The basis of the numerical sea state forecast system in the German Weather Service (DWD) is a spectral sea state model of the 3rd generation (3G-WAveModel). Spectral models describe the state of the swell using the so-called frequency-direction swell spectrum - this is the 2-dimensional distribution of the wave energy according to wave frequency (or wave period or wave number) and direction of propagation. In the current version, a resolution of 36 directions and 30 frequencies (wave periods between 1.5 and 24 seconds) is used. In the numerical model, the temporal development of the sea state spectrum is calculated at a large number of points on a grid stretched over the sea surface. Wave energy changes through the following physical processes: • Wave growth due to downward momentum flow from the wind field • Wave kinematics (advection, refraction) • Redistribution of energy between wavenumbers due to nonlinear interactions • Dissipation (internal friction and wave breaking) Similar to the chain of Atmospheric models (ICON, ICON-EU and ICON-D2) the sea state forecast system is divided into different forecast areas: The global model GWAM, the European model EWAM and the high-resolution coastal model CWAM. The model sea state is driven by analyzed and predicted 10m winds from the atmospheric models.
