Seismic and Meteorological Tsunami Contributions in the Manzanillo and Cabo San Lucas Seiches of September 14, 1995

Seismic generated sea waves from the September 14, 1995, Mexico earthquake are favored over the waves generated by hurricane Ismael to be the cause of the increase in seiche amplitudes in Manzanillo and Cabo San Lucas tide stations. The arguments are based on travel time computations from the seismic source and the moving meteorological source. A relatively sudden increase in seiche amplitudes is consistent with the arrival time of the seismic tsunami, and previously existent background is likely produced by incoming waves from the hurricane.     

渤海、黄海沿岸主要港湾假潮的基本特征

基于港湾假潮振动的基本原理，从渤海、黄海沿岸20个验潮站多年水位自记曲线资料获取假潮参数，并对其做了统计分析，给出了假潮的统计特征，还讨论了假潮形成的初步原因。结果表明，该海域主要港湾假潮出现具有明显的局地性、季节性和年际变化；夏季是大振幅假潮（振幅^1)≥80cm）的多发期，冬季基本上不出现；龙口港湾的大振幅假潮最为突出，最大振幅可达293cm；气压和风速、风向突变的良好配合，是发生大振幅假潮的主要原因。     

南海北部沿岸海洋站的假潮

收集和分析了南海北部沿岸15个海洋站所在港湾发生假潮的资料，并对其中较大振幅的典型假潮个例，进行了港湾基态自然周期分析和假潮振动的频谱分析。研究发现，假潮最大振幅振动多发生在天文潮的高潮或低潮期间；根据海湾地形尺度用梅立恩（Merian）公式计算得出的海湾基态自然周期；和根据资料曲线估计得出的假潮周期，以及功率谱分析得出的周期，三者非常接近。从本文研究所得结果来看，这些海湾的较大振幅假潮多发生在春、夏季，且都和风速、风向以及气压的剧烈变化有关。这些大气扰动在春天多为冷空气南下，在夏季多为热带气旋活动。研究认为，假潮的发生很可能是大气振动通过外海表面波作为中间机制，与港湾自由态振动之间产生耦合共振的结果。     

台风移行速度与登陆角度对风暴潮的影响:一个理想模型数值实验

The effects of hurricane forward speed(V) and approach angle(θ) on storm surge are important and a systematic investigation covering possible and continuous ranges of these parameters has not been done before. Here we present such a study with a numerical experiment using the Finite Volume Community Ocean Model(FVCOM).The hurricane track is simplified as a straight line, such that V and θ fully define the motion of the hurricane. The maximum surge is contributed by both free waves and a forced storm surge wave moving with the hurricane.Among the free waves, Kelvin-type waves can only propagate in the down-coast direction. Simulations show that those waves can only have a significant positive storm surge when the hurricane velocity has a down-coast component. The optimal values of V and θ that maximize the storm surge in an idealized semi-circular ocean basin are functions of the bathymetry. For a constant bathymetry, the maximum surge occurs when the hurricane approaches the coast from the normal direction when the free wave generation is minimal; for a stepped bathymetry, the maximum surge occurs at a certain acute approach angle which maximizes the duration of persistent wind forcing; a step-like bathymetry with a sloped shelf is similar to the stepped bathymetry, with the added possibility of landfall resonance when the free and forced waves are moving at about the same velocity. For other cases, the storm surge is smaller, given other parameters(hurricane size, maximum wind speed, etc.)unchanged.     

Signatures of ocean storms on seismic records in South China Sea and East China Sea

Ocean storms on seismic records reveal coupling mechanisms between Earth’s spheres. We analyzed temporal and spatial signatures of ocean storms on seismic records in the South China and East China Seas. The main points are: (1) the landing of ocean storms directly influences spectrum amplitudes of microseisms, showing an “increase-peak-decrease” pattern, whereas they exert no direct influence on Earth’s hum and the high-frequency noise; (2) for microseisms, spectrum amplitudes of short-period double-frequency microseisms are increased greatly during ocean storms’ landing, implying that storms preferably excite short-period ocean swells; (3) while the “increase-peak-decrease” pattern of spectrum amplitudes is observed for both short-period double-frequency microseisms and long-period double-frequency microseisms in South China Sea, the peak arrived and disappeared much earlier for long-period double-frequency microseisms, which can be explained by their causal mechanisms; (4) in East China Sea, only the spectrum amplitudes of short-period double-frequency microseisms show an “increase-peak-decrease” pattern and extraordinary spectrum pulses are observed reflecting thick sediments there; (5) spatial features of microseisms revealed from predominant polarization directions indicate that local coastlines play very important roles in deciding where ocean waves impact; (6) high-frequency noise is caused by local offshore wind-generated ocean waves instead of ocean storms; (7) the influence of ocean storm landing processes on microseisms can propagate through continents and is observed at inland stations; (8) seismic motions are excited more efficiently in horizontal directions when ocean waves impact seafloors. Our work clearly exhibits how effectively local ocean events are coupled with the Earth’s lithosphere in Chinese seas.     

