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Ocean Waves - Science topic

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There are two different waves of the gravitational field (GF):
1. Gravitational field transverse wave. What it reflects is the disturbance of the surrounding GF, and the transmission speed of this disturbance is equal to the slow light speed c. For example, the motion of the sun disturbs the GF generated by the center of the galaxy, causing transverse waves of GF around the sun.
2. Gravitational field longitudinal wave. It is generated by the gravitational source itself, and GF will transfer energy quickly, and this speed is much greater than the speed of light c.
When the gravitational source changes (position, mass), this change will first be reflected on the longitudinal wave of GF, and distant objects will feel the change of GF soon. At the same time, the disturbance of the gravitational source to other surrounding GFs will propagate to the surroundings at a slow speed c in the form of transverse waves.
To make an inappropriate analogy: when you throw a stone into a calm lake, you will observe slow water waves spreading around, which is the disturbance of the stone to the water surface, thus generating water waves. But in addition to water waves, there are sound waves in the water. The speed of the sound waves is much faster than that of the water waves, and the sound waves in the water arrive long before the slow water waves reach the shore.
A brief summary: the longitudinal wave of GF is produced by the gravitational source itself, and the transverse wave of GF is produced by the disturbance of the gravitational source to other surrounding GFs.
Newtonian gravity studies "sound waves in water", the longitudinal waves of GF.
Einstein GR studies the "water wave", that is, the transverse wave of GF.
I hope that you can understand the whole gravity from my simple narrative. You can also download my two papers on gravity here:
Kind regards,
Tony
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That is whole new paradigm, Tony!! Quite interesting to see like the two types of oscillations with electromagnetic waves that I am developing alongside Emory Taylor with upcoming paper publications on extending Maxwellian electromagnetism and the photon constant of c of light.
Yet another aspect that I am developing alongside my coauthors will be the concept of the weak gravity versus strong gravity, which would explain all the vagaries that four fundamental forces act on - the strong nuclear, the weak nuclear, gravitational, and electromagnetic types. Finally, they can all be unified together under this umbrella eventually!!
Thank you with your innovative paradigm!!
Sincerely,
Rajan Iyer
ENGINEERINGINC INTERNATIONAL OPERATIONAL TEKNET EARTH GLOBAL
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Can anybody suggest tool for simulation of water waves. Or how to do it in COMSOL?
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I suggest choosing Flowvision.
Create a calculation area. Water below, air above. Enter conditions at the boundaries. In the parallelepiped of air, it is possible to create wind (tangential stresses) and periodic changes in air pressure. Wave theory suggests two mechanisms - shear stresses and pressure changes. In the hydrodynamic model, only periodic changes in pressure above the water surface work. Shear stresses (wind) can be added to the calculation after the waves have formed. Consult the Flowvision website.
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hey,
I have data of waves taken nearby the coast , I would like to transform the wave data by reverse refracting to get an idea of the deep ocean waves. could anyone recommend a software or a technique to reverse refract wave data?
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In the water waves are more mechanical in form and have a slower propagation velocity than air. In the first 500 feet the phase velocity is non-linear. I would recommend COMSOL acoustic simulation toolbox.
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I need to simulate the movement of a moored buoy shell in the diameter of 20 cm with an internal ball (diameter in 3 cm) in the water wave, with the accuracy of engineering project.
I am familiar with Ansys Fluent and COMSOL. But I recently heard of more engineering software such as Ansys AWQA,and 3D Flow.
Thus, I need some recommendation and references on a suitable software. An explanation of the advantage and drawback on different software will be really appreciated.
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COMSOL is indeed great tool but i recommend CFD
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The most celebrated equation which describes water waves is the Korteweg-deVries(KdV) equation. Some of the assumptions which go into this equation are incompressibility, irrotationality, inviscid, small amplitude and long wavelength. I want to throw the assumption of irrotationality out. Instead, I want to replace it with the actual vorticity equation in 2D which is Dw/Dt=0, where D/Dt is the material derivative and w=v_x-u_y.
Can I still do the same asymptotic reduction to get a weakly nonlinear equation for waves? If not, Why not?
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Probably not and if you get something it would look neither quite good nor complete. Irrotationality condition is either implicitly (when derivation is done in terms of velocity potential) or explicitly (when the horizontal and vertical velocities are connected) is used in deriving Boussinesq-like equations--more generally depth-integrated wave equations. The use of irrotationality condition is in a sense obligatory to complete the system. I am attaching a paper of mine which shows these steps explicitly. See equations 15 to 23 but especially notice eqn. 18. I suppose there is no need to say KdV is a form of Boussinesq.
