Wave Spectra

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Tetsu Hara - One of the best experts on this subject based on the ideXlab platform.

  • wind profile and drag coefficient over mature ocean surface Wave Spectra
    Journal of Physical Oceanography, 2004
    Co-Authors: Tetsu Hara, Stephen E Belcher
    Abstract:

    The mean wind profile and the Charnock coefficient, or drag coefficient, over mature seas are investigated. A model of the Wave boundary layer, which consists of the lowest part of the atmospheric boundary layer that is influenced by surface Waves, is developed based on the conservation of momentum and energy. Energy conservation is cast as a bulk constraint, integrated across the depth of the Wave boundary layer, and the turbulence closure is achieved by parameterizing the dissipation rate of turbulent kinetic energy. Momentum conservation is accounted for by using the analytical model of the equilibrium surface Wave Spectra developed by Hara and Belcher. This approach allows analytical expressions of the Charnock coefficient to be obtained and the results to be examined in terms of key nondimensional parameters. In particular, simple expressions are obtained in the asymptotic limit at which effects of viscosity and surface tension are small and the majority of the stress is supported by Wave drag. This analytical model allows us to identify the conditions necessary for the Charnock coefficient to be a true constant, an assumption routinely made in existing bulk parameterizations.

  • numerical simulation of sea surface directional Wave Spectra under hurricane wind forcing
    Journal of Physical Oceanography, 2003
    Co-Authors: Ilju Moon, Isaac Ginis, C.w. Wright, Hendrik L Tolman, Tetsu Hara, Edward J. Walsh
    Abstract:

    Numerical simulation of sea surface directional Wave Spectra under hurricane wind forcing was carried out using a high-resolution Wave model. The simulation was run for four days as Hurricane Bonnie (1998) approached the U.S. East Coast. The results are compared with buoy observations and NASA Scanning Radar Altimeter (SRA) data, which were obtained on 24 August 1998 in the open ocean and on 26 August when the storm was approaching the shore. The simulated significant Wave height in the open ocean reached 14 m, agreeing well with the SRA and buoy observations. It gradually decreased as the hurricane approached the shore. In the open ocean, the dominant Wavelength and Wave direction in all four quadrants relative to the storm center were simulated very accurately. For the landfall case, however, the simulated dominant Wavelength displays noticeable overestimation because the Wave model cannot properly simulate shoaling processes. Direct comparison of the model and SRA directional Spectra in all four quadrants of the hurricane shows excellent agreement in general. In some cases, the model produces smoother Spectra with narrower directional spreading than do the observations. The spatial characteristics of the Spectra depend on the relative position from the hurricane center, the hurricane translation speed, and bathymetry. Attempts are made to provide simple explanations for the misalignment between local wind and Wave directions and for the effect of hurricane translation speed on Wave Spectra.

  • wind forcing in the equilibrium range of wind Wave Spectra
    Journal of Fluid Mechanics, 2002
    Co-Authors: Tetsu Hara, Stephen E Belcher
    Abstract:

    A new analytical model is developed for the equilibrium range of the spectrum of wind-forced ocean surface gravity Waves. We first show that the existing model of Phillips (1985) does not satisfy overall momentum conservation at high winds. This constraint is satisfied by applying recent understanding of the wind forcing of Waves. Waves exert a drag on the air flow so that they support a fraction of the applied wind stress, which thus leaves a smaller turbulent stress near the surface to force growth of shorter Wavelength Waves. Formulation of the momentum budget accounting for this sheltering constrains the overall conservation of momentum and leads to a local turbulent stress that reduces as the Wavenumber increases. This local turbulent stress then forces wind-induced Wave growth. Following Phillips (1985), the wind sea is taken to be a superposition of linear Waves, and equilibrium is maintained by a balance between the three sources and sinks of Wave action. These assumptions lead to analytical formulae for the local turbulent stress and the degree of saturation, B ( k ), of Waves in the equilibrium range. We identify a sheltering Wavenumber, k s , over which the local turbulent stress is significantly reduced by longer Waves. At low Wavenumbers or at low winds, when k [Lt ] k s , the sheltering is weak and B ( k ) has a similar form to the model of Phillips (1985). At higher Wavenumbers or at higher winds, k s , B ( k ) makes a transition to being proportional to k 0 . The additional constraint of conservation of momentum also yields a formula for the coefficient that appears in the solution for B ( k ). The Spectra for mature seas are calculated from the model and are shown to agree with field observations. In particular, our model predicts more realistic Spectral levels toward the high Wavenumber limit compared to the previous model of Phillips (1985). We suggest that the model may explain the overshoot phenomena observed in the Spectral energy levels as the fetch increases.

