The Andrea Wave Characteristics of a Measured North Sea Rogue Wave

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 ConocoPhillips. A double rogue wave was measured on Nov. 9, 2007 in a storm crossing the North Sea and named Andrea following a forecasting procedure between 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 Distributions: 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 crest/Hs factors around 1.0.