On the other hand, there is always Gerstner's rotational waves -exact solution- (see Lamb's Hyrodynamics, p. 421) but they really do not have a good reputation :) from the physical point of view.
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Hi 
What frequency specification should I use in order to create a low-pass filter (specifically filter order and Frequency specifications)? I have a huge data set which contains unfiltered water level data and it is in meters relative to mean lower water (MLW) at hourly intervals. The time stamp is in Julian data. 
thank you in advance. 
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Welcome!
Sure you found a solution but I have a proposal that may help solve such problems.
At first one has to draw the wave profiles as w(x,t)
There are two waves, time waves at certain position.
And there is also position waves.
At first you can analyze these waveforms using Fourier transform such as fft.
After which you may decide on the low pass filter characteristics by setting the cut off frequency and the stop frequency and the stop band attenuation.
Then give such specifications one can design the low pass filter. either analog of digital.
Best wishes
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I am interested to perform spectral analysis of a structure under random waves. could anyone suggest me a book or an example that starts from wave spectrum (such as
JONSWAP spectrum , P-M etc) to RAO. A complete example from formulation to numerical evaluation.
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I have been searching an open-source tsunami modelling code for calculating tsunami wave propagation using finite-fault model.
I looked into COMCOT (Cornell Multi-grid Coupled Tsunami Model) and found a couple of articles that has been done with using slip distribution models, but version 1.7 (that i can find on the internet) does not  seem to be working properly.
So in summary i am looking for a code that does;
1.Nested grid with different grid size
2.Using slip distribution model for calculating initial wave and propagation
3.Plotting maximum wave height 
4. Comparing the results with the observed tsunami data (tide-gauges or DART buoy records), tsunami wave heights versus time
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You can do all you mention with the Tsunami-HySEA code (https://edanya.uma.es/hysea/). You can download a open source version of the code (not allowing nested grids) at: https://edanya.uma.es/hysea/index.php/download. And you can find extensive references at: https://edanya.uma.es/hysea/index.php/references
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Liquid will move more and having more amplitude in comparison to Earth
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Oceanic waves (swell) generate in areas of persistent stormy and high wind speeds (wave factories) and propagate out from there, coalescing one with another during thousand of kilometers, finally reaching the nearshore of coastal waters as almost regularly spaced (here in SW Australia, 12-14 seconds, extremely long) packs of 2-5 similar waves, refracting against he seabed and breaking or surging or spilling on beachfaces.
Swell, is the oceanic expression of an aeolian (wind) process.
The Coriolis Effect, like wind and cyclones paths, has an impact on swell propagation across big distances, but "vibrations" of Earth, on swell do not have impacts.
Earthquakes, vulcanic eruptions and submarine landslides certainly cause anomalous waves (Tsunamis), but do not modify swell.
In conclusion, swell, as far as I know, is not affected by earthquakes.
Cheers
Nick
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Rogue waves are an important topic in water waves, plasma physics, nonlinear optics, Bose Einstein condensate and so on
How can we predict a rogue wave?
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Are you interested in theoretical aspects? If yes, decouple your system in the form of a dinNPDE. Apply preferably numerical simulations to see the wave amplitude behaviours, and so forth. Have a needful comparison with the latest reports on it.
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Hi every one,
I have a series of ocean wave data such as wave height, period, direction and etc. I want to analysis the data and draw association diagram such as wave rose,frequency distribution and etc. What software is useful for this purpose?
Thanks a lot.
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Hello Everyone.,
I am having confusion regarding to calculate wave breaker angle.
wave breaker angle was calculated as the difference between the peak wave direction and the direction normal to the depth contour.
I don't know how to find breaker angle in the right manner.
Thank you in advance, who helped me
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This paper may help in your research Direct Formula to Compute Wave Height and Angle at Incipient Breaking, JWPCE, ASCE
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Movement of submarines is expected to give disturbances in water.
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Dear Satish,
I have the following opinion :
The motion of a submarine could be similar to the propagation of an internal wave. The conservation of mass muss be zero for a wave perturbation in the vertical integration. In the case of a submarine you have mass transport (the submarine ) and the surface will be affected.
Regards
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Morning all,
My first post on Research Gate! Hopefully it helps my endeavor :] and warning, it is long!
As the title said, I am brainstorming topics for my thesis. I am enrolled as MS, but will pursue PhD afterwards.
I am extremely interested in WECs. I have worked on a solar-based project before, and, until enrolling at OSU, had never heard of water based renewable energies! I'm so enthralled by them that I am taking a graduate CE course to better understand water wave mechanics.
