Presence Of Surface Currents Research Articles

Statistical Analysis of Local Extrema in Rough Sea Surfaces Based on Computer Simulation

Introduction. Generalized extreme value (GEV) distributions represent a universal description of the limiting distribution of the normalized local maxima statistics for independent and identically distributed data series. Extreme value distributions are commonly classified into three different types representing different functional forms and thus varying in shape, also known as types I, II, and III. Thus, attribution of some observational data series to a particular type of its local maxima distribution, as well as fitting of the distribution parameters, provides certain information about the laws governing the underlying natural or technogenic process. Radar-based remote sensing techniques represent a ubiquitous tool for analyzing large patterns of the sea surface and determining the parameters of the waves. In turn, understanding the laws governing the extreme values in the rough sea surface obtained from their radar images followed by evaluation of their distribution parameters, depending on the wind speed and direction, as well as the presence of surface currents and swells, can be useful for predicting wave height. Aim. Analysis of the functional forms governing the local extreme value distributions in a rough sea surface for the given wind and swell parameters based on computer simulations. Materials and methods. For the rough sea surface simulated by an additive harmonic synthesis procedure, the local extreme value distribution was fitted using the least-mean-squares technique. The fitted parameters were then used for their classification according to the three predetermined types. Results. Computer simulations of a rough sea surface with combined wind and swell waves were performed. It is shown that the distribution of local maxima in the absence of swell waves could be well approximated by theWeibull (type III GEV) distribution, with the parameters explicitly depending on the wind speed. At the same time, no significant dependence on the sea depth was observed. On the contrary, in the presence of additional swell waves, the distribution of local extrema could be rather attributed to the Fréchet (type II GEV) distribution, with the parameters additionally depending on the angle between the wind and swell waves. Conclusion. The laws governing the distributions of local wave extrema in rough seas are in a good agreement with the theoretical GEV approximations, with the distribution parameters being deductible from the key features of the waves. This indicates the predictability of wave height extrema from sea surface measurements, which can be performed based on remote radar observations.

The potential of spaceborne GNSS reflectometry for detecting ocean surface currents

The detectability of ocean surface currents in global navigation satellite system reflectometry (GNSS-R) observations is analyzed. We use a large dataset of spaceborne GNSS-R measurements from NASA cyclone GNSS (CYGNSS) mission. The data is collocated with ocean wind and near-surface current measurements. Our analysis reveals clear responses of the GNSS-R σ0 to the presence of currents. The response depends on the wind conditions and is more prominent for wind speeds below 6 m/s. A current velocity of 0.5 m/s under an opposing wind can, on average, suppress the GNSS-R σ0 by 0.8 decibels for low incidence angles. The interaction of the same current with a codirectional wind can enhance σ0 by almost the same amount. This enhancement is most visible at high incidence angles. We develop a model that improves the prediction of the GNSS-R σ0 in the presence of surface currents. The detected signatures of wind–current interactions highlight the potential of GNSS-R sensors onboard small satellites for observing ocean surface currents.

Investigation of Underwater Wireless Optical Communications Links With Surface Currents and Tides for Oceanic Signal Transmission

This report investigated underwater wireless optical communication (UWOC) links in the presence of surface currents and tides. The scenario of surface currents and tides was generated by a sea water tank, a water pipe, and a water pump of different flow rates. It built the oceanic channel of UWOC varied with the salinity and flow rate in the UWOC transmission. The data rate of UWOC links based on a 520-nm laser diode and none-return-to-zero on-off keying (NRZ-OOK) modulation can achieve 3.4 Gb/s at a bit error rate (BER) of 3.78 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$\times 10^{-4}$</tex-math></inline-formula> in 1.8 m long sea water tank when a flow rate reached 1.08 m/s. The BER, eye-diagrams, and received signal optical power measurements showed that both the increasing salinity and flow rate cause the UWOC channel attenuation, thus degrade the performance of UWOC links.

The Nonlinear Instability of a Cylindrical Interface Between Two Hydromagnetic Darcian Flows

This paper investigates the nonlinear instability of an interface between two magnetic fluids separated by a cylindrical interface in porous media. The system is influenced by a uniform axial magnetic field. The magnetic field intensities allow a presence of surface currents at the interface. The transfer of mass and heat across the interface is considered. The solutions of linearized equations of motion, under the appropriate nonlinear boundary conditions, lead to a nonlinear characteristic equation that is governed the behavior of the interface deflection. Drawing on the linear stability theory, Routh–Hurwitz’s criteria are utilized to judge the stability criteria. The coupling of Laplace transforms and Homotopy perturbation techniques are adopted to obtain an approximate analytical solution of the interface profile. The nonlinear stability analysis resulted in two levels of solvability conditions. By means of these conditions, a Ginzburg–Landau equation is conducted. The latter equation represented the nonlinear stability configuration. The magnetic field intensity was plotted versus the wave number of the surface waves. Therefore, the stability picture was divided into stable and as well as unstable regions. Subsequently, the influence of the various physical parameters was addressed.