Numerical modeling of wind waves generated by tropical cyclones using moving grids

A version of the WAVEWATCH III wave model featuring a continuously moving spatial grid is presented. The new model option/version is intended for research into wind waves generated by tropical cyclones in deep water away from the coast. The main advantage of such an approach is that the cyclones can be modeled with spatial grids that cover much smaller areas than conventional fixed grids, making model runs with high spatial resolution more economically feasible. The model modifications necessary are fairly trivial. Most complications occur due to the Garden Sprinkler effect (GSE) and methods used to mitigate it. The basic testing of the model is performed using idealized wind fields consisting of a Rankine vortex. The model is also applied to hurricane Lili in the Gulf of Mexico in October 2002. The latter application shows that the moving grid approach provides a natural way to deal with hurricane wind fields that have a high-resolution in space, but a low resolution in time. Although the new model version is originally intended for tropical cyclones, it is suitable for high-resolution modeling of waves due to any moving weather pattern.     

Simulation of seiche oscillations in the Sea of Azov,using the finite element technique

A mathematical model based on the finite element technique is applied to study seiche oscillations in the Sea of Azov in an approximation of the linear theory for long waves. The model has been used to compute periods of the initial ten seiche modes.Translated by Vladimir A. Puchkin.     

Numerical modeling of the propagation of nonlinear acoustic-gravity waves in the middle and upper atmosphere

A numerical algorithm for modeling the vertical propagation and breaking of nonlinear acoustic-gravity waves (AGWs) from the Earth’s surface to the upper atmosphere is described in brief. Monochromatic variations in the vertical velocity at the Earth’s surface are used as an AGW source in the model. The algorithm for solving atmospheric hydrodynamic equations is based on three-dimensional finite-difference analogues of fundamental conservation laws. This approach selects physically correct generalized solutions to hydrodynamic equations. A numerical simulation is carried out in an altitude region from the Earth’s surface to 500 km. Vertical profiles of the background temperature, density, and coefficients of molecular viscosity and heat conduction are taken from the standard atmosphere models. Calculations are made for different amplitudes of lower-boundary wave forcing. The AGW amplitudes increase with altitude, and waves may break in the middle and upper atmosphere.     

Interface wave generated by an electromagnetic induction source

Interface waves such as the Scholte wave are a useful tool to study geoacoustic properties and can be conventionally generated by an explosive or a pneumatic source on/above the seafloor. This type of source, however, generates strong compressional waves in the water and sediment at the same time; these waves then disturb an observation of interface waves and leads to difficulty in processing. These sources are also relatively hard to control at sea from a viewpoint of repeatability and stability of interface waves to be generated. In addition, environmental problems caused by those sources is a concern. In this paper, an electromagnetic induction source whose vibrator plate hits the seafloor directly and excites interface waves is described. The capability of this source was evaluated both in a water tank and at seashore. The pulsed Scholte waves excited both by several types of electromagnetic induction source having a different shape of vibrator plate and by dropping weight were transmitted in sediment and received using geophones. As a result of comparison of measured signals, the pulse signal propagating from the source demonstrates a sharper rise time than that from dropping weight     

Numerical experiments of internal tides in Suruga Bay

Numerical experiments were performed to explain the observed results of the internal tides in Uchiura Bay. The experiments for the generation of the internal tides in Suruga Bay indicate that the internal tides, generated at the slopes in the bay, are not as large an amplitude as those observed in Uchiura Bay. However, when the semidiurnal internal tides incident through the mouth of Suruga Bay are considered, they are amplified. The amplitude at the head of Uchiura Bay is 6–12 times larger than that at the mouth of Suruga Bay under the summer density structure. Under the fall density structure, the amplitude ratio is approximately 4–6. The amplification of the semidiurnal internal tides in Uchiura Bay is considered to be due to resonance of the longitudinal internal seiche of Uchiura Bay. On the other hand, the calculated diurnal internal tides are not as large as those observed. Therefore, the diurnal internal tides are thought to already have these large amplitudes at the mouth of Suruga Bay. Therefore, from the observations and numerical experiments, it is concluded that the internal tides observed in Uchiura Bay are mostly the internal tides originating from the outer region of Suruga Bay, and the semidiurnal tides are the internal seiche which is resonantly amplified.     