The problem is, while I am an EE, I am not 500% interested in, for example:
- PE: for WECs, this might be designing a circuit with a super capacitor to help smooth the power output
- Control strategies through simulation: e.g. MPC in Simulink/WECsim
I have feared that maybe I chose wrong, and a ME/CE would suit me better, but all my advisors have told me that, "EEs wear multiple hats, we will surely find something for you"
Now to find what that something is!
I said "not 500%" because, I DO think those topics are interesting. I ask my friends hours on end what their PE circuit is, and to explain their method, and even go home and research to gain a fundamental understanding; I read a 50pg MPC document and covered each page top to bottom in notes you can't even tell where the original text is; I've attended WECSim courses to see how they are implementing control strategies
BUT it is not what I would want my thesis topic to be (I don't think, who knows maybe I start and I like it!)
I have talked to my advisor a lot, but am now reaching out to the power of the internet for various opinions...
So, does anyone have any ideas on what could be researched?
The only topic I found interesting when him and I were discussing were some WECs self adjusting to their environment.
Also, I know that most WECs are designed to work best at a certain frequency, but is there research out there regarding a WEC that could work at multiple frequencies, or change its' internal circuitry (control strategy) to better work at the average changing frequency of incoming waves?
Anything is really helpful. I am and will continue thoroughly researching and talking to many folks in the field to find something I am really passionate about, because in the end, that will make for a larger impact on the world.
Thanks,
- Chris
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Chris, a bit of friendly advice. You talk about WEC, but no where in your post do you mention what it stands for! Avoid acronyms.
WEC: Word Endurance Championships, World Energy Council, White Edge Crack, Washington Electric COOP, and the list goes on
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This area is my specialty?
Dr. Jim Wilson
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Interested
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I am focusing on the 3D-1D reduction of the Water-Waves equations in an open channel. What conditions should I impose on the intersection of the free surface and the bathymetry. I'm demanding if the condition of non-penetration at the bottom is sufficient ? thanks for your answers.
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There are two type of boundary cond. @ wall : "No Slip" & "Free Slip"
@ Bottom if you consider the " penetration " you might have vertical velocity, otherwise the perpendicular velocity would be zero; Horizontal Velocity is also the same as wall, "No Slip" with exponential profile in water column & "Free Slip"
@ free surface for 3D model you can use the VOF (Volume of Fluid) technique for better modelling the shape of the interface.
@ Upstream you could add wave maker through "piston" type or "plunger or paddle" type with relevant amplitudes, frequencies, & equations ...
@ Downstream it is better to assume a "low slope beach" to damp the energy, otherwise the channel should be long enough to avoid the impact of the "Reflection"
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There are many different forms of vortexes in the physical world. Here are two common examples:
Tornado vortex. These are common atmospheric disturbance in the Midwest in North America (see, e.g., the attached image).
Seashell vortex. Seashell sometimes are vortex-shaped (see, e.g., the attached image).
I am hoping that followers of this thread can post or point to other examples of vortexes in the physical world.
Here are several more examples more examples:
Photon vortex and electromagnetic vortexes.
This interest in vortexes is related to photon vortexes and electromagnetic vortexes.
Shape Vortexes [Geometry and Algebraic Topology].
From a shape theory perspective, it seems that many physical shapes are non-concentric, nesting cycles. In homology theory, the analogue of a physical vortexes is a collection of non-concentric, nesting 1-cycles. A 1-cycle is a collection of connected edges or surfaces that form a connected path so that one can find a path between any pair of vertices in the 1-cycle.
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The most common examples of vortical structures are found in classical fluids; at the laboratory scale, we can mention
  • Kelvin-Helmholtz billows
  • Hexagonal vortical patterns in fluids heated at the lower boundary
  • Von Karman vortex wakes
In the rotating stratified fluids in geophysical systems, we have a multitude of vortices at different scales; they include
  • tropical cyclones
  • Vorticity tubes associated with tornadoes
  • Rotors on the lee side of the mountain ranges
  • Lagrangian coherent vortical structures in the oceanic flow
The main characteristic of all these structures is that they do not keep their identity for a very long time, the main exception being perhaps the red spot of Jupiter which has preserved its integrity over the last few hundred years.
The completely new reality of vortex systems is in quantum fluids where they can last indefinitely.
On a larger scale, in space, practically all forms of matter organization come in the form of swirls with the most spectacular examples provided by rotating galaxies. It is fairly easy to conclude that the vortex movement is ubiquitous in all physical systems, from the smallest to the largest. There are even some attempts to connect the scales proposed in the framework of a Cantorian Superfluid Vortex hypothesis; these attempts are very similar, in many respects, to Lord Kelvin's old hypothesis explaining atoms as vortex structures.
