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Wave Model Description - WaveWatch III at the Met Office
The WaveWatch III model was intrduced as the Met Office's operational model in October 2008.
Contents
- Regional Wave Modelling - A Spectral Approach
- Forcing the Met Office wave models
- The WaveWatch III third-generation spectral wave model
- Operational Configurations
- References
1. Regional Wave Modelling – A Spectral Approach
National Met Services such as the Met Office have the responsibility of providing forecasts of dangerous sea conditions to mariners and coastal communities. These forecasts require knowledge of storms and sea conditions globally or over large regions of coast. At present, covering such large areas does not permit modeling that resolves waves on an individual basis, and instead the wave forecasting community aim to simulate statistical parameters that describe the wave field as an average in time and space (the sea-state). The tool of choice for this approach is to use spectral wave models.
Spectral models work by calculating the levels of wave energy that can be assigned to a two-dimensional frequency-direction domain (termed the wave spectrum) used to describe the average motion of the sea-surface under waves. Essentially the spectrum decomposes a given sea-state into a set of constituent sine waves, each with a different direction, period (inverse of frequency) and amplitude (energy). Field experiments have established families of wave spectra appropriate to different forcing circumstances, and upon which the evolution of spectra in wave models have been based. A seminal example of these field data is the JONSWAP experiment that recorded wave growth over a fetch in the North Sea (Hasselmann et al., 1973).
Standard integrated parameters representing wave conditions can be generated from the spectrum. Common examples are significant wave height, wave peak and mean period, mean wave direction and spreading. With knowledge of wind strength and direction, these integrated parameters can also be identified as wave field components defined as ‘wind-sea’ (the part of the wave field that is directly forced by the wind) or as freely propagating ‘swell’.
2. Forcing the Met Office wave models
The wave models are forced using hourly wind fields generated in Met Office Numerical Weather Prediction (NWP) models. These models describe a three-dimensional grid field of atmospheric variables (wind, temperature, pressure, moisture) both as an estimate of the atmospheric conditions in the present (the ‘analysis’) using a technique called data assimilation, and transported forward in time (the ‘forecast’) using a dynamical grid point model.
Since 1991 a Unified Model has been in use at the Met Office for both low-resolution climate modeling and high-resolution operational NWP. This system is regularly upgraded to take advantage of improvements in both NWP techniques and climate research. The model system comprises both assimilation process and grid-point model which can be run in global or regional configurations. Both sub-systems are developed and maintained by substantial teams. They are upgraded several times a year.
Data assimilation produces the analysis by combining up to the minute global observation data with the model’s background field (a forecast from an earlier model run) whilst taking account of the likely statistical errors in both. This is a key stage in the NWP process since subtle changes in these initial conditions can alter the subsequent short period forecast significantly. The Met Office model uses a variational assimilation method described by Lorenc et al. (2000).
The predictive part of the Unified Model uses a non-hydrostatic, fully compressible deep atmosphere formulation based on a terrain-following, height-based vertical coordinate (Davies et al., 2005).
Atmospheric model configurations used to force the Met Office wave models comprise a 40km resolution Global model, which provides forecasts out to 5 days ahead, and a 12km resolution North Atlantic European (NAE) configuration which forecasts out to 2 days ahead. In assessing an appropriate wave model spatial grid size, the resolution at which the forcing winds are provided is an important constraint. The forcing parameters provided to the wave models comprise wind speed and direction at a standard 10m height above sea level.
3. The WaveWatch III third-generation spectral wave model
In October 2008 the Met Office will superseded its second-generation spectral wave model with the third-generation spectral model WaveWatch III (WW3), which has been developed and released by the wave team at the US National Center for Environmental Prediction (NCEP). This update to the modeling scheme has been made since WW3 provides a modular coding system that is better suited to configuration management and porting requirements associated with modern supercomputing, and also presents an opportunity for the Met Office to make best use of research developments made within the expanding worldwide WW3 community.
The wave model is forced using hourly wind fields generated from the Met Office Numerical Weather Prediction (NWP) models, and must describe four key processes:
- growth of waves due to wind forcing
- dissipation of wave energy due to effects such as ‘whitecapping’ and bottom friction
- cascading of energy to lower frequencies through nonlinear interactions
- propagation of unforced ‘swell’ energy.
In the present Met Office configurations of WW3 we have used:
- the Tolman and Chalikov (1996) source term scheme; which comprises Chalikov and Belevich (1993) and Chalikov (1995) schemes for wave growth along with the dissipation scheme of Tolman and Chalikov (1996)
- the Discrete Interaction Aprroximation (aka 4-wave interaction) scheme for nonlinear energy transfer (following Hasselmann et al. 1985)
-
a Met Office second-order swell advection scheme – this was chosen to optimize model run time whilst ensuring that numerical errors in swell propagation (e.g. the so called ‘garden sprinkler effect’) are minimized.