Adriatic seiche decay and energy loss to the Mediterranean

A salient feature of sea level records from the Adriatic Sea is the frequent occurrence of energetic seiches of period about 21 h. Once excited by a sudden wind event, such seiches often persist for days. They lose energy either to friction within the Adriatic, or by radiation through Otranto Strait into the Mediterranean.The free decay time of the dominant (lowest mode) seiche was determined from envelopes of handpassed sea level residuals from three locations (Bakar, Split and Dubrovnik) along the Croatian coast during twelve seiche episodes between 1963 and 1986 by taking into consideration only time intervals when the envelopes decreased exponentially in time, when the modelled effects of along-basin winds were smaller than the error of estimation of decay time from the envelopes and when across-basin winds were small. The free decay time thus obtained was 3.2±0.5 d. This value is consonant with the observed width of the spectral peak.The decay caused by both bottom friction and radiation was included in a one dimensional variable cross section shallow water model of the Adriatic. Bottom friction is parameterized by the coefficient k appearing in the linearized bottom stress term _ρ0u (where u is the along-basin velocity and ρ₀ the fluid density). The coefficient k is constrained by values obtained from linearization of the quadratic bottom stress law using estimates of near bottom currents associated with the seiche, with wind driven currents, with tides and with wind waves. Radiation is parameterized by the coefficient f appearing in the open strait boundary condition ζ =auh/c (where ζ is sea level, h is depth and c is phase speed). This parameterization of radiation provides results comparable to allowing the Adriatic to radiate into an unbounded half plane ocean. Repeated runs of the model delineate the dependence of model free seiche decay time on k and a, and these plus the estimates of k allow estimation of a.The principle conclusions of this work are as follows.

1. (1) Exponential decay of seiche amplitude with time does not necessarily guarantee that the observed decay is free of wind influence.

2. (2) Winds blowing across the Adriatic may be of comparable importance to winds blowing along the Adriatic in influencing apparent decay of seiches; across-basin winds are probably coupled to the longitudinal seiche on account of the strong along-basin variability of across-basin winds forced by Croatian coastal orography.

3. (3) The free decay time of the 21.2 h Adriatic seiche is 3.2±0.5 d.

4. (4) A one dimensional shallow water model of the seiche damped by bottom stress represented by Godin's (1988) approximation to the quadratic bottom friction law ρ₀C_Du|u| using the commonly accepted drag coefficient C_D = 0.0015 and quantitative estimates of bottom currents associated with wind driven currents, tides and wind waves, as well as with the seiche itself with no radiation gives a damping time of 9.46 d; radiation sufficient to give the observed damping time must then account for 66% of the energy loss per period. But independent estimates of bottom friction for Adriatic wind driven currents and inertial oscillations, as well as comparisons between quadratic law bottom stress and directly measured bottom stress, all suggest that the quadratic law with C_D=0.0015 substantially underestimates the bottom stress. Based on these studies, a more appropriate value of the drag coefficient is at least C_D=0. In this case, bottom friction with no radiation leads to a damping time of 4.73 d, radiation sufficient to give the observed damping time then accounts for 32% of the energy loss per period.

     

Effect of ice cover on oscillations of fluid in a closed basin

Within the framework of the linear theory of long waves, the problem of the effect of ice cover on seiche oscillations of fluid in a two-dimensional constant-depth basin is solved. The eigenfrequencies and eigenfunctions of seiche oscillations are obtained for different boundary conditions at ice edges: rigid coupling and free edges. The forced oscillations of fluid and ice under the action of a moving disturbance of atmospheric pressure are investigated. The change in the stress of ice bending is considered and it is shown that the coast ice can be broken.     

南麂海域台风影响过程中的波浪特征

根据１９８３－１９８９年南麂海洋站在台风影响过程中的实测风和浪资料，分析了该海域的波浪特征。结果表明，这个海域的台风波浪通常是混合浪，在台风影响过程中出现的最大值波高，既有较大波陡的风浪，也有波陡较小的清浪；各向波高的均值变化不大，各向最大波高却有较大幅度的差距；本区的台风浪以４级波高占优，风浪以ＮＮＥ向、涌浪以Ｅ向为常浪向；波高为４级的风浪和涌浪，其周期分别在４．０－４．９Ｓ和７．０－７．９Ｓ之     

Quasi-linear model of interaction of surface waves with strong and hurricane winds

