, without consideration of its variation under different synoptic regimes. Wind is such a typical weather factor, as it not only induces error in rain gauge measurements but also causes the raindrops observed by weather radar to drift when they reach the ground. For this reason, as a first attempt, this study introduces the wind field into the uncertainty model and designs the radar rainfall uncertainty model under different wind conditions. We separate the original dataset into three subsamples according to wind speed, which are named as WDI (0–2 m/s), WDII (2–4 m/s) and WDIII (>4 m/s). The multivariate distributed ensemble generator is introduced and established for each subsample. Thirty typical events (10 at each wind range) are selected to explore the behaviours of uncertainty under different wind ranges. In each time step, 500 ensemble members are generated, and the values of 5th to 95th percentile values are used to produce the uncertainty bands. Two basic features of uncertainty bands, namely dispersion and ensemble bias, increase significantly with the growth of wind speed, demonstrating that wind speed plays a considerable role in influencing the behaviour of the uncertainty band. On the basis of these pieces of evidence, we conclude that the radar rainfall uncertainty model established under different wind conditions should be more realistic in representing the radar rainfall uncertainty. This study is only a start in incorporating synoptic regimes into rainfall uncertainty analysis, and a great deal of more effort is still needed to build a realistic and comprehensive uncertainty model for radar rainfall data. Copyright © 2014 John Wiley & Sons, Ltd.     

三维孔洞储层建模及其地震波场正演模拟

本文研究的内容有二,一是针对塔河油田六区东中、下奥陶统碳酸盐岩孔洞储层发育的特征,通过精细的层位标定、构造解释与时深转换,搭建深度域等时地层框架模型,然后采用协克里金算法,将波阻抗反演体作为协变量,对孔洞储集体的三维孔隙度分布进行确定性建模;二是应用二维横向各向同性介质中的弹性波波动方程,基于交错网格差分格式,考虑初始...     

Spatial evolution equation of wind wave growth

Early in the thirties of the last century, the former Soviet scientists proposed characteristic waves (such as significant wave, and mean wave) to manifest the behavior of wind waves and began to study growth of wind waves. Later on Sverdrup and Munk[1] u…     

Influence of high-resolution wind forcing on hydrodynamic modeling of the Gulf of Lions

The impact of the choice of high-resolution atmospheric forcing on ocean summertime circulation in the Gulf of Lions (GoL; Mediterranean Sea) is evaluated using three different datasets: AROME (2.5 km, 1 h), ALADIN (9.5 km, 3 h), and MM5 (9 km, 3 h). A short-term ocean simulation covering a 3-month summer period was performed on a 400-m configuration of the GoL. The main regional features of both wind and oceanic dynamics were well-reproduced by all three atmospheric models. Yet, at smaller scales and for specific hydrodynamic processes, some differences became apparent. Inertial oscillations and mesoscale variability were accentuated when high-resolution forcing was used. Sensitivity tests suggest a predominant role for spatial rather than temporal resolution of wind. The determinant influence of wind stress curl was evidenced, both in the representation of a mesoscale eddy structure and in the generation of a specific upwelling cell in the north-western part of the gulf.     

弹性介质中瑞雷面波有限差分法正演模拟

为研究瑞雷面波的形成机理及传播规律,促进瑞雷面波资料处理方法的发展,本文根据弹性波方程,采用交错网格有限差分数值求解算法,对浅层各向同性弹性介质进行了包括瑞雷面波和体波在内的全波场模拟. 提出了变系数吸收边界条件并将之应用于正演模拟,使边界条件的处理简单而高效,同时给出了角点的处理方法. 对工程勘察中常见的连续和层状介质模型进行了模拟,获得了更加接近实际情况的地震记录. 结合模拟记录,探讨了瑞雷面波的形成条件,同时讨论了震源埋深对面波能量的影响.     

