Why SAR wave mode data of ERS and Envisat are inadequate for giving the probability of occurrence of freak waves
Peter Janssen(1) and Werner Alpers(2)
Shinfield Park, Reading RG2 9AX,
Reading RG2 9AX,
(2) Universitaet Hamburg, Bundesstrasse 53, D-20146 Hamburg, Germany
In the last decade we have seen a rapid development in our understanding of the generation of extreme waves, termed here freak waves. These extreme sea states not only occur near strongly sheared currents but even in the open ocean. The most prominent explanation of the generation of freak waves in the open ocean is nonlinear focussing, a process that also causes the Benjamin-Feir Instability. Although predictions based on nonlinear focussing have been succesfully verified in the laboratory, it is of utmost importance that the theory is validated in the field.
There have been suggestions that SAR wave mode data could provide us with information on the occurrence probability of freak waves in the global ocean. However, it is well-known that the SAR imaging of ocean waves is, in general, a highly nonlinear process, mainly due to the velocity bunching caused by the wave motion. Furthermore, it can be expected that even for range travelling waves the SAR imaging mechanism is highly nonlinear when extreme waves are imaged due to the breakdown of the linear relationship between cross section modulation and wave amplitude. These nonlinearities cause strong distortions of the wave field, and thus imaging processing techniques applied to the image intensity distribution of ERS or Envisat wave mode SAR data which are aimed at locating freak waves in the global ocean are inadequate. The locations of the maxima in the image intensity or "groupiness" in the image intensity distribution do not, in general, mirror the locations of maxima in the wave amplitude or "groupiness" in the wave field. Thus it is impossible to obtain with this technique reliable information on the occurrence probability of extreme waves in the global ocean.
It is therefore strongly suggested to develop a new instrument to monitor this important phenomenon or to explore possibilities provided by other instruments. For example, in theory the Radar Altimeter provides us information on the probability distribution function of the sea surface, but clearly the feasibility of this approach needs to be investigated.