Wind-sea and swell partitioning are performed using a 'watershed' technique.
Depth information for the model grid uses a representative average for each cell. This assumption may prove important in some near coastal grid cells where the average depth may mask bathymetric features affecting the local distribution of wave energy. A cut-off depth is set in the model scheme at 200m, since at depths greater than this value shallow water effects are negligible even for wave energy in the lowest frequency range.
The importance of increased spatial resolution is clearest in the near coastal zone, since this allows a better representation of the coastline itself and will increasingly resolve shallow water bathymetric features. The trade off for making these resolution changes lies in run-time, with shorter calculation time-steps required for increased spatial resolution in order not to violate conditions for energy advection.
4. Operational configurations
The Met Office suite of operational global and regional nested wave models produces regularly updated wave forecasts with lead times of up to five days. Operationally the models are configured with a spectral resolution of 25 frequency bins and 24 directional bins, representing waves with a range of periods between 25 seconds and 3 seconds (deep-water wavelengths from 975 m to 15 m).
Wave conditions worldwide are forecast using the Global Wave Model on a 5/9 degree latitude by 5/6 degree longitude grid (approximately 60km square grid at mid-latitudes), with fields output at 3-hourly resolution to a lead time of 5 days (T+120). This model is forced using the Met Office's Global domain NWP 10m wind field and run twice daily based on 0000 and 1200 UTC analysis times. The extent of ice cover at high latitudes is updated daily using a global sea-ice analysis based on the Met Office OSTIA scheme.
Boundary conditions from the Global Wave Model are used as input to a North Atlantic European Wave Model, which uses a rotated 1/9 degree latitude by 1/6 degree longitude grid (approximately 12km) covering a region from approximately 68°W to 30°E and 25°N to 65°N. Two configurations of the NAE wave model are run. The first configuration is forced by high resolution NAE NWP 10m winds and is run four times daily using analysis times 0000, 0600, 1200 and 1800 UTC and provides hourly forecasts out to T+36. The second configuration (Extended NAE Model) is run twice daily (0000 and 1200 UTC analyses) forced by Global NWP 10m winds in order to provide 3-hourly forecast data out to T+120.
Data are output from the model comprise:
- 2D wave spectra
- Overall significant wave height (calculated from the entire spectrum)
- Peak wave period
- Overall mean wave period
- Overall mean wave direction
- Wind-sea significant wave height
- Wind-sea peak wave period
- Wind-sea mean wave direction
- Primary Swell significant wave height
- Swell mean peak period
- Swell mean wave direction
which are variously retained in commercially available fast-access hindcast archives and research based forecast model archives. Due to data handling constraints two-dimensional (frequency-direction) spectral data are output at specific model points only and are not archived.
Future versions of the model intend to provide additional outputs including:
- directional spreading
- secondary swell height, period and direction statistics
-
tertiary swell height, period and direction statistics.
5. References
Chalikov, D.V., 1995. The parameterization of the wave boundary layer. J. Phys. Oceanogr., 25, 1333-1349.
Chalikov, D.V. and M.Y. Belevich, 1993. One-dimensional theory of the wave boundary layer. Bound. Layer Meteor., 63, 65-96.
Davies, T., Cullen, M.J.P., Malcolm, A.J., Mawson, M.H., Staniforth, A., White, A.A., Wood, N., 2005. Q. J. R. Meteorol. Soc., 131, 1759-1782.
Hasselmann, S., K. Hasselmann, J. H. Allender and T. P. Barnett, 1985. Computations and Parameterizations of the Nonlinear Energy Transfer in a Gravity-Wave Specturm. Part II: Parameterizations of the Nonlinear Energy Transfer for Application in Wave Models. J. Phys. Oceaonogr., 15, 1378-1391.
Hasselmann, K., Barnett, T.P., Bouws, E., Carlson, H., Cartwright, D.E., Enke, K., Ewing, J.A., Gienapp, H., Hasselmann, D.E., Kruseman, P., Meerburg, A., Muller, P., Olbers, D.J., Richter, K., Sell, W., Walden, H., 1973. Measurements of wind-wave growth and swell decay during the Joint North Sea Wave Project (JONSWAP). Deutsches Hydrographisches Institut, Hamburg, UDC 551.466.31; ANE German Bight.
Lorenc, A., Ballard, S.P., Bell, R.S., Ingleby, N.B., Andrews, P.L.F., Barker, D., Bray, J.R., Clayton, A.M., Dalby, T.D., Li D., Payne T.J., Saunders, F.W., 2000. Q. J. R. Meteorol. Soc., 126, 2991-3012.
Tolman, H.L. and D.V. Chalikov, 1996. Source terms in a 3rd generation wind-wave model. J. Phys. Oceaonogr., 26, 2497-2518.
(Last Updated: 23-01-2009)