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Hans C. Graber
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Hans C. Graber
and
Ole S. Madsen

Abstract

A parametric windsea model for arbitrary water depths is presented. The model is derived from a conservation of energy flux formulation and includes shoaling, refraction, dissipation by bottom friction, as well as finite-depth modifications of the atmospheric input and nonlinear wave–wave interaction source terms. The importance of dissipation due to a rough ocean floor on the migration of the spectral peak frequency is discussed and compared with that caused by nonlinear energy transfer. Numerical simulations are used to systematically examine wave growth and the development of the spectral peak in a depth-limited ocean.

Two idealized situations of wave growth and propagation are considered to further understand the influence of bottom friction on the spectral dynamics. The first case studies the characteristics of fetch-limited wave growth in a steady, uniform wind as function of depth and bottom roughness. The second case examines the role of bottom dissipation on a fully developed deep-water spectrum propagating up a constant slope under a steady onshore blowing wind. For case 1 the growth curves and peak frequency development are plotted as a function of fetch, and wave spectra for infinite fetch and duration are shown for all depths and wave friction factors. For case 2 the evolution of total energy and peak frequency along the shelf slope are presented for stationary conditions as well as the stationary inshore energy spectra.

This numerical study reveals the following: (i) bottom friction is a finite-depth mechanism as important as the nonlinear energy transfer in controlling the spectral shape in shallow water, (ii) under the influence of bottom dissipation the positive energy transfer from wave–wave interactions to lower frequencies is reduced and causes the spectral peak to wander towards higher frequencies; (iii) equilibrium energy spectra in finite depth depend on depth and bottom roughness and occur when the nonlinear energy transfer and bottom friction source terms approximately balance each other.

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Clarence O. Collins III
,
C. Linwood Vincent
, and
Hans C. Graber

Abstract

Ocean wave spectra are complex. Because of this complexity, no widely accepted method has been developed for the comparison between two sets of paired wave spectra. A method for intercomparing wave spectra is developed based on an example paradigm of the comparison of model spectra to observed spectra. Canonical correlation analysis (CCA) is used to investigate the correlation structure of the matrix of spectral correlations. The set of N ranked canonical correlations developed through CCA (here termed the r-sequence) is shown to be an effective method for understanding the degree of correlation between sets of paired spectral observation. A standard method for intercomparing sets of wave spectra based on CCA is then described. The method is elucidated through analyses of synthetic and real spectra that span a range of correlation from random to almost equal.

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Hans C. Graber
,
Robert C. Beardsley
, and
William D. Grant

Abstract

Surface winds derived from atmospheric pressure fields are used as input to a finite-depth wind-wave model to predict the sea state during a cold air frontal passage over the Yellow and East China Seas, which occurred 15–18 November 1983. The predicted maximum wave-stress field near the bottom is used to examine the concept of turbulent wave intensities causing sediment resuspension. The temporal variability of the wave field at three sites is used to illustrate the dependence of the bottom response on depth within the Yellow Sea. Maps of the temporal and spatial distribution of index for initiation of sediment movement are computed for different noncohesive sediment materials during this storm period and compared to sedimentological results for this region.

This study demonstrates that wave action is a mechanism which can significantly influence the sediment transport pattern induced by the regional circulation existing in this marginal sea. The results also identify regions where winter storm-generated surface waves are too weak to affect bottom sediments. Although the spatial variability of sediment resuspension depends on sea state and sediment material, the predicted wave-induced bottom shear stresses during a characteristic winter storm show that fine-grained material can be re-suspended as far out as the 100 m isobath in the East China Sea. Temporal maps of the index of sediment movement further show that the critical shear stress is exceeded for silty sand over large regions of the East China Sea during the duration of the storm studied.

These numerical simulation results suggest that the present-day distribution of sediments in the Yellow and East China Seas is in part a direct consequence of storm-generated surface waves during the winter season. The numerical model results further suggest that erosion of sand along the Chinese and Korean coasts is largely determined by surface wave action. Furthermore, the present-day mud patch south of Cheju Island appears to be a consequence of the circulation pattern in the Yellow and East China Seas and the southeastward decrease in wave and tidal bottom stress.