For an in-depth discussion of all these fascinating topics, please see a very interesting article by Frank Wilczek entitled: "Beautiful Losers: Kelvin's Vortex Atoms". The text is available at:
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Linear wave theory for inviscid fluids gives a shallow water wave speed of c = (gy)^0.5. Is there an approximate solution that applies to viscous fluids? I am particularly interested in non-Newtonian fluids (sediment-water slurries). Can anyone point to experimental work on viscous/non-Newtonian fluids? Thanks!
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I am trying to simulate the gap resonance between two floating bodies and with a gap of 3m under the effects of ocean wave (direction longitudinal of the ships body) . I am trying to put surge, sway and yaw as fixed. There should be a damping effect on floating bodies due to viscous effect and radiation . How do I determine the linear and damping coefficient of such bodies. This is my first time simulating a multiphase flow and needed advice on this.
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You shoud add function ovject to your controlDict in order to read forces etc.
From the forces probably you can calc what u need.
Franco
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I have wave spectra time series and I use some partitioning and tracking algorithms to identify swell events and wind sea events. My aim is to model separately frequency time evolution in a swell event case and a wind sea event case.
- In case of swell event, since the swell celerity in deep water is g/(4pf), the frequency of waves generated at a single remote location and arriving at some observation point increases linearly with time. According to this, I choose an increasing linear model for pic frequency time evolution in a swell event.
- Now, what kind of model can I choose in wind sea event case? Actually I use a decreasing linear model assuming that the energy transferred by wind to sea water contribute to decrease the frequency. Is it a coherent hypothesis? Can you recommend another model to me?
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I have compiled waveDyMFoam successfully (OpenFOAM 2.4.0).
But when I use the solver to simulate a free floating ship model in regular waves (only heave motion allowed), the velocity of the ship increases rapidly, making the running process crashed. 
Has anyone  solved the problem?? 
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I am analysing a coastal time-series of hourly temperature and salinity measurements. I have calculated the density using Gibbs seawater toolbox (TEOS10). In addition I have also calculated the seawater density/stability ratio $R_{\rho}$,
     $R_{\rho} = \frac{\alpha \Delta\Theta}{\beta \Delta S_A}$.
Specifically, I want to distinguish between density changes affected by either changes in temperature or salinity. So far I have just seen articles analysing the spatial distribution of the density/stability ratio, either vertical or horizontal, and mentioning that when $-1 < R_{\rho} < 1$, the density is affected mostly by changes in salinity.
Could anyone please suggest further references, especially analysing this ratio using time-series?
Cheers!
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I think your interest to examine these temperature and salinity density/stability ratio on the coastline rather on the open water. If so, I suggest you look in other factors i.e., tidal chandes, local current circulation and bathymetry. 
These elements are affected local dwillers in many ways i.e., induce spawning
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please help me. i have developed a model in simulink to model a power generation system from ocean waves. i have used a three phase asynchronous generator block along with the a three phase RLC load in series and a three phase RLC load in parallel. The parameters for the generator block is known from literature but parameters for the two RLC loads are not known. Please help me to put suitable values to them. and also help me to obtain the variation of power generated in the generator with time.
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Hi Widitha
The shunt load is required for numerical stability of the model and should be set to 5% of the nominal value of the machine. In your case, you have a 1MVA machine so you should set the shunt load at 50kW.
The series load is set by you, according to your application. If you don't know what you are doing, I would suggest to set it to 80% of nominal power.
Hope this answer your question
Luc-André
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Air-sea interaction
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I had generated some sketches for the air-entrainment, bubble formation and buoyant degassing processes occurring during plunging breaking for my review article.
Here's a link to the paper. I hope this helps. Best regards. 
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I want to simulate open-channel flow in ANSYS CFX with a cylinder inside the water, which has a distance from bottom and free surface. I want to simulate free surface waves due to the existence of cylinder. The inlet water is assumed to be laminar. I have done this project by using the method in tutorial 9 of ANSYS CFX but the result is not acceptable. As you can see in the picture, free surface waves appeared between cylinder and inlet, which should be appeared between cylinder and outlet. (Flow direction is from left to right.)
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Are you using a ramp to initialize the flow? From your image I guess that an abrupt flow initialization may give this effect.
Try to check also your timestep and CFL during the simulation as well as the discretization around the free surface.
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The drag coefficient increase with wind speed up to a limited value (~35m/s),the roughness reduces to virtually zero by ~80m/s wind speed. (Leo Holthuijsen's JGR paper 2012). An idealized condition, if hurricane wind speed bigger than 70m/s (category 5), the drag coefficient is very low (towards zero), how is the ocean waves? how does wind energy/momentum go to the ocean currents?
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I do not think we know what is happening at 70 m/s... The sea is white, the air is full of spray...