  • optical measurements of capillary gravity Wave Spectra using a scanning laser slope gauge
    Journal of Atmospheric and Oceanic Technology, 1995
    Co-Authors: Erik J Bock, Tetsu Hara
    Abstract:

    Abstract A description of a new scanning laser slope gauge (SLSG) is given and the results obtained from both laboratory wind-Wave tank and field measurements are presented. The device relies on the measurements of two components of surface slope to compute spatial and temporal lags that are used to estimate the full three-dimensional slope spectrum. The device is capable of resolving frequencies up to 34.7 Hz and Wavelengths in the range between 7.9 × 10−3 and 3.08 × 10−1 m. The technique makes use of a two-dimensional laser scanner that samples the perimeter of a circle of 0.154-m diameter (an unfilled aperture). Both laboratory and field results indicate the device is well suited to measure the full three-dimensional Spectra of capillary-gravity Waves and is capable of providing ground-truthing measurements for the verification of remotely sensed ocean surface features.

  • in situ measurements of capillary gravity Wave Spectra using a scanning laser slope gauge and microWave radars
    Journal of Geophysical Research, 1994
    Co-Authors: Tetsu Hara, Erik J Bock, D R Lyzenga
    Abstract:

    Capillary-gravity Wave Spectra are measured using a scanning laser slope gauge (SLSG), and simultaneously by X and K band Doppler radars off the Chemotaxis Dock at the Quissett campus of the Woods Hole Oceanographic Institution at Woods Hole, Massachusetts. Wave Spectral densities estimated from the radar measurements using the Bragg theory agree with those measured using the SLSG at the Bragg Wavenumber to within a few decibels, suggesting that Bragg scattering theory is valid for the conditions of this experiment. The observed degree of saturation of capillary-gravity Waves is in reasonable agreement with measurements by Jahne and Riemer (1990) obtained from measurements in a large wind-Wave flume at intermediate wind speeds, but our data indicate a higher degree of saturation at very low wind speeds. The rate at which the slope-frequency spectrum falls off, however, is much lower in the field than in laboratories, even at moderate winds, suggesting long Waves are responsible for a large Doppler shift of capillary-gravity Waves. Close examination of combined Wavenumber-frequency slope Spectra also reveals significant smearing of the Spectra in the frequency domain due to long Waves. These observations confirm that spatial measurements (Wavenumber Spectra measurements) are essential for characterizing short capillary-gravity Waves, since this strong Doppler shift will dramatically change apparent frequency Spectra.

G B Hospodarsky - One of the best experts on this subject based on the ideXlab platform.

  • statistical distribution of emic Wave Spectra observations from van allen probes
    Geophysical Research Letters, 2016
    Co-Authors: Xiaojia Zhang, Vassillis Angelopoulos, Jacob Bortnik, W. Li, W S Kurth, G B Hospodarsky
    Abstract:

    It has been known that electromagnetic ion cyclotron (EMIC) Waves can precipitate ultrarelativistic electrons through cyclotron resonant scattering. However, the overall effectiveness of this mechanism has yet to be quantified, because it is difficult to obtain the global distribution of EMIC Waves that usually exhibit limited spatial presence. We construct a statistical distribution of EMIC Wave frequency Spectra and their intensities based on Van Allen Probes measurements from September 2012 to December 2015. Our results show that as the ratio of plasma frequency over electron gyrofrequency increases, EMIC Wave power becomes progressively dominated by the helium band. There is a pronounced dawn-dusk asymmetry in the Wave amplitude and the frequency spectrum. The frequency spectrum does not follow the commonly used single-peak Gaussian function. Incorporating these realistic EMIC Wave frequency Spectra into radiation belt models is expected to improve the quantification of EMIC Wave scattering effects in ultrarelativistic electron dynamics.

