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Jicai Zhang - One of the best experts on this subject based on the ideXlab platform.

  • a feature point scheme for improving estimation of the temporally varying Bottom Friction coefficient in tidal models using adjoint method
    Ocean Engineering, 2021
    Co-Authors: Daosheng Wang, Jicai Zhang
    Abstract:

    Abstract Refined estimation of Bottom Friction coefficient (BFC) is significantly important for coastal engineers to determine precisely the hydrodynamic conditions and sediment transport rates, which can be realized by assimilating observations of sea surface elevation using the adjoint method. In this study, a new method, named as feature point scheme (FPS), is developed to further improve the estimation of temporally varying BFC. In FPS, BFCs at some feature points are assumed to be independent and BFCs at other temporal points are obtained by interpolating BFCs at feature points. By assimilating artificial observations in twin experiments, it is demonstrated that FPS can improve the estimated temporally varying BFC, especially with the nonuniformly distributed feature points and nonuniform spline interpolation (nonuniform FPS). The M2 tide and four principal tidal constituents in the Bohai Sea are then simulated by assimilating real satellite-retrieved observations with FPS. In all the experiments, the performance of the tidal model with nonuniform FPS outperforms those using other schemes. The experimental results indicate that the temporally varying BFC is related to the tidal constituents. Moreover, the temporal variation of estimated BFC has a significantly negative correlation with the current speed, which is possibly related with the sediment transport.

  • study on linear and nonlinear Bottom Friction parameterizations for regional tidal models using data assimilation
    Continental Shelf Research, 2011
    Co-Authors: Jicai Zhang, Ping Wang, Ya Ping Wang
    Abstract:

    Abstract Data assimilation technique (adjoint method) is applied to study the similarities and the differences between the Ekman (linear) and the Quadratic (nonlinear) Bottom Friction parameterizations for a two-dimensional tidal model. Two methods are used to treat the Bottom Friction coefficient (BFC). The first method assumes that the BFC is a constant in the entire computation domain, while the second applies the spatially varying BFCs. The adjoint expressions for the linear and the nonlinear parameterizations and the optimization formulae for the two BFC methods are derived based on the typical Largrangian multiplier method. By assimilating the model-generated ‘observations’, identical twin experiments are performed to test and validate the inversion ability of the presented methodology. Four experiments, which employ the linear parameterization, the nonlinear parameterizations, the constant BFC and the spatially varying BFC, are carried out to simulate the M2 tide in the Bohai Sea and the Yellow Sea by assimilating the TOPEX/Poseidon altimetry and tidal gauge data. After the assimilation, the misfit between model-produced and observed data is significantly decreased in the four experiments. The simulation results indicate that the nonlinear Quadratic parameterization is more accurate than the linear Ekman parameterization if the traditional constant BFC is used. However, when the spatially varying BFCs are used, the differences between the Ekman and the Quadratic approaches diminished, the reason of which is analyzed from the viewpoint of dissipation rate caused by Bottom Friction. Generally speaking, linear Bottom Friction parameterizations are often used in global tidal models. This study indicates that they are also applicable in regional ocean tidal models with the combination of spatially varying parameters and the adjoint method.

  • numerical study on spatially varying Bottom Friction coefficient of a 2d tidal model with adjoint method
    Continental Shelf Research, 2006
    Co-Authors: Xianqing Lu, Jicai Zhang
    Abstract:

    Abstract Based on the simulation of M 2 tide in the Bohai Sea, the Yellow Sea and the East China Sea, TOPEX/Poseidon altimeter data are assimilated into a 2D tidal model to study the spatially varying Bottom Friction coefficient (BFC) by using the adjoint method. In this study, the BFC at some grid points are selected as the independent BFC, while the BFC at other grid points can be obtained through linear interpolation with the independent BFC. Two strategies for selecting the independent BFC are discussed. In the first strategy, one independent BFC is uniformly selected from each 1°×1° area. In the second one, the independent BFC are selected based on the spatial distribution of water depth. Twin and practical experiments are carried out to compare the two strategies. In the twin experiments, the adjoint method has a strong ability of inverting the prescribed BFC distributions combined with the spatially varying BFC. In the practical experiments, reasonable simulation results can be obtained by optimizing the spatially varying independent BFC. In both twin and practical experiments, the simulation results with the second strategy are better than those with the first one. The BFC distribution obtained from the practical experiment indicates that the BFC in shallow water are larger than those in deep water in the Bohai Sea, the North Yellow Sea, the South Yellow Sea and the East China Sea individually. However, the BFC in the East China Sea are larger than those in the other areas perhaps because of the large difference of water depth or Bottom roughness. The sensitivity analysis indicates that the model results are more sensitive to the independent BFC near the land.

Alex E Hay - One of the best experts on this subject based on the ideXlab platform.