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Do any one know how to find 'Directional spreading index, n' or
'Directional standard deviation, DSD' of Ocean waves? 
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Thank you Fabrice Ardhuin and Nguyen Thanh Luan...
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Free SAR Wave Mode Data Download
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You will find some info on Sentinel 1 wave mode here ... http://oceanwavesremotesensing.ifremer.fr/
As for the level 2 access, just ask Alexis mouche <alexis.mouche@ifremer.fr> or Romain Husson at CLS.
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Hi all,
I'm looking for references regarding the implementation of WW3 for the NE Atlantic by IFREMER. I have access to the hindcasts through the IFREMER ftp but I can't find any reference regarding the implementation. Can anyone help with this. 
Thanks in advance!
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Hi guys,
the latest hindcasts are at
if you are looking at rectangular grids, yes the latest reference is Rascle & Ardhuin (2013). For the triangle-based grids, there are extra coastal effects described in Roland and Ardhuin (Ocean Dynamics 2014) :
And if you are interested in infragravity waves
the proper paper would be Ardhuin et al. (Ocean Modelling 2014)
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I'm working on modelling the propagation and dissipation of waves in both the marginal ice zone and also in the ice pack. My understanding is that in the ice pack we may get both scattering (from changes in ice thickness as described in papers by Squire for example) and dissipation which may occur due to the viscosity in the water (this I know how to deal with) and also the creep effect in the ice. I've found the 1973 paper by Wadhams about this but I imagine that there may be more recent updates on the ice properties ... and that these properties may be different for steady or oscillating loads. I just started to read a few papers by Timco, but I am a bit lost.
So, basically I was going to use Wadhams' theory, with some updates on the mechanical properties of sea ice. Any advice is welcome.
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Thanks to all. I finally found an answer in Cole et al. (JGR 1998): "Cyclic loading and creep response of aligned first-year sea ice" which is generally consistent with Wadhams (JGR 1973), in proposing a reduced B coefficient for creep. This is now in version 5.10 of WAVEWATCH III (to be released in a few months) ... and was used for this paper:
But yes it is in general a function of ice temperature and thickness and floe sizes are very important parameters too. 
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What are the open source software for Tsunami modeling? 
thanks and regards
sowmya
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TUNAMI is code written by Prof. Immamura. It has TUNAMI N1, N2, N3 and F1, F2. N stands for  near field and F stands for far field. Code is available online. YOu may refer the following document:
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Lower resolution CTD can miss too much of the detail maybe? 
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Not sure about 1KHz, but if you wanted to callibrate before casting them, controll environment is the best as Mr jf Argentino suggested. And as for more the manufacture information must be kept in mind for accuracy n resolution for those sensor. 
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I dont have any buoy data at the particular locations, anybody pls let me the suggetion
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You can export a sample .dfs2 file to ASCII to see the structure of header, body, etc.
Something like this:
--------------------------------
"Title" "File Title"
"Dim" 2
"Geo" "LONG/LAT" 46 36 0
"Time" "EqudistantTimeAxis" "2015-01-01" "00:00:00" 848 21600
"NoGridPoints" 73 97
"Spacing" 0.125 0.125
"NoStaticItems" 0
"NoDynamicItems" 3
"Item" "u" "Wind speed" "m/s"
"Item" "v" "Wind speed" "m/s"
"Item" "mslp" "Pressure" "hectoPascal"
NoCustomBlocks 1
"M21_Misc" 1 7 0 -1E-030 -900 -99 -1E-030 -1E-030 -1E-030
"Delete" -1E-030
"DataType" 0
"tstep" 0 "item" 1 "layer" 0
2.652323 2.653296 2.673731 2.694167 2.715089 2.735525 2.755961 2.776396 2.797319 2.801698 2.798292 2.795372 2.791966 2.788561 2.785641 2.782235 2.778829 2.771531 2.763746 2.756447 2.749149 2.741850 2.734065 2.726767 2.712657 2.684922 2.657675 2.629941 2.602207 2.574472 2.546738 2.519491 2.484945 2.446993 2.409041 2.371575 2.333623 2.295671 2.257719 2.219767 2.215388 2.211982 2.209063 2.205657 2.202251 2.199332 2.195926 2.200305 2.224633 2.248961 2.273289 2.297618 2.321946 2.346274 2.370602 2.404662 2.445533 2.485918 2.526789 2.567174 2.608045 2.648917 2.689302 2.753528 2.819701 2.885873 2.952046 3.018219 3.084878 3.151051 3.219656 3.297020 3.373897
.
.
.
------------------------------
Write your code via MATLAB/FORTRAN or any program languages you're able to use
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Charnock parameter is approximately saturated under high wind speeds.Will Charnock parameter be decreased under gale-force winds? What is the role of the wave age and sea spray on this possible decrease?
