Rough Sea Surface Research Articles

Experiments on the sound transmission at the water-air interface for different source-interface distances

The water-air interface is a nearly perfect reflecting boundary for acoustic waves due to the high impedance contrast between the two media. In many marine applications, ranging from geophysical measurements to bioacoustics, the sea surface has an important impact. A rough sea surface topography leads to complex scattering and interference for acoustic frequencies with similar wavelength or higher than the wavelength of the interface. In geophysical applications and infrasound the frequencies of interest often have larger wavelengths. At the same time, the source within these applications is mostly located close to the interface with respect to its wavelength. In this case, an increased transmission could be expected as experiments with acoustic transducers demonstrate [D. C. Calvo et al., J. Acoust. Soc. Am. 134, 3403-3408 (2013)]. We design two experiments with sources close to the interface while changing the distance between the source and the water-air surface. For the first experiment, a water gun, which creates large cavities, is placed in water. Second, a signal gun, as used in athletics, is positioned in air. The acoustic transmission from both sides is measured and investigated. We find an increasing transmission coefficient for lower frequencies and decreasing distance between source and interface.

Numerical analysis on the effect of the sea surface roughness on the acoustic transmission loss in the atmosphere

Atmospheric sound propagation over the sea surface is affected by several factors including wind, temperature profile, and sea state. Preliminary numerical studies using a Crank-Nicolson parabolic equation solution approach have demonstrated that sea roughness introduces excess transmission loss. This excess loss induced by the sea roughness can be represented instead by a flat surface with a compensatory effective impedance. Calculation of transmission loss for the flat surface is computationally more efficient but requires the evaluation of the effective impedances for any given sea state. In this study, a 2-D finite-difference time-domain solver is used to determine the effective impedances for several sea states. The effect of sea roughness on transmission loss is then quantified by using the Crank-Nicolson parabolic equation.

Effects of wind waves on horizontal array performance in shallow-water conditions

We analyze the influence of statistical effects of the propagation of an acoustic signal excited by a tone source in a shallow-water channel with a rough sea surface on the efficiency of a horizontal phased array. As the array characteristics, we consider the angular function of the array response for a given direction to the source and the coefficient of amplification of the signal-to-noise ratio (array gain). Numerical simulation was conducted in to the winter hydrological conditions of the Barents Sea in a wide range of parameters determining the spatial signal coherence. The results show the main physical effects of the influence of wind waves on the array characteristics and make it possible to quantitatively predict the efficiency of a large horizontal array in realistic shallow-water channels.

Equations for normal-mode statistics of sound scattering by a rough elastic boundary in an underwater waveguide, including backscattering

Underwater sound scattering by a rough sea surface, ice, or a rough elastic bottom is studied. The study includes both the scattering from the rough boundary and the elastic effects in the solid layer. A coupled mode matrix is approximated by a linear function of one random perturbation parameter such as the ice-thickness or a perturbation of the surface position. A full two-way coupled mode solution is used to derive the stochastic differential equation for the second order statistics in a Markov approximation.

Implementation of a two-way coupled atmosphere-ocean wave modeling system for assessing air-sea interaction over the Mediterranean Sea

It is generally accepted that the fluid layer surrounding Earth should be considered as a single system at short and longer spatiotemporal scales. The current knowledge on the complex mechanisms of atmosphere-ocean interactions is still insufficient. This translates to the simulation of atmosphere and ocean as a single fully coupled system constructing multi-scale, multi-model integrated modeling systems. In this study, a new two-way coupled atmosphere-ocean wave modeling system is introduced with overarching aim to thoroughly unveil the impact of waves on the sea surface roughness and the atmospheric properties. The newly developed system consists of the Weather Research Forecasting (WRF) model with Chemistry (WRF-Chem) and Hydrology (WRF-Hydro) as the atmospheric component and the Wave model (WAM) as the ocean wave component. WRF and WAM models are coupled using the OASIS Model Coupling Toolkit (OASIS3-MCT) that enables models to communicate and exchange the information required to refine their simulation results. CHAOS (Chemical Hydrological Atmospheric Ocean wave System) has been tested in a high-impact cyclonic system over the Mediterranean Sea employing one-way and two-way coupling simulations to assess the air-sea interactions, currently neglecting the chemical and hydrological capabilities. The encapsulation of the ocean waves in the atmospheric surface layer processes modifies the characteristics of atmospheric flow and determines the ocean wave generation. A remarkable finding is that the coupling of the two systems affects the air-sea momentum, enthalpy and moisture transfer determining the evolution of the cyclonic system. The interactions along the air-sea interface are also reflected in the vertical structure of troposphere and its properties. CHAOS in two-way coupling mode shows an overall improvement of the forecast skill up to 20% over the sea while positively affects the atmospheric predictability over the land.