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Ge Peng
,
Christopher N. K. Mooers
, and
Hans C. Graber

Abstract

Thirteen-month records for the period of April 1994–April 1995 from eight (out of nine) Coastal-Marine Automatic Network (C-MAN) stations in south Florida are analyzed statistically to study alongshore variability of observed atmospheric variables. The surface variables largely are statistically homogeneous and coherent along the Straits of Florida. The maximum correlation for hourly wind components between adjacent stations (separated alongshore by 30–117 km) ranges from 0.9 to 0.75, respectively. However, there is a lack of coverage in the cross-shore direction; hence, a redistribution of C-MAN stations in the cross-shore direction should be considered to provide better spatial coverage of surface atmospheric variables in the south Florida region.

Surface winds from the National Centers for Environmental Prediction (NCEP) 80-km grid, η (Eta) Model analysis for the same period are compared statistically with observations from an air–sea interaction buoy and a C-MAN station in the south Florida coastal region. The η winds represent the low-frequency winds (periods between 3 days and 3 weeks) fairly well (e.g., the coherence exceeds 0.8 and the phase difference is less than 15°) but generally are weaker in magnitude than are the observed winds. The difference can be up to 2 m s−1 for the monthly mean and 1 m s−1 for the seasonal mean. The histogram of the η winds in winter has a single large peak instead of multiple peaks as occur in those of the observed winds. Southward bias in the η winds exists in summer.

The η Model simulates well the flow patterns of a tropical cyclone and an extratropical cyclone on the regional scale but lacks local spatial variability. As demonstrated, local spatial variability can be represented better by a blend of model and observed winds than by either the model-based or observed local surface winds alone.

These issues need to be reexamined periodically with upgraded versions of NCEP’s operational models.

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Björn Lund
,
Christopher J. Zappa
,
Hans C. Graber
, and
Alejandro Cifuentes-Lorenzen

Abstract

Surface wave measurements from ships pose difficulties because of motion contamination. Cifuentes-Lorenzen et al. analyzed laser altimeter and marine X-band radar (MR) wave measurements from the Southern Ocean Gas Exchange Experiment (SOGasEx). They found that wave measurements from both sensors deteriorate precipitously at ship speeds 3 m s−1. This study demonstrates that MR can yield accurate wave frequency–direction spectra independent of ship motion. It is based on the same shipborne SOGasEx wave data but uses the MR wave retrieval method proposed by Lund et al. and a novel empirical transfer function (ETF). The ETF eliminates biases in the MR wave spectra by redistributing energy from low to high frequencies. The resulting MR wave frequency–direction spectra are shown to agree well with laser altimeter wave frequency spectra from times when the ship was near stationary and with WAVEWATCH III (WW3) model wave parameters over the full study period.

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Rafael J. Ramos
,
Hans C. Graber
, and
Brian K. Haus

Abstract

The capability of phased-array HF radar systems to sample the spatial distribution of wave energy is investigated in different storm scenarios and coastal configurations. First, a formulation introduced by D. E. Barrick to extract significant wave height Hs from backscatter Doppler spectra was calibrated and subsequently tested (to assess bias and uncertainty) with data from seven different buoy/gauge stations collected during three different field experiments. Afterward, Hs observations were obtained for selected sampling locations within the radar effective domain (in all experiments), and a filtering technique based on wavelet transform characterization and decomposition was applied. The accuracy of the filtered radar-derived observations was assessed by comparing these estimates to results from independently calibrated wave propagation models. It was found that the HF radar accurately measured the energy field induced by different storm events. The filtering technique minimized the contribution of unrealistic features introduced by the presence of defective sampling, which is intrinsic to radar remote sensing at this frequency, and it proved to be central for the use of the HF radar as a tool to identify wave energy trends and potential zones of wave energy concentration in coastal areas. These findings show that the sampling capabilities of radar systems may be greatly enhanced because reliable wave energy estimates can be obtained in addition to conventional surface current measurements. This is particularly important in locations such as harbor entrances where in situ measuring devices cannot be deployed.