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Hi Georges:
At high winds (above 25ms-1) the sea surface roughness saturates hence the Charnock coefficient becomes constant. This does not mean that for higher winds  the momentum flux becomes constant because it increases as the wind speed squared. A good reference on surface roughness saturation is the paper by Donelan et al. 2004 published in Geophys. Res. Lett. Measurements in hurricanes have shown a decrease of drag coefficient at very strong winds this topic is discussed in papers by Emanuel, Soloviev and others.
Regards
Héctor
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Hi
I try to use optimal interpolation and kalman filter method of data assimilation for water wave data with fortran and matlab. but before using these methods in wave modeling, I want to test my fortran and matlab codes with simple examples. so I tried to estimate a supposed simple quadratic equation with methods mentioned above but the results are Disappointing. could u plz give me a numerical example that helps me to learn these methods?
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Please see the report arXiv:1503.04313v1.pdf
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I have been working on synthetic aperture radar data for retrieving ocean wave spectra, and i was using python for the process i was successful only to the extent of reading raster data, i would like to know whether python has any module for sar data processing as further processing of SAR data is getting complex
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You could try Nansat (https://github.com/nansencenter/nansat). Should at least get you a bit further than just reading the data. It works quite well for ASAR and Radarsat-2, and also Sentinel-1 should be ok.
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I have been working on some SAR data Sets like RISAT-1 and RADARSAT-2 , I have extracted the pixel information using python and I would like to know how this information can be used to extract wave and wind information over open ocean
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Hi,
There are a few oceanography and climate libraries for python. You should check a list like this:
As for extracting the exact parameters you're requesting I would advise checking scientific literature (since I don't know how to do it). A quick google search for "RISAT-1 ocean waves" revealed this article:
You can request the full article here in research gate. I'm sure a more extensive search would reveal a lot more info on the subject.
Best of luck.
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This is the definition everyone uses, including myself. But just because everyone uses does not justify that it is valid.
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Dear Paul,
The definition is totally irrational. I avoid using it because I find it wholly misleading. Using Hs as a reference is also irrational but less so since nearly everybody seems to believe it with religious zeal, and it has become an unfortunate unavoidable fixture in oceanography and ocean engineering that is impossible to get rid of and replace with the rms surface displacements as the more logical choice. To an outsider well informed in statistics and stochastic processes, this will look a rather nonsensical parameter for scaling various features of the sea-surface elevations.
There are more rational ways for describing wave occurrences that do not seem to follow expectations defined in terms of a reliable wave-height model. These are basically outliers that can be defined on a logical basis as a joint event that a wave height several times larger than the surface rms occurs, and it does so more frequently than what we predict from a well-established wave-height frequency distribution. In other words, the definition must include a threshold representing a relatively large magnitude as well as some higher rate of occurrence. Otherwise, wave-heights satisfying thresholds such as H//Hs > 2-3 or more are not necessarily abnormal since they are predicted by the Gaussian model, obviously with very low frequencies of occurrence.
An apparent difficulty with the alternative definition implied here is that how one decides on a relative threshold magnitude that would be of importance in theory and practice. My experience is that even the so-called abnormal waves do not violate the Miche-Stokes type upper bounds. Relatively low or small waves do, but we are not interested in these in the present context. Given the spectrum, the largest wave that we might observe based on the Gaussian or some non-linear theory is that implied by this upper bound. If one uses the spectral average period (T01) associated with the largest waves in a sea state, the period of the largest ones seem to converge to a value 1-1.25 * T01 approximately, depending on the frequency bandwidth. The corresponding upper limit wave height then follows immediately from the Miche-Stokes bound. That is one possible clue to the threshold magnitude needed. Consequently, the relative frequency of that threshold is easily  estimated from the theoretical wave-height frequency curve. If the wave height and associated occurrence frequency of a large wave actually observed exceeds the two thresholds both noticeably, say about 5-10% or so, then one can contemplate if the observed wave is an abnormal occurrence. 
The bigger dilemma is that there is presently no way for experimentally verifying the existence of these "abnormal waves" under oceanic conditions. They seem to be isolated occurrences observed as outliers in some relatively short wave record, but it is ignored that the wave field where that record comes from actually contains zillions of other waves. Fixed-point observations are archaic and simply misleading for extremes to begin with in realistic seas characterized with a directional wide-band spectral content. So, what is needed is much larger spatial-temporal observations of the wind-disturbed sea-surface over long intervals of time. That has not become a routine yet. Numerical and experimental simulations in wave tanks do not explain what might be happening in the ocean. There is a lot more to discuss on the general topic, but enough now.