Effects of Breaking Waves on Composite Backscattering from Ship-Ocean Scene**Supported by the National Natural Science Foundation of China under Grant No 61372004, the Fundamental Research Funds for the Central Universities, and the Foundation of Science and Technology on Electromagnetic Scattering Laboratory.

The existence of the sea surface is bound to affect the electromagnetic (EM) scattering from marine targets. When dealing with the composite scattering from targets over a sea surface by applying high-frequency EM theories, the total scattering field can be decomposed into three parts in low sea states, namely, the direct scattering from the sea surface, the direct scattering from targets and the coupling scattering between the sea surface and targets. With regard to high sea states, breaking waves make the direct scattering from the sea surface and the coupling scattering more complicated. To solve this issue, a scattering model is proposed to analyze the composite scattering from a ship over a rough sea surface under high sea states. To consider the effect of breaking waves, a three dimensional geometric model is adopted together with Ufimtsev’s theory of edge waves for the scattering from a breaker. In addition, the coupling scattering between targets and breaking waves is taken into account by considering all possible scattering paths. The simulated results indicate that the influence of breaking waves on the scattering field from the sea surface and on the coupling field is non-negligible, and the numerical results also show the effectiveness of the proposed scattering model.

Feasibility of GNSS-R Ice Sheet Altimetry in Greenland Using TDS-1

Radar altimetry provides valuable measurements to characterize the state and the evolution of the ice sheet cover of Antartica and Greenland. Global Navigation Satellite System Reflectometry (GNSS-R) has the potential to complement the dedicated radar altimeters, increasing the temporal and spatial resolution of the measurements. Here we perform a study of the Greenland ice sheet using data obtained by the GNSS-R instrument aboard the British TechDemoSat-1 (TDS-1) satellite mission. TDS-1 was primarily designed to provide sea state information such as sea surface roughness or wind, but not altimetric products. The data have been analyzed with altimetric methodologies, already tested in aircraft based experiments, to extract signal delay observables to be used to infer properties of the Greenland ice sheet cover. The penetration depth of the GNSS signals into ice has also been considered. The large scale topographic signal obtained is consistent with the one obtained with ICEsat GLAS sensor, with differences likely to be related to L-band signal penetration into the ice and the along-track variations in structure and morphology of the firn and ice volumes The main conclusion derived from this work is that GNSS-R also provides potentially valuable measurements of the ice sheet cover. Thus, this methodology has the potential to complement our understanding of the ice firn and its evolution.

Numerical Calculation and Analysis of the Normal Relationship of the Dual-Frequency Altimeter Backscatter Coefficient

To improve effectively sea surface wind speed retrieval accuracy when it is raining, it is very interesting to find out a more accurate normal relationship between Ku band backscatter coefficient s0ku and C band backscatter coefficient s0c for a dual frequency altimeter (13.58GHz/5.25GHz). In this paper an electromagnetic scattering model with a rough surface profiles is established. In terms of the established rough sea surface model s0ku and s0c are calculated by electromagnetic numerical methods of method of moment (MOM) and Kirchhoff approximatation (KA) when the rms of the rough surface is various. The simulation results are in a good agreement by these two different methods: MOM and KA. Those calculated results show that the relationship between s0ku and s0c obtained by calculating electromagnetic scattering from Gaussian random rough sea surface profiles exists positive differences from the normal relationships while those obtained by calculating electromagnetic scattering from Exponential rough sea surface profiles exist negative differences from thoses. It is obvious that the calculated relationship can be in a good agreement with an empirical normal relationship by adjusting compositions of those ocean spectrums. It is a good way to find out the normal relationship between s0ku and s0c by theoretical calculation and analysis, and also is an effective way to study the ocean spectrum and sea states.