  • turbulence intensity in the wave boundary layer and Bottom Friction under mainly flat bed conditions
    Journal of Geophysical Research, 2007
    Co-Authors: John P Newgard, Alex E Hay
    Abstract:

    [1] Variations with wave energy of near-bed turbulence and the wave Friction factor are investigated in the near-shore zone for bed states spanning low-steepness sand ripples and flat bed, and for wave energies extending well into the sheet flow regime. The measurements were made using a 1.7-MHz pulse-coherent Doppler profiler in ca. 3-m mean water depth. Near-bed turbulence intensities, phase-averaged over the highest-1/3 waves, peak at phases between 10° and 55° after the wave crest, this phase decreasing with increasing wave Reynolds number. Wave Friction factors computed from near-bed vertical turbulence intensity fall within the range predicted by existing semi-empirical formulae, and exhibit broadly similar trends. At the higher end of the observed wave energy range (i.e., in the sheet flow regime), however, the measured Friction factors increase with sea-and-swell energy faster than the predictions. This anomalous increase is correlated with infragravity wave energy and with mean cross-shore current speed, but not with other forcing parameters including mean long-shore current speed, wave skewness, wave asymmetry and wave breaking frequency. It is argued that the anomaly is partly due to additional near-bed turbulence associated with infragravity waves, and therefore that these data are not inconsistent with Wilson's (1989) parameterization for Bottom roughness in oscillatory sheet flow. Peak near-bed turbulence intensities are independent of wave Reynolds number for Re ≲ 1.2 × 106, but proportional to Re for Re ≳ 1.2 × 106, this abrupt change possibly indicating a critical dependence on Re of turbulence production in the WBL over flat or nearly flat mobile beds.

  • vertical distribution of wave shear stress in variable water depth theory and field observations
    Journal of Geophysical Research, 2006
    Co-Authors: Qingping Zou, Anthony J Bowen, Alex E Hay
    Abstract:

    [1] A generalized analytical model is developed to describe the vertical distribution of wave-induced shear stress in the presence of Bottom slope, Bottom Friction and depth-induced wave breaking in the shoaling region and surf zone. The theory is compared to data obtained on an unbarred beach in 3.7-m mean water depth over a 2° Bottom slope. Bottom slope and Bottom Friction are incorporated in the theory by including the wave energy dissipation by Bottom Friction in a potential wave theory for variable water depth, and coupling the wave theory with a wave Bottom boundary layer (WBBL) theory over a sloping Bottom. The effect of wave breaking is included through a periodic bore dissipation model. Previous studies have considered some, but not all, of these effects. Each of the three components, sloping Bottom, Bottom Friction and wave breaking, exhibits a different vertical structure. The contributions due to Bottom slope and Bottom Friction attain maximum strength in the WBBL, and decay with distance above the bed, approaching smaller but nonzero values at the surface. In contrast, the contribution from wave breaking is maximal at the surface, and decays linearly with depth, becoming zero at the bed. The vertical structure of the cross-shore (u) and vertical (w) components of wave orbital velocity was measured using a coherent Doppler acoustic profiler. The measured velocity fields were used to obtain the ensemble averaged wave shear stress profiles that extend across the WBBL to a height of 30 cm above the bed. Close to the seabed, the observed and predicted 〈uw〉 profiles are similar, confirming the sensitivity of wave stress to Frictional and Bottom slope effects within the WBBL. Farther from the bed, however, the observed profiles fall off more rapidly with height, as w approaches quadrature with u faster than predicted. Wave breaking induced shear stresses were not observed.

  • near bed turbulence and Bottom Friction during sandyduck97
    Journal of Geophysical Research, 2003
    Co-Authors: C E Smyth, Alex E Hay
    Abstract:

    [1] Remote acoustic measurements of turbulence intensity profiles are investigated as a function of wave energy and bedstate, from low energy vortex ripples to high energy flat bed. Outside the wave boundary layer, velocity power spectral densities increase with increasing wave energy for all bedstates at frequencies across the wave and turbulence bands up to the Nyquist frequency of the measurements, 8 to 10 Hz. The power spectra of the horizontal and vertical velocity exhibit the −5/3 slope characteristic of inertial subrange turbulence. As the seafloor is approached, the slopes of the vertical velocity spectra in this subrange become progressively less steep, reaching values between −1.2 and −0.6 within the wave boundary layer where the spectral densities are independent of bedstate and incident wave energy. Consistent with laboratory turbulence spectra showing similarly reduced spectral roll-off for the transverse velocity component at microscale Reynolds numbers below 1000 [Sreenivasan, 1996], these field observations indicate that near-bed turbulence generated by irregular waves above a mobile bed is probably anisotropic. Ensemble-averaged vertical turbulence intensity profiles exhibit a peak within the wave boundary layer at heights of O(1 cm) above Bottom for all bedstates. The peak is less pronounced for higher energy bedstates. Consistent with the observed spectral convergence within the boundary layer, these peak average turbulence intensities are relatively independent of bedstate, varying by no more than 50% despite a factor of 7 variation in average wave energy. This remarkable observation can be understood from the corresponding decrease in the physical roughness of the bed, associated with the different observed bedstates, as wave energy increases. Estimated wave Friction factors are highest for low-energy rippled beds and smallest for flat bed conditions, and within the uncertainty of the measurements, are generally consistent with predictions from the model by Tolman [1994].