Spatial distribution of phytoplankton cells in small elongated lakes subject to weak diurnal wind forcing

The horizontal distribution of phytoplankton in a medium-size canyon type reservoir, forced by weak winds of 3–4 ms^?1, is largely driven by the interaction of the large-scale circulation and processes regulating the vertical distribution of algal cells in the water column. These drivers, in turn, are subject to diurnal variations, making our understanding of the horizontal distribution of phytoplankton a challenging task. A three-dimensional physical-–ecological model is used to understand the spatial distribution of algae and the role of diurnal variations in the physical–biological drivers. The model was used to demonstrate that the large-scale circulation induced during the day is more efficient generating patchiness than the circulation existing at night, when convectively driven turbulence homogenizes the upper layers. Different spatial distributions develop for different populations and under different forcing scenarios, characterized in terms of the directionality of wind forcing, wind magnitude and the lags between winds and diel heat fluxes. The time scales needed so that algal biomass in the surface exhibit significant longitudinal gradients—T _P —will vary depending on the algal group, and its ability to regulate its vertical position. These scales are shorter for those species that either migrate actively in the water column or exhibit positive buoyancy (Cryptophytes or Positive buoyant algae). In El Gergal, these scales T _P are on the order of a few days. Synoptic changes in the meteorological forcing, like the passing of a front, could potentially change the longitudinal distribution of algal biomass if they persist for periods of time longer than T _P .     

Internal tide modelling and the influence of wind effects

Initially the development of shallow sea three-dimensional barotropic tidal models is briefly reviewed with a view to determining what were the key measurements that allowed progress in this field and rigorous model validation. Subsequently this is extended to a brief review of baroclinic tidal models to try to determine a “way forward” for baroclinic model development. The difficulty of high spatial variability, and wind influence are identified as possibly important issues that must be considered in validating baroclinic tidal models. These are examined using a three-dimensional unstructured grid model of the M₂ internal tide on the shelf edge region off the west coast of Scotland. The model is used to investigate the spatial variability of the M₂ internal tide, and associated turbulence energy and mixing in the region. Initial calculations are performed with tidal forcing only, with subsequent calculations briefly examining how the tidal distribution is modified by down-welling and up-welling favourable winds. Calculations with tidal forcing only, show that there is significant spatial variability in the internal tide and associated mixing in the region. In addition, these are influenced by wind effects which may have to be taken into account in any model validation exercise. The paper ends by discussing the comprehensive nature of data sets that need to be collected to validate internal tidal models to the same level currently attained with three dimensional barotropic tidal models.     

The importance of aspect for modelling the hydrological response in a glacier catchment in Central Asia

Understanding how explicit consideration of topographic information influences hydrological model performance and upscaling in glacier dominated catchments remains underexplored. In this study, the Urumqi glacier no. 1 catchment in northwest China, with 52% of the area covered by glaciers, was selected as study site. A conceptual glacier‐hydrological model was developed and tested to systematically, simultaneously, and robustly reproduce the hydrograph, separate the discharge into contributions from glacier and nonglacier parts of the catchment, and establish estimates of the annual glacier mass balance, the annual equilibrium line altitude, and the daily catchment snow water equivalent. This was done by extending and adapting a recently proposed landscape‐based semidistributed conceptual hydrological model (FLEX‐Topo) to represent glacier and snowmelt processes. The adapted model, FLEX^G, allows to explicitly account for the influence of topography, that is, elevation and aspect, on the distribution of temperature and precipitation and thus on melt dynamics. It is shown that the model can not only reproduce long‐term runoff observations but also variations in glacier and snow cover. Furthermore, FLEX^G was successfully transferred and up‐scaled to a larger catchment exclusively by adjusting the areal proportions of elevation and aspect without the need for further calibration. This underlines the value of topographic information to meaningfully represent the dominant hydrological processes in the region and is further exacerbated by comparing the model to a model formulation that does not account for differences in aspect (FLEX^G,nA) and which, in spite of satisfactorily reproducing the observed hydrograph, does not capture the influence of spatial variability of snow and ice, which as a consequence reduces model transferability. This highlights the importance of accounting for topography and landscape heterogeneity in conceptual hydrological models in mountainous and snow‐, and glacier‐dominated regions.