A quasi-linear model for determining the aerodynamic drag coefficient of the sea surface and the growth rate of surface waves under a hurricane wind is proposed. The model explains the reduction (stabilization) in the drag coefficient during hurricane winds. This model is based on the solution of the Reynolds equations in curvilinear coordinates with the use of the approximation of the eddy viscosity, which takes into account the presence of the viscous sublayer. The profile of the mean wind velocity is found with consideration for nonlinear wave stresses (wave momentum flux), whereas wave disturbances induced in air by waves on the water surface are determined in the context of linear equations. The model is verified by comparing the calculation results with experimental data for a wide range of wind velocities. The growth rate and drag coefficient for hurricane winds are calculated both with and without consideration for the shortwave portion of the windwave spectrum. On the basis of calculations with the quasi-linear model, a simple parametrization is proposed for the drag coefficient and the growth rate of surface waves during hurricane winds. This model is convenient for use in models of forecasting winds and waves.     

Numerical modeling of the propagation and strengthening of tsunami waves near the Crimean Peninsula and the Northeast Coast of the Black Sea

The linear model of long waves is used for the evaluation of the parameters of tsunami waves along the South Coast of Crimea, in the near-Kerch zone, and near the northeast coast of the Black Sea. Our numerical investigations are carried out for 24 probable locations of the elliptic zones of tsunami generation over the continental slope of the basin. The amplitude characteristics of tsunamis are computed for 27 sites of the Black-Sea coast. It is shown that significant strengthening of tsunami waves is possible in the course of their propagation toward the coast. The highest waves are formed at the sites of the coast closest to the seismic source. The dependence of the intensity of tsunami waves along the Black-Sea coast on the location of the seismic source and its magnitude is analyzed.     

Seiche oscillations in Lake Baikal

The variations in the free surface of Lake Baikal at three stations (Bol’shie Koty, Listvyanka, and Baikal’sk) are measured. A modern recording method and an advanced technique of record processing are used. Based on 1-year-long observation data, the amplitudes of seiche oscillations and their seasonal changes are analyzed. It is found, in particular, that 67-min seiches are manifested in different seasons. Numerical calculations of seiches in Lake Baikal are made with the use of up-to-date bathymetric data on one-dimensional, plan, and spherical models. Spatial structures of oscillations with periods of 277, 152, 84, 67, and 59 min, corresponding to the well-expressed peaks of power spectral density, are studied. It is shown that the first four periods correspond to uninodal, binodal, trinodal, and quadrinodal longitudinal seiche modes of Lake Baikal. The periods of three solutions can correspond to the value of 59 min. The first of them is the seiche of the lake’s South Basin, and two others are characterized by significant amplitude growth in the Small Sea and Chivyrkui Bay.     

Numerical Study of Two-Dimensional Focusing Waves

Two-dimensional focusing waves are generated and investigated by numerical method. The numerical model is developed by introducing the wave maker boundary on the high-order spectral （HOS） method proposed by Dommermuth and Yue in 1987 and verified by theoretical and experimental data. Some cases of focusing waves considering different parameters such as assumed focusing amplitudes, frequency bandwidth, central frequency and frequency spectrum are generated. Characteristics of the focusing wave including surface elevations, the maximum crest, shift of focusing points and frequency spectra are discussed. The results show that the focusing wave characteristics are strongly affected by focusing amplitudes, frequency bandwidth, central frequency and frequency spectrum.     

Wind-generated waves in Hurricane Juan

We present numerical simulations of the ocean surface waves generated by hurricane Juan in 2003 as it reached its mature stage (travelling from deep waters off Bermuda to Nova Scotia and making landfall near Halifax) using SWAN (v.40.31) nested within WAVEWATCH-III (v.2.22; denoted WW3) wave models, implemented on multiple-nested domains. As for all storm-wave simulations, spectral wave development is highly dependent on accurate simulations of storm winds during its life cycle. Due to Juan’s rapid translation speed (accelerating from 2.28 m s⁻¹ on 27 September, 1200 UTC to 20 m s⁻¹ on 29 September, 1200 UTC), an interpolation method is developed to blend observed hurricane winds with numerical weather prediction (NWP) model winds accurately. Wave model results are compared to in situ surface buoys and ADCP wave data along Juan’s track. At landfall, Juan’s maximum waves are mainly swell-dominated and peak waves lag the occurrence of the maximum winds. We explore the influence of surface waves on the wind and show that the accuracy of the wave simulation is enhanced by introducing swell and Stokes drift feedback mechanisms to modify the winds, and by limiting the peak drag coefficient under high wind conditions, in accordance with recent theoretical and experimental results.