Offshore wind resource assessment of Persian Gulf using uncertainty analysis and GIS

This paper, focused on offshore wind resource assessment of the Persian Gulf using uncertainty method. In this method, for better estimation of wind energy potential, the uncertainties due to wind speed, sea surface roughness, air density and wind turbine power production were considered. For this purpose, first, a 25-year (1984–2008) ECMWF (European Center for Medium-Range Weather Forecasts) data from over 550 points in the region were collected. Then by analyzing the data, the probability of density of each parameter was established. Next; a Monte Carlo simulation was used for long-term simulation of wind field and energy production. The results showed that the Persian Gulf is capable of generating over 2980 GWh per year which is 5.3% lower than results of the conventional analysis. A detailed GIS (Geographic Information System) analysis depicted that middle and southern parts of the region have a high level of wind energy potential. Also, an economic analysis showed that by 15% interest rate, offshore wind farms cannot compete the global electricity production rates; however, by decreasing the interest rate to 5%, the southern, southwestern, and a part of northern shoreline are capable of generating offshore wind electricity in the range of global offshore electricity production costs.

Intense deformation field at oceanic front inferred from directional sea surface roughness observations

AbstractFine‐scale current gradients at the ocean surface can be observed by sea surface roughness. More specifically, directional surface roughness anomalies are related to the different horizontal current gradient components. This paper reports results from a dedicated experiment during the Lagrangian Submesoscale Experiment (LASER) drifter deployment. A very sharp front, 50 m wide, is detected simultaneously in drifter trajectories, sea surface temperature, and sea surface roughness. A new observational method is applied, using Sun glitter reflections during multiple airplane passes to reconstruct the multiangle roughness anomaly. This multiangle anomaly is consistent with wave‐current interactions over a front, including both cross‐front convergence and along‐front shear with cyclonic vorticity. Qualitatively, results agree with drifters and X‐band radar observations. Quantitatively, the sharpness of roughness anomaly suggests intense current gradients, 0.3 m s−1 over the 50 m wide front. This work opens new perspectives for monitoring intense oceanic fronts using drones or satellite constellations.

How rough sea affects marine seismic data and deghosting procedures

ABSTRACTMost seismic processing algorithms generally consider the sea surface as a flat reflector. However, acquisition of marine seismic data often takes place in weather conditions where this approximation is inaccurate. The distortion in the seismic wavelet introduced by the rough sea may influence (for example) deghosting results, as deghosting operators are typically recursive and sensitive to the changes in the seismic signal. In this paper, we study the effect of sea surface roughness on conventional (5–160 Hz) and ultra‐high‐resolution (200–3500 Hz) single‐component towed‐streamer data. To this end, we numerically simulate reflections from a rough sea surface using the Kirchhoff approximation. Our modelling demonstrates that for conventional seismic frequency band sea roughness can distort results of standard one‐dimensional and two‐dimensional deterministic deghosting. To mitigate this effect, we introduce regularisation and optimisation based on the minimum‐energy criterion and show that this improves the processing output significantly. Analysis of ultra‐high‐resolution field data in conjunction with modelling shows that even relatively calm sea state (i.e., 15 cm wave height) introduces significant changes in the seismic signal for ultra‐high‐frequency band. These changes in amplitude and arrival time may degrade the results of deghosting. Using the field dataset, we show how the minimum‐energy optimisation of deghosting parameters improves the processing result.

Normal-mode statistics of sound scattering by a rough elastic boundary in underwater wave-guide in a full coupled mode approach, including back-scattering

The underwater sound scattering by rough sea surface, ice, or rough elastic bottom is studied. The effects of scattering from a rough elastic boundary are included in a coupled mode propagation model by the analytical equation for solid-state layer impedance. A full two-way coupled mode solution was used to derive the stochastic differential equations for the second order statistics in a Markov approximation for a transport theory. The coupled mode matrix was approximated by a linear function of one random parameter such as ice-thickness or the surface perturbation. A one parameter Gaussian model has a form of a two-way differential stochastic equation for the correlation of normal mode coefficients. The derived equation relates the correlation matrix of mode coefficients (for both directions) with the correlation function of the rough boundary and the power spectrum of the its slopes. The theory gives a solution for sound attenuation and horizontal coherence over long-range propagation along random interfaces. The result can be used for the estimation of sound attenuation in long-range under-ice propagation or attenuation of seismic waves in an underwater wave-guide with random bathymetry.