On C-Band Quad-Polarized Synthetic Aperture Radar Properties of Ocean Surface Currents

We present new results for ocean surface current signatures in dual co- and cross-polarized synthetic aperture radar (SAR) images. C-band RADARSAT-2 quad-polarized SAR ocean scenes are decomposed into resonant Bragg scattering from regular (non-breaking) surface waves and scattering from breaking waves. Surface current signatures in dual co- and cross-polarized SAR images are confirmed to be governed by the modulations due to wave breaking. Due to their small relaxation scale, short Bragg waves are almost insensitive to surface currents. Remarkably, the contrast in sensitivity of the non-polarized contribution to dual co-polarized signals is found to largely exceed, by a factor of about 3, the contrast in sensitivity of the corresponding cross-polarized signals. A possible reason for this result is the co- and cross-polarized distinct scattering mechanisms from breaking waves: for the former, quasi-specular radar returns are dominant, whereas for the latter, quasi-resonant scattering from the rough breaking crests governs the backscatter intensity. Thus, the differing sensitivity can be related to distinct spectral intervals of breaking waves contributing to co- and cross-polarized scattering in the presence of surface currents. Accordingly, routinely observed current signatures in quad-polarized SAR images essentially originate from wave breaking modulations, and polarized contrasts can therefore help quantitatively retrieve the strength of surface current gradients.

Resistive wall mode in cylindrical plasmas in the presence of surface currents

AbstractStability of the resistive wall mode in cylindrical plasmas confined by surface currents is investigated for the δ-function and step-function equilibrium surface-current profiles. For the former, it is shown that the perturbations oscillate and even decay for all locations of the initial perturbation. The entire system is stable and the plasma flow has little effect. For the step-function surface-current distribution, it is found that the thicker the surface current layer, the more stable is the system even if the largest initial perturbation is located on the rational surface, but the plasma flow also has little effect on the system.

The Effect of Current-Induced Stress Perturbation On Baroclinic Rossby Waves in a Continuously Stratified Model

Abstract A shear stress at the air–sea interface is induced by the presence of surface currents. In the case of isotropic wind, a quadratic stress law leads to a stress curl proportional to and opposing the surface current vorticity. This causes a spindown effect on the surface vorticity field at a rate proportional to the characteristic wind speed. This effect of surface vorticity spindown or friction on propagating baroclinic Rossby waves is analyzed in a continuously stratified system with well-mixed boundary layers above and below it. In this system, both bottom friction and surface friction modulate the baroclinic Rossby waves in the stratified interior each time a wave group is reflected at the lower and the upper interfaces. The reflection coefficient and the corresponding spatial attenuation rate are calculated for various values of wave frequency, wind speed, and interior stratification. Both bottom friction and surface friction are important dissipation mechanisms for the baroclinic Rossby waves...

Air‐sea boundary layer dynamics in the presence of mesoscale surface currents

In the presence of surface currents, a shear stress at the air‐sea interface is induced by the surface currents. In the case of a unidirectional current, a quadratic stress law leads to a stress curl proportional to and opposing the surface current vorticity even with a uniform wind. This causes a spindown effect on the surface vorticity field at a rate proportional to the wind speed. In the steady state, or in slowly varying processes which can be treated as parametrically developing quasi‐steady states, the surface‐layer potential vorticity modulation causes upwelling and downwelling patterns associated with the surface‐current vorticity. These effects are analyzed for an idealized jet current, and for a physical situation characteristic of a Gulf Stream boundary ring along the Florida Keys, where the induced transport patterns may be important for onshore transport of fish and spiny lobster larvae, as well as for onshore transport to the Florida Keys of general flotsam transported past them by the Gulf Stream. The spindown time scale (t*) for a 1.5‐layer system is H/( ρ′cdVa) for a surface jet on the deformation radius scale (where H is the thickness of the surface layer, Va the surface wind speed, ρ′ the air to water density ratio and cd the surface drag coefficient) and increases for large horizontal scales in proportion to the current width squared. For a typical wind speed of 5 m/s and a density normalized drag coefficient ρ′cd= 2 × 10−6, t* is on the order of 1 month for a 30‐m surface layer. In the more general case of a stratified interior water column, the vorticity spindown directly affects only the potential vorticity of the surface layer and generally leads to subsurface velocity and vorticity maxima for mesoscale eddies and jets.

The East Australian current, 1965–1968

Surface dynamic topographies from eight cruises are presented. They show additional examples of the complex, variable circulation patterns already reported for this area. Direct measurements of currents using neutrally-buoyant floats showed that the reference level, or level of no horizontal motion, may be at least as deep as 2300 m, instead of 1300 m as assumed hitherto. Below 3000 m, the observed currents were in the range 2–7 cm/sec, and showed no clear evidence of being coherent with surface currents. The floats were tracked relative to a pinger anchored near the bottom. This method was satisfactory even in the presence of surface currents as strong as 200 cm/sec.