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Video in French

Ocean Wave Model - ECWAM

Purpose

The ECMWF Ocean Wave Model (ECWAM) describes the development and evolution of wind generated surface waves and their height, direction and period.  Its domain extends across the full globe. 

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ECWAM is solely concerned with ocean wave forecasting and does not model the ocean itself: dynamical modelling of the ocean is done by NEMO.

Structure

 ECWAM evaluates the 2-dimensional surface wave spectrum, in both oceanic and coastal (but not inshore) waters.  This describes how much wave energy is present for given sea wave frequencies and associated propagation directions.  The part of the spectrum under the direct influence of the local wind is called “wind-wave" or "wind-sea”; the remaining part is usually referred to as “swell”.

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• wave advection,
• wave refraction,
• wind-wave generation,
• wave dissipation due to white capping and bottom friction,
• non-linear wave interactions.

Interaction with atmospheric and ocean models

ECWAM has two-way interaction with the Atmospheric models:

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Note: ECMWF uses LIM2 which is an earlier version of the Louvain-la-Neuve sea ice model currently available (Version 3.6)

Wave Data Assimilation

• ECWAM assimilates space-borne altimeter wave height data.
• ECWAM does not assimilate buoy wave data; instead, these data serve as an independent check on the quality of modelled wave parameters.

Output from ECWAM

ECWAM is run as:

• ENS-WAM twice daily giving forecasts to Day15 associated with ensemble based on 00UTC and 12UTC data times.  Global coverage 0.25^o x 0.25^o latitude-longitude grid (Ocean wave model ensemble 15-day forecast (Set IV ENS-WAM).
• Extended ENS-WAM daily giving extended forecasts from Day16 to Day46 associated with the extended range forecasts based on 00UTC data times.  Global coverage 0.5^o x 0.5^o latitude-longitude grid (Atmospheric Model Ensemble extended forecast (Set VI - ENS extended), section VI-v-c).
• SEAS-WAM monthly for forecasts to 7 months ahead associated with the seasonal forecast model (System 5).  Global coverage 1.0^o x 1.0^o latitude-longitude grid (Atmospheric model Seasonal 7-month forecast (Set V - SEAS), section V-v-e).
• SEAS-WAM quarterly for forecasts to 1 year ahead associated with the seasonal forecast model (System 5).  Global coverage 1.0^o x 1.0^o latitude-longitude grid (Atmospheric model Seasonal 7-month forecast (Set V - SEAS), section V-v-e).

       Output is in the form of wave and swell height, direction and period.  Also available are wave energy flux, mean direction and magnitude (important for assessment of the impact of the waves on coastlines and offshore structures). 

Graphical and chart presentation of wave forecasts

Fig2.2.1:ECMWF forecast entry page.

Wave output on ecCharts.

Fig2.2.2:  Procedure to load wave parameter charts on ecCharts.  Click on "Show Layers List" icon (1); Select "Add Layers" option (2); Input Wave into the "Layer select" box (3); Select desired chart by clicking on the icon (4).

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Fig2.2.3: Wave parameter charts available on ecCharts (see Fig 2.2.1 and Fig2.2.2 above) and may be displayed by clicking on the desired icon.

Wavegram output on ecCharts

Wavegrams are also available to show a time series of significant wave height, mean wave direction, and mean wave period for any sea location.

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Fig2.2.4: Wavegrams for any oceanic location are available on ecCharts. Choose location using the Probe icon (1); Click on "Views" (2); Select "Meteograms" option on the dropdown menu (3); Select "More" on the option page that appears(4); input Wave into the "Meteogram select" box (5); select desired chart(s) by clicking on the icon (6).

Wave output on Open Charts.

Fig2.2.5: Menu to select wave parameter charts from Open Charts (See Fig2.2.1 above).  Select "Range" (here medium and extended ranges); Select "Ocean Waves".

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Fig2.2.6B: Seasonal forecast charts for Tropical Storm, Hurricane, Typhoons frequencies are available on Open Charts by selecting Long option in the menu (See Fig2.2.5)  and then may be displayed by clicking on the desired icon.

Convention for describing wave direction

Users should note that by convention the direction of waves (and hence also wave energy flux) is described as the direction the waves are moving towards.  This is opposite from the convention for wind direction which is defined as where the winds are coming from.  Thus a southwesterly wind blows from the southwest; the corresponding wind-sea moves towards the northeast and is a thus described as a northeasterly wind-sea.

Wave Height definitions

The wave height is the distance between trough and crest.  However, many waves co-exist at the surface of the ocean and their distribution is given by the 2D wave spectrum.  From this distribution, the significant wave height is defined as 4 times the square root of the integral over frequency and direction of the wave spectrum.  It can be shown to correspond to the average wave height of the one-third highest waves, commonly known as H_1/3.  The mean wave direction is the spectrally averaged propagation direction of the waves (weighted by amplitude).