Best regards...Aziz
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We are looking into time series of vertical buoy (Datawell) or radar displacement measurements of the ocean surface and are trying to discriminate between outliers and extreme waves. Has anybody got experience with this task?
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... thanks a lot!
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Clearly freak waves happen all the time in the ocean, but only being noticed when someone nearby encounters it or becomes a victim of it because it happens unexpectedly.  
The unexpectednes is a fact everyone recognizes, but no one can do much about it.
The main failure of the conventional definition of greater than twice significant wave height is simply its failure reflecting unexpectedness. Hence this question!
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I think Christian's answer covers a lot of ground.  Is the wave a freak because it is different from the wave field it is in, is it an extreme wave (which is the public idea) or is it a wave that produces unusual forces/motions/effects.  All are 'freaks' in some way and size does not really matter :)
A real 'freak' would be defined as inexplicable in terms of existing theoretical paradigms.
If the question is about defining a probability in philosphical terms then maybe we are into the idea of 'surprise' - I think there's a very old book by Spanner - it was new when I was young.
Or there is a field of study that deals with the probability of events that never happen where the boundary limit is estimated by the duration within which nothing has been observed.
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Nonlinear Schrodinger equation is a completely integrable Hamiltonian system. It admits infinite number of conserved quantities. It also admits rational solution (or rogue wave solution).The main character of this solution is unstable and it appears nowhere without a trace. So my question is what happen to the energy of the rogue wave when it goes off.
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I am a believer in the use of NLSE for optical phenomena not for surface waves.  There is some work on the importance of transverse shear flow in generating these.  When you say "goes off" do you mean that the wave is suddenly dissipating?   I did a little calculation recently that gives the energy of such a bound wave group as borrowed from the surface shear energy so the vanishing of the shear allows the wave to spread into the spectral components and simply become lower and wider.  
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Why the usual(spilling wave) assumption of the wave height inside the surf zone being proportional to the water depth will conflict with the important matter of the distribution of the applied longshore stress across the surf zone?
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I've been perplexed by the state of this field for a while.  I thought I would visit the topic while studying hydrodynamics of quantum gases to get some classical background but I found a lot of enduring superficial analysis and inconsistent assumptions.  I don't claim to be certain of the answer.  I wrote a series of papers that I recently sent off.  Tell me if they address the general topic you are asking about in any meaningful way.  
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Thanks in advance.
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The nonhydrostatic effects are important when the vertical acceleration is non-negligible. Boussinesq (1872) describes mathematically this kind of equations when the vertical velocity component is included in the integrated motion equations. In the literature, contributions related to barotropic and homogene water have been published in the literature in the last 50 years ( e.g. Peregrine 1967; Abbot et. al. ,1978; Hauguel 1980; Shapper, 1986).
Boussinesq J., 1872. Theory des ondes et des remous Qui se propagent le long d?un canal rectangulaire horizontal, en communiquant au liquide contenu dans ce canal des vitesses sensiblement pareilles de la surface au fund. J. Math. Pures Appl., vol. 17, pp. 55-108.
Abbot M.B., Peterson H.M., Skoovgaard, 1978. On the Numerical Modelling of Short Waves in Shallow Water. Journal of Hydraulic Research, vol. 16, pp. 173Peregrine D.H., 1967. Long waves on a beach. Journal of Fluid Mechanics, vol. 27, pp. 815-827.
Shapper H., 1986. Ein Beitrag zur numerischen Berechnung von nichtlinearen kurzen Flachwasserwellen mit verbesserten Differenzenverfahren. Report No. 21, Institut fuer Stroemungsmechanik und Elektronischen Rechnen im Bauwesen, Universitatet Hannover. -203. 
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statistical and spectral analysis of ocean waves
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To play around with data this MATLAB toolbox is a good base:
MACE toolbox from Nobuhito Mori:
You might also have look here for Nobuhito Mori.
Greetings
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For a moored structure in waves, calculation of wave drift damping of the body is of vital importance for calculation of the slow drift motion. However, how to calculate the wave drift damping in a more concise way?
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Please try to see relevant papers by Dr. Ir. Johan Weichers. He is one of the pioneers in this field. You can also see some works by Prof. Grant Hearn. These works are all done in late 80s and early 90s.
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I normally work with the degrib dos command line which is practical to extract data from ECMWF as you can have many years and different variables in a single file, if you reduce the geographical location for which you are extracting the data. The problem is that I downloaded some wave data from NCEP, and for each wave characteristic you get a file for a single month (eg. nww3.hs.200905.grb, for significant wave height values for May 2009).