Recent development and application of inverted echo sounders in observational physical oceanography

Since the first experiment carried out by Rossby in Bermuda, inverted echo sounders have been used in physical oceanography observations for half a century. The inverted echo sounder measures the round-trip acoustic travel time from the sea floor to the sea surface, thus acquiring vertically integrated information on the thermal structure, from which the first baroclinic mode of thermocline motion can be inferred. Arrays of inverted echo sounders have been deployed almost all over the global ocean to observe internal waves, mesoscale eddies, western boundary currents, etc., providing valuable targeted data for physical oceanographers. Acoustic aspects of the inverted echo sounder performance have been recently examined. Sources of error affecting instrument performance include tidal effects, barotropic adjustment, ambient acoustic noise and sea surface roughness. The latter two effects are explored with a simulation that includes surface wave reconstruction, acoustic scattering based on the Kirchhoff approximation, wind generated noise, sound propagation, and the instrument's signal processing circuitry. Not only does the analysis enhance our understanding of the acoustic travel time data but also suggests new approaches to extend the application of inverted echo sounders, for example, for sensing wind over the sea. New deployments of inverted echo sounders and recent developments in hardware system and auxiliary accessories are also introduced.

SST Dependence of Ku- and C-Band Backscatter Measurements

The normalized radar cross section (NRCS) measured by satellite ocean radar systems is representative of the sea surface roughness at the scale of gravity-capillary waves, which are not only dominated by winds, but also modulated by some secondary factors such as sea surface temperature (SST) and sea surface salinity (SSS). In this paper, the variations of NRCS due to SST changes, depending on scatterometer radar frequency, polarization, and incidence angle, are investigated on the basis of a physics-based radar backscatter model and a dataset of collocated ASCAT C-band and RapidScat Ku-band scatterometer measurements. The study shows that the SST effects are substantial at Ku-band, but rather negligible for C-band NRCS measurements. Furthermore, the SST effects are wind speed dependent and more pronounced in VV polarization and at higher incidence angles. SSS effects, due to dielectric constant, surface tension, and dynamic viscosity variations, on scatterometer winds are limited (within 1%). This study concludes that it is necessary to take SST into account in scatterometer wind retrieval for radar wavelengths smaller than C-band.

Facet-Based Investigation on Microwave Backscattering From Sea Surface With Breaking Waves: Sea Spikes and SAR Imaging

A complete facet model for the backscattering from rough sea surface with breaking waves is proposed in this paper. In consideration of the spatial distribution of breaking waves on sea surface, the model is able to give a good interpretation to the “super events” under high sea states, such as sea spikes and high polarization ratios. Based on the proposed model, normalized radar cross section plots of sea surface under backscattering configuration are calculated and compared with measured data. The comparisons show that the proposed model is tractable to estimate the scattering from electrically large ocean surface under high sea states with accuracy and efficiency. In addition, the non-Gaussian statistics and spatial correlation properties of sea clutter are analyzed under different range resolutions and incident angles. The results show that high kurtosis value, which means a sea spike phenomenon, mostly happens in lower grazing angle and higher range resolution cases. The comparisons of simulated SAR images of sea surface with and without breaking waves also reveal that sea spikes and high polarization ratios are caused by breaking waves. All the simulation results prove that our model not only is able to explain the physical mechanism of the scattering but also can be applied to the analyses of statistical properties of sea clutter under high sea states.

Rough sea surface implications on receiver deghosting

Efficient removal of ghost events is the key to achieving broadband seismic data. On the receiver-side, accurate ghost removal can be performed when both pressure and vertical particle velocity information is available. However, deghosting pressure-only measurements by spectral division with a flat sea surface or a statistical ghost function is still commonly applied. In this paper, we evaluate the performance of wavefield separation and deghosting with a flat sea surface (or statistical) ghost function for a rough weather marine seismic data acquisition. The behaviour of the pressure ghost function under rough sea conditions is analysed in comparison to the flat and the statistical pressure ghost functions. The three pressure ghost functions (i.e., true, flat, and statistical) converge to the same amplitude and phase values for the lower frequencies, but they diverge from each other for higher frequencies. The error created by wavefield separation and deghosting by spectral division is quantified, using synthetic and real seismic data. Wavefield separation provides deterministic and full receiver-side deghosted up-going wavefields, while both flat and statistical deghosting methods result in significant errors with implications in pre- and post-stack evaluation.

