GOOS Products and Services Bulletin

Wave Modelling at
The Met Office

Martin Holt and Jon Turton
Met Office, UK

For many years the Met Office has run second-generation global and regional spectral wave models to provide forecasts of sea state, supporting a range of user applications. There are three operational wave model configurations: global, European and UK waters, with different areas and resolutions. All use the same wave model formulation, and include the effects of shallow water on the wave energy spectrum.

Global wave model

• Coverage from 80.28° N to 79.17° S on a regular latitude–longitude grid, with a resolution of 5/6° longitude by 5/9° latitude; includes all sea areas, and takes ice edge information from the global numerical weather prediction (NWP) model.
• Run twice daily from 00 UTC and 12 UTC data times to five days ahead, using hourly NWP forecast winds. The winds from global NWP are at the same spatial resolution as the global wave model.
• Able to assimilate observations of wave height from satellite radar altimeter.

European wave model

• Covers the area from 30.75° N to 67° N and 14.46° W to 41.14° E (covering the north-west European shelf-seas, the Baltic, Mediterranean and Black Sea) with a resolution of approximately 35 km.
• Run twice daily from 00 UTC and 12 UTC data times out to two days ahead, using hourly NWP forecast winds.
• Takes boundary data from the global wave model, allowing swell from the Atlantic to propagate in.

UK waters wave model

• Covers the north-west European continental shelf from 12° W, between 48° N and 63° N at a resolution of 1/9° longitude by 1/6° latitude (approximately 12 km).
• Also includes the effect of time-varying currents on the waves, using forecast currents from the operational storm-surge model.
• Runs four times daily from 00, 06, 12 and 18 UTC, taking hourly surface winds from mesoscale NWP to give a 48-hour forecast. A second run of the UK waters wave model is also made to give a five-day forecast, but does not include the effects of currents.

Wave model output

For output the wave model spectra are processed to give values of significant wave height, wave period and direction for each of total sea, wind-sea and swell. Products from the Met Office's wave models are available through both the WMO Global Telecommunications System (GTS) and over the internet (e-mail or ftp) from the Met Office's Data and Products Distribution System (DPDS).

• Products available on GTS are at a reduced resolution (2.5° by 2.5° for the global wave model and 1.25° by 1.25° for the European wave model).
• Higher resolution products for a greater number of times are available via DPDS.

Sea state

The sea state at any point may be thought of as the sum of many individual waves, each of a particular direction and frequency. This can be represented as the wave energy spectrum, where the wave energy in each frequency and each direction is known. The Met Office wave models divide the wave energy spectrum at each grid point into 13 frequency components and 16 direction components. The lowest model frequency is at 0.04 Hz (25 seconds period or 975 m wavelength), and the highest frequency resolved by the model is 0.324 Hz (three seconds period or 15 m wavelength). The effect of waves at higher frequencies is included in the calculation of source terms.

The wave models account for growth of waves due to wind input, dissipation of energy by breaking waves, and transfer of energy between spectral components by non-linear interactions. Wave energy is advected from one grid point to the next at the group velocity. All the models include some shallow-water physics, namely bottom friction, refraction and shoaling.

Work carried out under the recently completed MAXWAVE project (funded under the European Community 5th Framework Programme) has led to development of some preliminary diagnostics of likelihood of occurrence of extreme or damaging waves based on the 'peakedness' of the wave energy spectrum.

Swell

Knowledge of the expected wave period is as important as wave height in calculating the response of a vessel or oil rig to sea state. If wave energy is present at periods close to the resonant period of the vessel, then the vessel motion will be enhanced.

Long-period swells can arrive in one of two ways – they can be generated locally under strong winds, in which case the weather and sea state on site will already be extreme, or alternatively the long-period swell could have been generated several days earlier, and may arrive having crossed the Atlantic. In this case the weather on site may be calm, and the amplitude of the long-period swell may be small. Because of the effect of wave dispersion (longer-period wave energy travels faster) such remotely generated long-period swell may arrive without warning, as the longest-period swell arrives first.

Such swells are very difficult to predict manually, and so the forecast relies on the output from the global wave model. In order to predict near-shore wave conditions accurately it is important to account for the effect of incoming swell. The figure opposite shows modelled long period swell from Hurricane Gert, some 48 hours after generation.

Validation against ENVISAT along-track altimeter data, and spectral validation against ENVISAT and ERS-2 Synthetic Aperture Radar (SAR) datasets provide global assessment of the prediction of long period swell.

Met Office Global wave model prediction of waves of 21 seconds period
from Hurricane Gert (1800 on 22 September 1999)

Near-shore wave and surf zone forecasting

In collaboration with HR Wallingford the Met Office is able to provide site-specific near-shore wave forecasts for a range of coastal applications. The Met Office wave model provides forecast values of offshore wave conditions, which are applied to HR's ray tracing models to derive near-shore conditions. A two-dimensional surf zone model can also be applied to derive surf zone diagnostics, including the depth of water when breaking occurs, height of breakers and breaker type.

Future developments

Work is ongoing towards implementing the third-generation wave model (WAM) as the next operational wave model, with an increase in the resolution of the global wave model to around 30 km and the UK waters model to around 2 nautical miles.

For coastal waters there is ongoing development, with Proudman Oceanographic Laboratory, of a coupled wave and shelf-seas model with full treatment of time-varying currents and water depth interacting with the waves.

Future work for the near-shore wave model will include extending the system to forecast wave overtopping on coastal structures.

For further information on the Met Office wave models, and how to obtain wave model data, see 
http://www.metoffice.gov.uk/research/ncof/products.html.

© Met Office, UK