R. Padilla-hernandez - One of the best experts on this subject based on the ideXlab platform.

  • Wave-current interaction in coastal waters: Effects on the Bottom-shear stress
    Journal of Marine Systems, 2007
    Co-Authors: P Rosales, Francisco J. Ocampo-torres, Pedro Osuna, Jaak Monbaliu, R. Padilla-hernandez
    Abstract:

    Abstract Computer simulations of wave and current fields in the southern North Sea were made with a coupled-models system to study the influence of wave–current interactions on the Bottom-shear stress in coastal waters. A third-generation wave-spectral model is coupled with a tide-surge model, which provides current and water level information to take into account wave–current interactions, to calculate the Bottom stress. Two different expressions for Bottom Friction are used; one derived from the JONSWAP experiment, and a second given by Christoffersen and Jonsson that takes into account wave–current interaction at the Bottom. The coupled-models system is applied to four nested grids to achieve fine spatial resolution near the Belgium coast. Two events of moderate to high waves are analyzed. Those two events are associated with different wind regimes; SW winds for the first period and NW for the second. The calculations of Bottom-shear stress when taking into account wave–current interactions are compared with reference runs where only waves are considered to calculate the energy dissipation at the Bottom. Small differences in the Bottom-shear stress results are observed mainly related to the water-level variation caused by tides, when coupled and uncoupled runs using the JONSWAP expression were compared. However, when wave–current interactions are taken into account using the expression of Christoffersen and Jonsson, the calculated maximum Bottom stress is usually doubled for coupled-model runs compared to the reference runs. The results clearly show that the formulation of the Bottom-Friction dissipation that accounts for the effect of wave–current interaction has quite a significant effect on the determination of the Bottom-shear stress.

Ya Ping Wang - One of the best experts on this subject based on the ideXlab platform.

  • study on linear and nonlinear Bottom Friction parameterizations for regional tidal models using data assimilation
    Continental Shelf Research, 2011
    Co-Authors: Jicai Zhang, Ping Wang, Ya Ping Wang
    Abstract:

    Abstract Data assimilation technique (adjoint method) is applied to study the similarities and the differences between the Ekman (linear) and the Quadratic (nonlinear) Bottom Friction parameterizations for a two-dimensional tidal model. Two methods are used to treat the Bottom Friction coefficient (BFC). The first method assumes that the BFC is a constant in the entire computation domain, while the second applies the spatially varying BFCs. The adjoint expressions for the linear and the nonlinear parameterizations and the optimization formulae for the two BFC methods are derived based on the typical Largrangian multiplier method. By assimilating the model-generated ‘observations’, identical twin experiments are performed to test and validate the inversion ability of the presented methodology. Four experiments, which employ the linear parameterization, the nonlinear parameterizations, the constant BFC and the spatially varying BFC, are carried out to simulate the M2 tide in the Bohai Sea and the Yellow Sea by assimilating the TOPEX/Poseidon altimetry and tidal gauge data. After the assimilation, the misfit between model-produced and observed data is significantly decreased in the four experiments. The simulation results indicate that the nonlinear Quadratic parameterization is more accurate than the linear Ekman parameterization if the traditional constant BFC is used. However, when the spatially varying BFCs are used, the differences between the Ekman and the Quadratic approaches diminished, the reason of which is analyzed from the viewpoint of dissipation rate caused by Bottom Friction. Generally speaking, linear Bottom Friction parameterizations are often used in global tidal models. This study indicates that they are also applicable in regional ocean tidal models with the combination of spatially varying parameters and the adjoint method.

Zhang Yang - One of the best experts on this subject based on the ideXlab platform.

  • boussinesq equations in orthogonal curvilinear coordinate system
    Marine Environmental Science, 2007
    Co-Authors: Zhang Yang
    Abstract:

    In this paper,based on the mass conservation and Euler's equation,the modified form of Boussinesq equations is derived,which includes the effects of Bottom Friction,wave breaking and subgrid turbulent mixing.Through transferring the governing equation and boundary condition by the orthogonal curvilinear coordinate,a 2D wave model in the orthogonal curvilinear coordinate systems is established.The numerical model is tested by the computing wave field for several experimental terrains,and agreement between the model results and available experimental data is found to be quite reasonable.It demonstrates that the model is able to simulate wave shoaling,refraction,diffraction and reflection et al.

  • research on boussinesq equations in orthogonal curvilinear coordinate system
    Science & Technology Review, 2006
    Co-Authors: Zhang Yang
    Abstract:

    In this paper, based on the mass conservation equation and Euler's equation, the modified form of Boussinesq equations is derived, which includes the effects of Bottom Friction, wave breaking and subgrid turbulent mixing. Through transferring the governing equations and boundary condition by orthogonal curvilinear coordinate, a 2D wave model in orthogonal curvilinear coordinate systems is established. The numerical model is tested by computing wave field for several experiment terrain, and agreement between model results and available experimental data is found to be quite reasonable, which demonstrates the model's ability to simulate wave shoaling, refraction, diffraction and reflection et etc.