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Fig2.2.14: In this example the colours west of Ireland denote a low-point in wave heights, or potentially a form of 'weather window' for certain types of marine/shipping operations.  Equally this EFI can signify periods with anomalously big waves (yellow to red shading).

Considerations when using output from ECWAM

Interaction of wind-sea and swell

Use of the mean wave height and direction is the simplest method of describing the forecast wave regime in a given area and it is easy to be beguiled into just using this output for forecasts to customers.  However, the mean wave direction and height is made up of contributions from wind-sea and swell with different wave periods and they interact in a complex manner.  It is important to investigate the forecast wind-sea and swell separately to give an understanding of likely sea conditions in an area (e.g. for a ship requiring a particularly smooth passage) or at a location (e.g. an oil rig).  

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Fig2.2.15E(right): The forecast mean wave directions derived from the wind-sea and mean swell (as shown in Fig2.2.15A) superimposed on the previous chart (Fig2.2.15D left).  This illustrates the important additional information that is gained from consideration of the wind-sea and mean swell forecasts together.  The mean wave directions (FIG2.2.15A) give no indication of that a sea passage to the west of Portugal is likely to be through confused rough seas.

Sea-surface currents

The interaction of waves with sea-surface currents is not yet included in the operational version of the model.  In particular areas, (e.g. Gulf Stream or Agulhas current), the current effect may give rise to localised changes of up to a metre in the wave height.

Users should also note that whilst ECMWF does provide some ocean current output, from its ocean model (as "sea water velocity fields"), the current 1/4 degree resolution of that model is insufficient to allow strong gradients in western boundary currents to be captured. This means that stronger currents that are observed around the world tend to be underestimated in this output, sometimes substantially so.

Shoaling

Shoaling is the deformation of waves as they move from the ocean into shallow waters causing the waves to become steeper, increase in height, and have shorter wavelength .  The basic equations in ECWAM do represent the effect when the waves propagate from deep to shallow water, but the effect is not dramatic over most coastal waters.   Waves inshore and at the beach, where shoaling is very strong, are not represented since the resolution of ECWAM (~ 10km) cannot represent the actual beach slope.  Wave products near coasts, and, to a lesser extent, within small and enclosed basins (e.g. Baltic Sea) may be of lower quality than for the open ocean.  This may be due to incomplete resolution the detail of the coast by the land-sea mask.  Small islands too may not be identified and hence allow waves to propagate unhindered.   Note, however, that the wave model has a scheme that attempts to represent the impact of unresolved islands on the global propagation of waves. 

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 Fig2.2.17: The same example chart of wind-wave and swell as in Fig2.2.16, magnified near the Azores.  There are some areas around the islands where wave parameters are not forecast (i.e. where some of the grid points used in interpolation of wave data are on land) but the detail of coast may not be fully resolved.  ECWAM shows re-build of wind waves to the lee of the islands as the wind fetch increases and also the penetration of larger waves through the inter-island straits.

Near sea ice

Sea ice is not static but forms or extends with low air temperature or sea-surface temperatures, and can move with winds and sea current.  NEMO passes information to ECWAM regarding the extent and movement of the sea ice field forecast by LIM2, allowing a more realistic definition of what is open sea throughout the forecast period.  In the current operational version of the wave model, the interaction between waves and sea-ice is not actually represented.  Where sea ice cover >30% all wave parameters are set to missing (i.e. no valid values).  Wave products near ice-edges may be of lower quality than for the open ocean.  This may be due to uncertainty in sea-ice cover, or in the detail of an ice edge and consequently also in the boundary of the water area.  Spurious areas of ice or incorrect extent of ice will act as if a coastline or island and stop waves from propagating correctly, possibly decaying the waves completely and incorrectly sheltering an otherwise exposed location.

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Fig2.2.19: Significant height of combined wind waves and swell (Hs).  The coloured areas show the difference between the heights derived from the 2d spectra (used where >30% sea ice cover is forecast) and from the wave model (as if open sea).  Some large differences are evident, illustrating the need to treat the values of wave height with caution where sea ice is present.

Waves near tropical storms

In the IFS, there is an active two-way coupling between the atmosphere and ocean waves - surface wind stress generates the waves and in turn the waves modulate the wind stress.  The ECWAM generally forecasts realistic wave parameters (wave height, period etc). 

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When considering forecast wave parameters in the vicinity of typhoons, hurricanes etc., it should be remembered that IFS still has difficulties in producing some intense tropical cyclones and their subsequent motion.

Additional Sources of Information

(Note: In older material there may be references to issues that have subsequently been addressed)

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