  • statistical distribution of emic Wave Spectra observations from van allen probes
    Geophysical Research Letters, 2016
    Co-Authors: Xiaojia Zhang, Vassillis Angelopoulos, Jacob Bortnik, W. Li, W S Kurth, C A Kletzing, R M Thorne, G B Hospodarsky
    Abstract:

    It has been known that electromagnetic ion cyclotron (EMIC) Waves can precipitate ultrarelativistic electrons through cyclotron resonant scattering. However, the overall effectiveness of this mechanism has yet to be quantified, because it is difficult to obtain the global distribution of EMIC Waves that usually exhibit limited spatial presence. We construct a statistical distribution of EMIC Wave frequency Spectra and their intensities based on Van Allen Probes measurements from September 2012 to December 2015. Our results show that as the ratio of plasma frequency over electron gyrofrequency increases, EMIC Wave power becomes progressively dominated by the helium band. There is a pronounced dawn-dusk asymmetry in the Wave amplitude and the frequency spectrum. The frequency spectrum does not follow the commonly used single-peak Gaussian function. Incorporating these realistic EMIC Wave frequency Spectra into radiation belt models is expected to improve the quantification of EMIC Wave scattering effects in ultrarelativistic electron dynamics.

  • electron densities inferred from plasma Wave Spectra obtained by the Waves instrument on van allen probes
    Journal of Geophysical Research, 2015
    Co-Authors: W S Kurth, G B Hospodarsky, S De Pascuale, J B Faden, C A Kletzing, S A Thaller, J R Wygant
    Abstract:

    The twin Van Allen Probe spacecraft, launched in August 2012, carry identical scientific payloads. The Electric and Magnetic Field Instrument Suite and Integrated Science suite includes a plasma Wave instrument (Waves) that measures three magnetic and three electric components of plasma Waves in the frequency range of 10 Hz to 12 kHz using triaxial search coils and the Electric Fields and Waves triaxial electric field sensors. The Waves instrument also measures a single electric field component of Waves in the frequency range of 10 to 500 kHz. A primary objective of the higher-frequency measurements is the determination of the electron density ne at the spacecraft, primarily inferred from the upper hybrid resonance frequency fuh. Considerable work has gone into developing a process and tools for identifying and digitizing the upper hybrid resonance frequency in order to infer the electron density as an essential parameter for interpreting not only the plasma Wave data from the mission but also as input to various magnetospheric models. Good progress has been made in developing algorithms to identify fuh and create a data set of electron densities. However, it is often difficult to interpret the plasma Wave Spectra during active times to identify fuh and accurately determine ne. In some cases, there is no clear signature of the upper hybrid band, and the low-frequency cutoff of the continuum radiation is used. We describe the expected accuracy of ne and issues in the interpretation of the electrostatic Wave spectrum.

Daoyi Chen - One of the best experts on this subject based on the ideXlab platform.

  • wind profiles and Wave Spectra for potential wind farms in south china sea part i wind speed profile model
    Energies, 2017
    Co-Authors: Daoyi Chen, Qian Yi, Sunwei Li
    Abstract:

    With the setting of wind energy harvesting moving from coastal waters to deep waters, the South China Sea has been deemed to offer great potential for the construction of floating wind farms thanks to the abundance of wind energy resources. An engineering model describing the wind profiles and Wave Spectra specific to the South China Sea conditions, which is the precondition for offshore wind farm construction, has, however, not yet been proposed. In the present study, a series of numerical simulations have been conducted using the Weather Forecast and Research model. Through analyzing the wind and Wave information extracted from the numerical simulation results, engineering models to calculate vertical profiles of wind speeds and Wave Spectra have been postulated. While the present paper focuses on the wind profile model, a companion paper articulates the Wave spectrum model. For wind profiles under typhoon conditions, the power-law and log-law models have been found applicable under the condition that the Hellmann exponent α or the friction velocity u * are modified to vary with the wind strength. For wind profiles under non-typhoon conditions, the log-law model is revised to take into consideration the influence of the atmospheric stability.

  • Wind Profiles and Wave Spectra for Potential Wind Farms in South China Sea. Part II: Wave Spectrum Model
    Energies, 2017
    Co-Authors: Yichao Liu, Daoyi Chen
    Abstract:

    Along with the commercialization of offshore wind energy in China, the South China Sea has been identified as ideal for constructing offshore wind farms, especially for farms consisting of floating wind turbines over deep waters. Since the wind profiles and Wave Spectra are somewhat primitive for the design of an offshore wind turbine, engineering models describing the wind and Wave characteristics in the South China Sea area are necessary for the offshore wind energy exploitation given the meteorological, hydrological, and geographical differences between the South China Sea and the North/Norwegian Sea, where the commonly used wind profile and Wave spectrum models were designated. In the present study; a series of numerical simulations were conducted to reveal the Wave characteristics in the South China Sea under both typhoon and non-typhoon conditions. By analyzing the simulation results; the applicability of the Joint North Sea Wave Project (JONSWAP) spectrum model; in terms of characterizing the wind-induced Wave fields in the South China Sea; was discussed. In detail; the key parameters of the JONSWAP spectrum model; such as the Phillips constant; Spectral width parameter; peak-enhancement factor, and high frequency tail decay; were investigated in the context of finding suitable values.

