[1] A model for wave and wind stress prediction is constructed. The source functions that drive the space-time evolution of the energy spectra are developed in form based on theory and laboratory and field experiments. The calibration factors (proportionality constants of the source functions) are determined from a comparison of modeled and observed significant height and mean period. The observations are for the month of January 2005 and are derived from an array of laser range finders mounted on a bridge between two platforms in the Ekofisk oil field in the North Sea. The model calculates the form stress on the waves and adds it vectorially to the sheltering-modified skin stress. The resulting drag coefficient versus wind speed is shown to have the observed structure: low in light winds, increasing in moderate winds, and increasing more slowly in very strong winds. Modeled spectral shapes in the four quadrants of Hurricane Bonnie (1998) match the Scanning Radar Altimeter measurements. Modeled spectral properties in Hurricane Ike (2008) are compared against NDBC buoy estimates with good results. Drag coefficients in the mixed seas produced by hurricanes show dependence on wave age of the wind sea, swell propagation direction, and water depth. The need for wave and stress modeling for atmosphere-ocean coupling is emphasized. The new wave model has all the necessary attributes to be the basis for such a coupler.
Wave profiles have been measured with a system of four Optech lasers mounted on a bridge at the oil production site Ekofisk in the central North Sea since 2003, operated by ConocoPhitlips. A double rogue wave was measured on Nov. 9, 2007 in a storm crossing the North Sea and named Andrea following a forecasting procedure betM>een the Norwegian Meteorological Institute and ConocoPhillips. This wave, named here the "Andrea wave," is comparable in height and characteristics to the well known Newyear wave (or Draupner wave) measured in 1995 by Statoil. Front steepness is higher. That the same profile is measured by all four lasers is a good indication that the shape of the wave has been captured correctly, but one may still ask if this crest is that of blue, green, or white water. That is, how much of the height is related to presence of foam or sea spray? We attempt to answer this using the information of intensity of the return signals, which has been related to wave breaking and sea spray in recent studies by Toffoli et al. (2011, "Estimating Sea Spray with a Laser Altimeter," J. Atmos. Oceanic Technol., 28(9), pp. 1177-1183. Measurements of the average intensity of the return signal do not indicate presence of sea spray in the incoming part of the wave, but high intensity of return after the passage of the crest indicates presence of sea spray or foam on the parts of the waves exposed to winds. Cameras following the sea surface at measuring position with information on the return signal as given here would most probably increase our understanding of what is measured. Exceedance probability of crests and heights show a deviation from the second order distribution as given by Forristall (2000, "Wave Crests Di.stributions: Observations and Second-Order Theory," J. Phys. Oceanogr. 30(8), pp. 1931-1943 for the one percent highest waves in an apparently stable 3 h period including the Andrea wave. The deviation already starts at crestlHsfactors around 1.0.
Wind field ensembles from six CMIP5 models force wave model time slices of the northeast Atlantic over the last three decades of the 20th and the 21st centuries. The future wave climate is investigated by considering the RCP4.5 and RCP8.5 emission scenarios. The CMIP5 model selection is based on their ability to reconstruct the present (1971–2000) extratropical cyclone activity, but increased spatial resolution has also been emphasized. In total, the study comprises 35 wave model integrations, each about 30 years long, in total more than 1000 years. Here annual statistics of significant wave height are analyzed, including mean parameters and upper percentiles. There is general agreement among all models considered that the mean significant wave height is expected to decrease by the end of the 21st century. This signal is statistically significant also for higher percentiles, but less evident for annual maxima. The RCP8.5 scenario yields the strongest reduction in wave height. The exception to this is the north western part of the Norwegian Sea and the Barents Sea, where receding ice cover gives longer fetch and higher waves. The upper percentiles are reduced less than the mean wave height, suggesting that the future wave climate has higher variance than the historical period.
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