The Experts below are selected from a list of 7512 Experts worldwide ranked by ideXlab platform
Angelique Dremeau - One of the best experts on this subject based on the ideXlab platform.
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dispersive grid free orthogonal matching pursuit for modal estimation in ocean acoustics
International Conference on Acoustics Speech and Signal Processing, 2020Co-Authors: Thomas Pavietsalomon, Julien Bonnel, Clement Dorffer, Barbara Nicolas, Thierry Chonavel, Angelique DremeauAbstract:Considering low-frequency acoustic sources, shallow-water environments act as modal dispersive waveguides. In this context, the signal can be described as a sum of a few modal components, each of them propagating with its own Wavenumber. When dealing with broadband sources, Wavenumber-frequency (f-k) diagrams constitute popular representations naturally enabling modal separation. Based on a Fourier transform, they require however a large number of sensors to resolve Wavenumbers with a high-resolution. This limitation can be overcame by adding some physical priors to the processing method. In the continuation of previous works, we propose here a new grid-free algorithm allowing a super-resolution of the (f-k) diagram by benifiting from the sparse nature of the Wavenumber Spectrum and embedding the broadband behavior of the Wavenumbers within the algorithm. The method is validated on simulated data.
Ian R. Young - One of the best experts on this subject based on the ideXlab platform.
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The form of the asymptotic depth-limited wind-wave Spectrum part III: directional spreading
Coastal Engineering, 2010Co-Authors: Ian R. YoungAbstract:Abstract The directional spreading of both the Wavenumber and frequency spectra of finite-depth wind generated waves at the asymptotic depth limit are examined. The analysis uses the Wavelet Directional Method, removing the need to assume a form for the dispersion relationship. The paper shows that both the Wavenumber and frequency forms are narrowest at the spectral peak and broaden at Wavenumbers (frequencies) both above and below the peak. The directional spreading of the Wavenumber Spectrum is bi-modal above the spectral peak. In contrast, the frequency Spectrum is uni-modal. This difference is shown to be the result of energy in the wind direction being displaced from the linear dispersion shell. A full parametric relationship for the directional spreading of the Wavenumber Spectrum is developed. The analysis clearly shows that typical dispersion relationships are questionable at high frequencies and that such effects can be significant. This result supports greater attention being focussed on the routine recording of Wavenumber spectra, rather than frequency spectra.
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the form of the asymptotic depth limited wind wave Spectrum part ii the Wavenumber Spectrum
Coastal Engineering, 2009Co-Authors: Ian R. Young, Alexander V BabaninAbstract:Abstract Data from a spatial array of wave gauges is analysed using the Wavelet Directional Method (WDM) to directly determine the Wavenumber Spectrum. The data shows that the asymptotic depth-limited Wavenumber Spectrum can be represented as a two-parameter form, which is far simpler than the corresponding frequency Spectrum. The WDM analysis shows that there are significant nonlinear processes active in the finite depth water, which results in energy being “smeared” across a range of Wavenumbers and frequencies around the standard dispersion shell. As a result, the Wavenumber Spectrum has much less peak enhancement than seen in the frequency Spectrum obtained with standard Fourier analysis. In addition, the Wavenumber Spectrum does not have the clear harmonic previously observed in the finite depth frequency Spectrum. This result demonstrates that the harmonic is nonlinearly phase-locked to the spectral peak.
Thomas Pavietsalomon - One of the best experts on this subject based on the ideXlab platform.
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dispersive grid free orthogonal matching pursuit for modal estimation in ocean acoustics
International Conference on Acoustics Speech and Signal Processing, 2020Co-Authors: Thomas Pavietsalomon, Julien Bonnel, Clement Dorffer, Barbara Nicolas, Thierry Chonavel, Angelique DremeauAbstract:Considering low-frequency acoustic sources, shallow-water environments act as modal dispersive waveguides. In this context, the signal can be described as a sum of a few modal components, each of them propagating with its own Wavenumber. When dealing with broadband sources, Wavenumber-frequency (f-k) diagrams constitute popular representations naturally enabling modal separation. Based on a Fourier transform, they require however a large number of sensors to resolve Wavenumbers with a high-resolution. This limitation can be overcame by adding some physical priors to the processing method. In the continuation of previous works, we propose here a new grid-free algorithm allowing a super-resolution of the (f-k) diagram by benifiting from the sparse nature of the Wavenumber Spectrum and embedding the broadband behavior of the Wavenumbers within the algorithm. The method is validated on simulated data.
Xiaohui Zhou - One of the best experts on this subject based on the ideXlab platform.