This implies that for a 12 years series I have for each location to convert 144 files from grib to txt or dat. As I need also to get peak directions and peak periods this would mean that 432 files need to be converted. Considering that I have to do it for other 4 locations the total number will rise to 2160 files.
For sure there should be a quicker way of doing it! All suggestions in how to do it will be appreciated.
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wgrib, which is available from NCEP is another useful tool for dumping grib to different formats.  Here are some tricks:  ftp://ftp.cpc.ncep.noaa.gov/wd51we/wgrib/tricks.wgrib
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Look at the attachment picture and observe that there exists a strong analogy to the global map of Earth: The blue areas are relevant to the continents while the red ones correspond to the oceans. Is there a possibility of CMB to be just a microwave reflection of radiation and not the so called cosmic microwave background radiation?
More images for CMB here:
For more arguments see also:
Robitaille P.-M., WMAP: A Radiological Analysis, http://www.ptep-online.com/index_files/2007/PP-08-01.PDF
and other references from Stephen Crothers post in:
The original CMB picture was taken from:
Bennett C.L. et al, First-year Wilkinson Microwave Anisotropy
Probe (WMAP) observations: preliminary maps and basic
results.Astrophys. J. Suppl. Ser., 2003, v. 148(1), 1–27
My modest opinion is that Africa, South America and Australia are visible in all CMB pictures.
What is your opinion about?
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Arno: I don't know who described the CMB as homogeneous, but I have here on my bookshelf Weinberg's 1972 book, Gravitation and Cosmology, which has an entire chapter devoted to the topic of the CMB, including a detailed discussion of its fluctuations and anisotropies, along with whatever scant early observational data was available at the time.
Then, as early as 1983, there was already a space-based (Soviet) experiment producing a whole-sky CMB map clearly showing the anisotropy due to the Earth's motion and smaller fluctuations (https://web.archive.org/web/20130331094338/http://epizodsspace.airbase.ru/bibl/nauka-v-ussr/1992/vzglad.html). The experiment was sufficiently sensitive to detect a quadrupole moment (https://en.wikipedia.org/wiki/RELIKT-1).
Ten years ago, the WMAP satellite was already collecting data for three years. Indeed, their first data release took place one year earlier (see, e.g., http://lambda.gsfc.nasa.gov/data/map/Tegmark/toh_wmap_wiener.jpg). So the suggestion that "ten years ago [...] background radiation was described as homogeneous" seems to me strongly contradicted by the actual, well-documented history of the subject. Indeed, I am not aware of any serious suggestion in physical cosmology that the CMB would be homogeneous; on the contrary, it was understood very early on that the temperature fluctuations of the CMB would be strongly correlated with density fluctuations of matter in the early universe. By 1972, when Weinberg's famous book was published, this was well-established science.
Just to be clear, I like weird ideas. I worked on a few of them myself. But it is not enough for an idea to be weird... it also has to show agreement with observation, be self-consistent, and it must be an improvement over existing ideas in terms of its predictive power. And while the inertia of the "mainstream" and the resistance of a mediocre "establishment" to new thoughts can be annoying at times, all too often, when maverick ideas fail on these counts, their supporters tend to blame some "mainstream" conspiracy instead of doing the right thing, namely revising or discarding a hypothesis that is proven to be untenable.
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Please compare the strengths as well as limitations of these two models. Also the answer should consider the recent advancement as far as possible in both the models.
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One of the other possible answer could be that Boussinesq equation can model nonlinear waves but Mildslope equations can model only linear waves.
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Most of the field experiments reported use a single array of instruments to measure the surf zone wave and current dynamics. I would like to know if anyone has studied the alongshore variation of the wave and current along a stretch of beach/surf zone.
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See Apotsos et al. [JGR 2008] (http://onlinelibrary.wiley.com/doi/10.1029/2007JC004514/abstract) where they analyzed alongshore setup and flow variability in the surf zone.
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Concerning the analysis of a three dimensional water wave in the ocean, is there any relation (displayable by the equations) between wave refraction and fluid particle trajectories, or a relation to show the water current refraction?
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Hi José, the reference is: Constantin, Adrian, and Joachim Escher. "Particle trajectories in solitary water waves." Bulletin of the American Mathematical Society 44.3 (2007): 423-431.
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When running a wave model with these winds I suddenly get large biases in 2007. I did a quick fix by reducing some wave model parameters. This was OK for the wave heights, but the mean period seem to suffer a change, possibly because there is more swells in 2007 and following years... So, hum, my next likely step is to look at wind histograms and possibly to a quantile-quantile correction.
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I actually double checked in the Chawla et al. paper (in press in Ocean Modelling) and there is a clear creep up of the 80 to 99 percentiles of the CFSR wind after 2006, not of the mean.