Observation and Modeling of High Individual Ocean Waves and Wave Groups Caused by a Variable Wind Field

Kurzfassung

The impact of the gustiness on surface waves under storm conditions is investigated with focus on the appearance of rogue waves. The observations from optical and SAR satellites indicate structures of open cells moving across the North Sea during many storms characterized by extremely high individual waves measured near the German coast. According to measurements the footprint of the cell produces a local increase in the wind field at sea the surface, moving as a system with a constant propagation speed. Wave groups with the same speed grow continuously under the influence of this traveling gust. Growth continues until it is balanced by negative source terms as internal friction nonlinear interaction and breaking events. Slow moving gusts support therefore slow wave groups which have short wave length. Breaking occurs already at low wave heights. To create monster waves by this mechanism we need a fast moving gust, so that the wave length is larger than 150m, corresponding to periods larger than 10 s, and group velocities larger then 7.5m•s-1. The optical and microwave satellite data to connect the atmospheric turbulence and the extreme waves measured are used. The parameters of open cells (size, propagation speed) and there influence on the surface wind are determined and used for numerical spectral wave simulation. The test cases with different conditions are simulated with a spectral numerical wave model to identify the influence of a single and of several open-cells. The North Sea with 1nm horizontal resolution with focus on the German Bight was taken as simulation area. To take into account the rapid moving gustiness structure the input wind field is updated each 5min. The results show that a single moving open cell can cause the significant wave height increase in order of meters within the cell area, and especially in a small area of 2 -5km. For example, if one individual cell travels with 15m•s-1 over a sea surface with a wind-sea of Hs=3m (peak at 8sec) and a swell of Hs=2m (peak at 15sec), it produces a local increase of 3.5m. The results of this study can be important for ship safety and coastal protection. Unfortunal the open-cell caused gustiness can not be incorporated in present operational wave forecasting systems, because it needs an update of the wind field, which is presently not available. However, the scenario simulations for cell structures, observed by optical satellites can be done and applied for warning messages.