Operational wave forecasts in the southern north sea

Caroline Gautier, Sofia Caires

Chair(s): dr. Sanchez Arcilla

Tuesday 30 june 2015

11:45 - 12:00h at Central America (level 0)

Themes: (T) Special session, (ST) Coasts at threat in Europe

Parallel session: 5H. Special session: Coasts at threat in Europe

Wave forecasts are paramount for the safety of coastal regions. Not only high water levels but also waves can pose a thread for flood defenses. Proper wave forecasts are thus essential in flood forecasting. Furthermore, larger ports need wave forecasts to optimize their tidal window. Also for predicting workability offshore, wave forecasts are needed. After a year of pre-operational use, from 2015 on the SWAN-North Sea model will be the operational wave forecast model of the Dutch part of the North Sea. It consists of two grids. The first coarse grid covers a large area (-12º to +9º E; 48º to 64ºN) and computes boundary conditions for the detailed nested model domain. The SWAN-North Sea simulations run continuously within FEWS-North Sea. Four times a day, a run is started, with a forecast horizon of 48 hours. SWAN-North Sea makes use of wind fields from the Dutch meteorological Institute (KNMI), wave boundary spectra from the operational wave model of the European Centre for Medium-Range Forecast (ECMWF) and water level and current fields from the operational hydrodynamic models WAQUA-DCSM and WAQUA-ZUNO. Sensitivity tests indicated that the model performs best with the whitecapping formulation according to Komen et al. (1984, J. Phys. Oceanogr.). Based on a six month validation at twelve locations, the models relative bias is 4% for significant wave height Hm0 and -30% for low frequency wave height HE10. The scatter indices are respectively 18% and 47%. The low frequency wave height HE10, defined as the significant wave height based on the domain 0.03 Hz - 0.10 Hz, turns out to be a hard parameter to simulate. Because of the steep low frequency flank of the wave spectrum, a small deviation in spectral shape has large effect on the forecast quality of this parameter. However, it is an important parameter determining to a large extend the motion of large vessels. Given the less good performance of the model for this parameter the best way to correct the model forecast spectra and hence the HE10 using observations, is being investigated.