High waves in Draupner seas — Part 2: Observation and prediction from synthetic radar images

In Part 1, Van Groesen et al. (J Ocean Eng Mar Energy, 2017), a numerical study of extreme waves in so-called Draupner seas showed that extreme crest heights of 18 m, one-and-a-half times the significant wave height, occur in a time span of 20 min on average in every area less than 1 km(Formula presented.). Such extreme, steep waves are dangerous for ships and offshore structures. In this Part 2, we demonstrate that using synthetic images of an X-band radar such high waves can be predicted around 60 s before their actual appearance. It will be shown that a recent dynamic average and evolution scenario (DAES) that has demonstrated to lead to good reconstruction of the sea from distorted shadowed radar images of synthetic seas with moderate wave heights can also be applied to the high seas using nonlinear evolution codes. Taking at one instant a reconstructed sea state to calculate the nonlinear sea in future times leads to a qualitatively good prediction that can warn ships of freak waves before their appearance.