The Met Office operational wave forecasting system: the evolution of the Regional and Global models

Abstract. The Met Office operational wave modelling system is an operational forecast system run four times daily at the Met Office to provide global and regional forecasts up to 7 days ahead. The underpinning model uses a recent development branch of the spectral wave model WAVEWATCH III® (version 7.12) that includes a number of updates developed at the Met Office. Code contributions include the Spherical Multiple-Cell (SMC) grid, rotated pole grid formulation for mid latitudes, enhancements to OASIS coupling and updates to the netCDF postprocessing. Here we document and describe the technical details behind the Met Office operational system of WAVEWATCH III® configurations with a view to further development. These include a global forecast deterministic model (GS512L4EUK) and two regional models nested one-way covering the Northwest (NW) European shelf and UK waters (AMM15SL2) as well as an Atlantic wave ensemble (AS512L4EUK). GS512L4EUK and AS512L4EUK are based on a four-tier SMC 25-12-6-3 km grid refinement where currents are not included. AMM15SL2 is run operationally both as a standalone forced model and as the wave component of a two-way ocean-wave coupled operational system FOAM-AMM15. The AMM15SL2 baseline configuration is based on a two-tier SMC grid that focuses on the shelf seas around the United Kingdom (3 km resolution) where coastal cells have 1.5 km resolution and wave-current interaction is included. Results from a 2-year hindcast demonstrate the ability of the baseline configurations to reproduce both in-situ and satellite wave observations. Model-observations correlation is above 0.94–0.96 with standard deviations of differences that correspond to maximum 13–25 % of the observed mean bulk wave diagnostics, demonstrating the quality and accuracy of the system. Evidence of resolution dependent differences in wave growth was observed, leading to slightly overestimated significant wave heights when replicating coastal mid-range conditions by AMM15SL2, and better suited to replicate the extremes. Additionally, the inclusion of wave-current interaction in AMM15SL2 tends to larger spread on the observation-model differences. Hence, although a positive impact of the surface currents is not always shown in the overall statistics of the significant wave height, the addition of currents helps to significantly improve the prediction of the wave direction and period near the coast (>20 % improvement), which has implications in beach safety, risk to coastal overtopping and shoreline evolution. Future system developments such as the use of sea point wind forcing, the optimisation of the models in line with model resolution, the utilisation of SMC multigrid and data assimilation are discussed.