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Naoto Ebuchi
,
Hans C. Graber
, and
Michael J. Caruso

Abstract

Wind vectors observed by the QuikSCAT/SeaWinds satellite mission are validated by comparing with wind and wave data from ocean buoys. Effects of oceanographic and atmospheric environment on scatterometer measurements are also assessed using the buoy data. Three versions of QuikSCAT/SeaWinds wind data were collocated with buoy observations operated by the National Data Buoy Center (NDBC), Tropical Atmosphere Ocean (TAO), and Pilot Research Moored Array in the Tropical Atlantic (PIRATA) projects, and the Japan Meteorological Agency (JMA). Only buoys located offshore and in deep water were analyzed. The temporal and spatial differences between the QuikSCAT/SeaWinds and buoy observations were limited to less than 30 min and 25 km. The buoy wind speeds were converted to equivalent neutral winds at a height of 10 m above the sea surface. The comparisons show that the wind speeds and directions observed by QuikSCAT/SeaWinds agree well with the buoy data. The root-mean-squared differences of the wind speed and direction for the standard wind data products are 1.01 m s−1 and 23°, respectively, while no significant dependencies on the wind speed or cross-track cell location are discernible. In addition, the dependencies of wind speed residuals on oceanographic and atmospheric parameters observed by buoys are examined using the collocated data. A weak positive correlation of the wind speed residuals with the significant wave height is found, while dependencies on the sea surface temperature or atmospheric stability are not physically significant.

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Björn Lund
,
Hans C. Graber
,
Katrin Hessner
, and
Neil J. Williams

Abstract

The ocean wave signatures within conventional noncoherent marine X-band radar (MR) image sequences can be used to derive near-surface current information. On ships, an accurate near-real-time record of the near-surface current could improve navigational safety. It could also advance understanding of air–sea interaction processes. The standard shipboard MR near-surface current estimates were found to have large errors (of the same order of magnitude as the signal) that are associated with ship speed and heading. For acoustic Doppler current profilers (ADCPs), ship heading errors are known to induce a spurious cross-track current that is proportional to the ship speed and the sine of the error angle. Conventional mechanical gyrocompasses are very reliable heading sensors, but they are too inaccurate for shipboard ADCPs. Within the ADCP community, it is common practice to correct the gyrocompass measurements with the help of multiantenna carrier-phase differential GPS systems. This study shows how a similar multiantenna GPS-based ship heading correction technique stands to improve the accuracy of MR near-surface current estimates. Changes to the standard MR near-surface current retrieval method that are necessary for high-quality results from ships are also introduced. MR and ADCP data collected from R/V Roger Revelle during the Impact of Typhoons on the Ocean in the Pacific (ITOP) program in 2010 are used to demonstrate the MR currents’ accuracy and reliability.

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Charles L. Vincent
,
Hans C. Graber
, and
Clarence O. Collins III

Abstract

Buoy observations from a 1999 Gulf of Mexico field program (GOM99) are used to investigate the relationships among friction velocity u *, wind speed U, and amount of swell present. A Uu *sea parameterization is developed for the case of pure wind sea (denoted by u *sea), which is linear in U over the range of available winds (2–16 m s−1). The curve shows no sign of an inflection point near 7–8 m s−1 as suggested in a 2012 paper by Andreas et al. on the basis of a transition from smooth to rough flow. When observations containing more than minimal swell energy are included, a different Uu * equation for U < 8 m s−1 is found, which would intersect the pure wind-sea curve about 7–8 m s−1. These two relationships yield a bilinear curve similar to Andreas et al. with an apparent inflection near 7–8 m s−1. The absence of the inflection in the GOM99 experiment pure wind-sea curve and the similarity of the GOM99 swell-dominated low wind speed to Andreas et al.’s low wind speed relationship suggest that the inflection may be due to the effect of swell and not a flow transition. Swell heights in the range of only 25–50 cm may be sufficient to impact stress at low wind speeds.

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