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anisotropy of the sea surface height Wavenumber Spectrum from altimeter observations
Scientific Reports, 2019Co-Authors: Shihong Wang, Fangli Qiao, Dejun Dai, Xiaohui ZhouAbstract:In this paper, the zonal and meridional sea surface height (SSH) Wavenumber spectra are systematically calculated using along-track and gridded altimeter products, and the slopes of the SSH Wavenumber spectra over the mesoscale band, which is defined by the characteristic length scale of mesoscale signals, are estimated. The results show that the homogeneous spectral slopes calculated from the along-track and gridded altimeter datasets have a similar spatial pattern, but the spectral slopes from gridded altimeter data are generally steeper than that from the along-track data with an averaged difference of 1.5. Significant differences are found between the zonal and meridional spectra, which suggest that SSH Wavenumber spectra are indeed anisotropic. Furthermore, the anisotropy exhibits strong regional contrast: in the equatorial region, the zonal Spectrum is steeper than its corresponding meridional Spectrum, while in the eastward-flowing high EKE regions the meridional Spectrum is steeper than its zonal counterpart. The anisotropy of SSH Wavenumber spectral slopes implies that EKE distributes anisotropically in different directions, and this distribution is closely associated with the generation and nonlinear evolution of mesoscale movements.
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global Wavenumber Spectrum with corrections for altimeter high frequency noise
Journal of Physical Oceanography, 2015Co-Authors: Xiaohui Zhou, Dongping Wang, Dake ChenAbstract:The altimetry Wavenumber spectra of sea surface height (SSH) provide a unique dataset for testing of geostrophicturbulence.WhileSSHspectralslopesofk 211/3 andk 25 areexpectedfromtheoriesandnumerical simulations, the altimetry spectra from the original unfiltered and instrument noise‐corrected data often are too shallow, falling between k 22 and k 23 . In this study, the possibility that the flattened spectral slopes are partly due to contamination by unresolved high-frequency (,10 days) motions is tested. A spatiotemporal filter based on empirical orthogonal function expansion (EOF) is used to remove the temporally incoherent signals. The resulting spectral slopes are much steeper than in the previous studies. Over 70% of the revised global spectral estimates, excluding the tropics, are above k 23 . Moreover, in high energy regions like the Gulf StreamandKuroshio,thespectralslopesare aboutk 25 , whichis consistentwiththeclassicalquasigeostrophic (QG) turbulence. The spectral slopes are validated with the eddy kinetic energy (EKE) spectra from shipboard acoustic Doppler current profiler (ADCP) measurements in the high and low energy regions.
Detlef Stammer - One of the best experts on this subject based on the ideXlab platform.
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satellite altimetry the marine geoid and the oceanic general circulation
Annual Review of Earth and Planetary Sciences, 1998Co-Authors: Carl Wunsch, Detlef StammerAbstract:For technical reasons, the general circulation of the ocean has historically been treated as a steady, laminar flow field. The recent availability of extremely highaccuracy and high-precision satellite altimetry has provided a graphic demonstration that the ocean is actually a rapidly time-evolving turbulent flow field. To render the observations quantitatively useful for oceanographic purposes has required order of magnitude improvements in a number of fields, including orbit dynamics, gravity field estimation, and atmospheric variability. With five years of very high-quality data now available, the nature of oceanic variability on all space and time scales is emerging, including new findings about such diverse and important phenomena as mixing coefficients, the frequency/Wavenumber Spectrum, and turbulent cascades. Because the surface elevation is both a cause and consequence of motions deep within the water column, oceanographers soon will be able to provide general circulation numerical models tested against and then combined with the altimeter data. These will be complete three-dimensional timeevolving estimates of the ocean circulation, permitting greatly improved estimates of oceanic heat, carbon, and other property fluxes.
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the global frequency Wavenumber Spectrum of oceanic variability estimated from topex poseidon altimetric measurements
Journal of Geophysical Research, 1995Co-Authors: Carl Wunsch, Detlef StammerAbstract:Two years of altimetric data from the TOPEX/POSEIDON spacecraft have been used to produce preliminary estimates of the space and time spectra of global variability for both sea surface height and slope. The results are expressed in terms of both degree variances from spherical harmonic expansions and in along-track Wavenumbers. Simple analytic approximations both in terms of piecewise power laws and Pade fractions are provided for comparison with independent measurements and for easy use of the results. A number of uses of such spectra exist, including the possibility of combining the altimetric data with other observations, predictions of spatial coherences, and the estimation of the accuracy of apparent secular trends in sea level.