The Dependence of Signal Coherence on Sea-Surface Roughness for High and Low Duty Cycle Sonars in a Shallow-Water Channel

It is anticipated that high duty cycle (HDC) sonars will typically maintain the same bandwidth as the pulsed active sonars (PASs) that they might replace. This will significantly increase their time–bandwidth product, but may not produce the increased gain anticipated, if there are coherence limitations of the acoustic channel. To compare performance of HDC with conventional PAS in the littorals, a set of experiments was conducted as part of the Target and Reverberation Experiment in spring 2013 (TREX13). This paper presents the results of an examination of short-range single surface-reflection echoes, and longer range target echoes from an air hose. The Pearson product–moment correlation coefficient (Pearson's $R$ ) was used to confirm significance of the results. Measurements showed that for an 18-s HDC pulse, the mean (coherent) component of the specular arrival decreased by as much as 5 dB as root mean square (rms) surface roughness increased, whereas the 0.5-s PAS pulse echoes showed no correlation with roughness. The standard deviations of the mean levels were used to examine the incoherent (scattered) component of the specular arrivals. The incoherent component of the specular arrival increased with the product of the surface correlation length and the square of the rms roughness, for both HDC and PAS, with the PAS data having a 1-dB higher standard deviation. A normal mode propagation model and a rough surface scattering model used in conjunction with a simple model that accounts for coherence loss from the matched filter were successfully used to interpret the results.

Investigation of Sea Surface Effect on Shallow Water Reverberation by Coupled Mode Method

The effects of rough sea surface on the long-range bottom reverberation in shallow seas are studied by the coupled mode reverberation theory. The scattering effect caused by irregular rough sea surface is described by couple coefficients. The decaying rules of long-range bottom reverberation level are simulated at different sea states, and the rough sea surface effect on the coherence of distant bottom reverberation is also discussed. It is indicated that irregular upper boundary has changed the propagation effect of the shallow water waveguide, and bottom reverberation, which is dominated among other kinds of reverberation in shallow water, is affected by the sea surface scattering as the increasing sea state. Compared with other literatures, the emphasis of this paper is to present the mechanism of rough sea surface scattering by describing the transfer of energy between different modes, and the details of energy transitions between different modes which are caused by sea surface scattering are presented for different sea states. With the increasing sea state, stronger mode coupling caused by surface scattering would affect the intensity and its space coherence of bottom reverberation obviously.

Long‐term impacts of ocean wave‐dependent roughness on global climate systems

AbstractOcean surface waves can play an active role in climate systems, but they are often ignored in Global Climate Models (GCMs). Wave‐dependent surface roughness was implemented within the Atmospheric GCM (MRI‐AGCM) using the spectral wave model WAVEWATCH III. Two types of wave‐dependent roughness, due to wave steepness and to wave age, were considered. Climate simulations with wave‐dependent roughness were compared to simulations with just wind speed‐dependent roughness. In climate simulation with wave steepness‐dependent roughness, the spatial distribution of roughness is correlated to that of swell dominance. In simulation with wave age‐dependent roughness, the spatial distribution of roughness is correlated to that of wind direction stationarity. Both simulations show reduced roughness in the tropics, which leads to an enhancement of surface wind speeds by up to 15%; these enhanced wind speeds are closer to observations compared with the baseline simulation with just wind speed‐dependent roughness. We find that the reduced roughness and the enhanced wind speeds in the tropics lead to significant changes in atmospheric circulation, as in Hadley circulation and precipitation. The characteristic responses of the Hadley circulation and precipitation to changing sea surface roughness are presented.

Analysis of current–topography interaction in remote sensing imaging procedures for shallow water topography

Underwater topography can be imaged using remote sensing methods. Numerous studies have been conducted to understand the imaging mechanism of shallow water topography. However, current–topography interaction constitutes the weakest link in the remote sensing imaging mechanism of underwater topography. In addition, few studies focused on different topography patterns. Current velocity is highly correlated with bathymetry, and velocity gradient is used as an indicator of sea surface roughness. In this research, we analyzed the distribution characteristics of current velocity and velocity gradient in different cases using a 3-dimensional (3-D) hydrodynamic model to discuss the current–topography interaction part of the remote sensing imaging mechanism, especially the modulation by current parallel or normal to the underwater topography, both positive and negative. Results showed remarkable agreement between 3-D current analysis and surface velocity analysis. Parallel and normal currents had different responses to different topography types. The distribution of surface current gradient showed opposite features over negative and positive topography in parallel and normal current fields. This can be used to distinguish negative topography from positive topography with different current directions to a certain extent.