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Phenomenal seas*

Image of the Month - April 2012

Top: altimeter significant wave heights (SWH) estimated along the tracks of 4 altimeters (Jason-1 and -2, ERS-2, Envisat) on February 13, 2011 (left) and February 14, 2011 (right). The black square in the left (right) panel indicates the location of the most extreme sea states measured during these two days by the Envisat (Jason-2) altimeter, 18.1 m and 20.1 m respectively. Bottom: Altimeter (black) SWH values estimated along the Envisat (left) and Jason-2 (right) tracks, and computed from the WaveWatch-3 numerical model and ECMWF (red), NCEP (green) and NCEP+10% (blue) winds. (Credits Ifremer)

Peak periods as calculated by the WaveWatch 3 model, from SAR, wave buoy, and seismometers observations. The background shows the model values at 12:00 on the 15, as the longest swells were encroaching on the west coast of Scotland. The square symbols represent the wave buoy data, the size of the symbol signifying the SWH at the time of the maximum peak period and the color the value of the peak period at this time. Beside each symbol is printed the time of arrival of the maximum peak period at each buoy. The circle gives the location of the SAR observations and diamond symbols represent the seismic stations, also colored according to the peak periods observed. (Credits Ifremer)

Several storms with hurricane-force winds cross the North Atlantic Ocean every year. And, indeed we saw that the highest waves were recorded there (Image of the Month, June 2011). However, observations of winds and waves during these storms is a particular challenge because there are not many in situ observations in the middle of the ocean, so there are few opportunities to validate remote sensing techniques. In fact, significant wave height altimeter measurements over 15 m are considered to have a lower level of confidence due to the lack of validation data. Moreover, models are reputed to underestimate the severity of extreme events, partly due to low spatial resolution but also due to underestimation of these values by the wind fields used to force the models.

The Quirin storm occured in February 2011, and extreme values of significant wave heights were observed in the North Atlantic Ocean. To study this particular storm, observations from satellite altimeters, in situ buoys, and ground-based seismometers (which record the seismic noise from storms as it propagates far from the source) were used, as well as a numerical wave model. This study showed consistency between these different sources, both for significant wave heights in the open ocean generated in the storm area and for observations of the swell field it produced as it reached European shores a day or two later.

 
(* Following the
World Meteorological Organization, a "phenomenal sea" is a sea with wave heights of more than 14 m.)
 

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References:

  • Hanafin, J.A.,  Y. Quilfen, F. Ardhuin, J. Sienkiewicz, P. Queffeulou, M. Obrebski, B. Chapron, N. Reul, F. Collard, D. Corman, E.B. de Azevedo, D. Vandemark, E. Stutzmann (2012): Phenomenal sea states and swell from a North Atlantic Storm in February 2011: a comprehensive analysis, BAMS (in revision)
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