Abstract
At 18:20 November 30, 2018, a triple rogue wave group was recorded in the central North Sea. These three consecutive rogue waves, subsequently called “Justine Three Sisters”, were recorded at a single point by a SAAB REX radar. The Norwegian Meteorological Institute’s operational wave forecast model and WAMOS marine radar’s measurements show that they appeared in a crossing sea condition with angle between wind sea and swell being 60 degrees, with swell energy much lower than the wind sea energy but with approximately the same peak frequency. We use the nonlinear wave model HOSM (Higher Order Spectral Method) to investigate frequency of occurrence of such an event in the ocean. Input to the simulations has been a wave frequency-directional spectrum generated by the operational wave forecast model of the Norwegian Meteorological Institute having 4 km resolution. The investigations show that occurrence of three consecutive rogue waves at a single point is a very seldom event in the ocean, which can, however, be reproduced in time domain HOSM simulations if sufficient number of realizations is performed. With the HOSM model being able to capture essential physics of ocean waves, we can assume to predict occurrence frequency from simulations. The study demonstrates also the effect of sampling variability on sea surface elevation and illustrates limitation of single point measurements, using the sea state in which “Justine Three Sisters” occurred as an example. Importance of using spacetime statistics in description of ocean waves as well as in design and operations of marine structures is also discussed.
Rogue wave patterns in the nonlinear Schrödinger equation