W. Li - One of the best experts on this subject based on the ideXlab platform.

  • statistical distribution of emic Wave Spectra observations from van allen probes
    Geophysical Research Letters, 2016
    Co-Authors: Xiaojia Zhang, Vassillis Angelopoulos, Jacob Bortnik, W. Li, W S Kurth, G B Hospodarsky
    Abstract:

    It has been known that electromagnetic ion cyclotron (EMIC) Waves can precipitate ultrarelativistic electrons through cyclotron resonant scattering. However, the overall effectiveness of this mechanism has yet to be quantified, because it is difficult to obtain the global distribution of EMIC Waves that usually exhibit limited spatial presence. We construct a statistical distribution of EMIC Wave frequency Spectra and their intensities based on Van Allen Probes measurements from September 2012 to December 2015. Our results show that as the ratio of plasma frequency over electron gyrofrequency increases, EMIC Wave power becomes progressively dominated by the helium band. There is a pronounced dawn-dusk asymmetry in the Wave amplitude and the frequency spectrum. The frequency spectrum does not follow the commonly used single-peak Gaussian function. Incorporating these realistic EMIC Wave frequency Spectra into radiation belt models is expected to improve the quantification of EMIC Wave scattering effects in ultrarelativistic electron dynamics.

  • statistical distribution of emic Wave Spectra observations from van allen probes
    Geophysical Research Letters, 2016
    Co-Authors: Xiaojia Zhang, Vassillis Angelopoulos, Jacob Bortnik, W. Li, W S Kurth, C A Kletzing, R M Thorne, G B Hospodarsky
    Abstract:

    It has been known that electromagnetic ion cyclotron (EMIC) Waves can precipitate ultrarelativistic electrons through cyclotron resonant scattering. However, the overall effectiveness of this mechanism has yet to be quantified, because it is difficult to obtain the global distribution of EMIC Waves that usually exhibit limited spatial presence. We construct a statistical distribution of EMIC Wave frequency Spectra and their intensities based on Van Allen Probes measurements from September 2012 to December 2015. Our results show that as the ratio of plasma frequency over electron gyrofrequency increases, EMIC Wave power becomes progressively dominated by the helium band. There is a pronounced dawn-dusk asymmetry in the Wave amplitude and the frequency spectrum. The frequency spectrum does not follow the commonly used single-peak Gaussian function. Incorporating these realistic EMIC Wave frequency Spectra into radiation belt models is expected to improve the quantification of EMIC Wave scattering effects in ultrarelativistic electron dynamics.

D R Lyzenga - One of the best experts on this subject based on the ideXlab platform.

  • in situ measurements of capillary gravity Wave Spectra using a scanning laser slope gauge and microWave radars
    Journal of Geophysical Research, 1994
    Co-Authors: Tetsu Hara, Erik J Bock, D R Lyzenga
    Abstract:

    Capillary-gravity Wave Spectra are measured using a scanning laser slope gauge (SLSG), and simultaneously by X and K band Doppler radars off the Chemotaxis Dock at the Quissett campus of the Woods Hole Oceanographic Institution at Woods Hole, Massachusetts. Wave Spectral densities estimated from the radar measurements using the Bragg theory agree with those measured using the SLSG at the Bragg Wavenumber to within a few decibels, suggesting that Bragg scattering theory is valid for the conditions of this experiment. The observed degree of saturation of capillary-gravity Waves is in reasonable agreement with measurements by Jahne and Riemer (1990) obtained from measurements in a large wind-Wave flume at intermediate wind speeds, but our data indicate a higher degree of saturation at very low wind speeds. The rate at which the slope-frequency spectrum falls off, however, is much lower in the field than in laboratories, even at moderate winds, suggesting long Waves are responsible for a large Doppler shift of capillary-gravity Waves. Close examination of combined Wavenumber-frequency slope Spectra also reveals significant smearing of the Spectra in the frequency domain due to long Waves. These observations confirm that spatial measurements (Wavenumber Spectra measurements) are essential for characterizing short capillary-gravity Waves, since this strong Doppler shift will dramatically change apparent frequency Spectra.