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Ocean Wave Model - ECWAM

Purpose

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

ECWAM is coupled to:

• the atmospheric forecast model at all configurations (ensemble control forecast, medium range ensemble, sub-seasonal range ensemble, seasonal ensemble) with resolution matching each model.
• the dynamic ocean model NEMO and SI3 sub-program.   (Note: the IFS analysis and the short range forecasts that are part of the IFS analysis cycle are not coupled with ECWAM).
  

ECWAM is solely concerned with ocean wave forecasting.  It does not model the ocean itself.  Dynamical modelling of the ocean is done by NEMO.

Ensemble structure

ECWAM evaluates the 2-dimensional surface wave spectrum for both oceanic and coastal (but not inshore) waters.  The wave information is output in 36 directions of propagation and 29 wave frequencies at 6hr intervals.  These describe the extent, severity and timing of the forecast wave energy and highlight risk areas.

The horizontal resolutions of ECWAM versions are the same as each of the atmospheric models with which they interact (~9km with ensemble control forecast and medium range forecasts, ~36km with sub-seasonal range and seasonal models).  Heat, moisture and momentum fluxes between atmosphere and ocean are included, depending on the sea state.  The land-sea masks match up with the atmospheric model versions in Cy50r1.

Changes in the wave spectrum are derived from the processes of:

• wave advection.
• wave refraction.
• wind-wave generation.
• wave dissipation due to white capping and bottom friction.
• non-linear wave interactions.
• impact of bathymetry.  Improved description of the depth of the ocean floor (particularly in coastal waters).  This improves coastal wave realism. 
• interaction with sea-ice.  Waves are attenuated when sea-ice concentrations >30% (rather than completely blocked in Cy49 and earlier). This improves wave realism near the ice edges.
• surface currents.  Currents modify forecasts of waves for refraction and result in reduced smoothing of wave fields.
• waves are attenuated within the ice using the SI³ model when sea ice concentrations are above 30% improving realism near the ice edges.

• a revised and improved bathymetry enables better description of waves, particularly in shelving coastal areas.

   

Interaction with atmospheric and ocean models

ECWAM has two-way interaction with the Atmospheric models:

• ECWAM supplies surface roughness (according to the forecast sea state) which is used in the evaluation of boundary layer winds.
• Atmospheric models supply surface wind conditions, via surface stress, and this dominates sea-surface wave development.

ECWAM has two-way interaction with NEMO and sub-program SI3:

• ECWAM supplies surface stress, Stokes drift, and turbulent energy flux to the ocean surface.
• NEMO and sub-program SI3 supply information on the concentration and thickness of ice.

Wave data assimilation

• ECWAM increased number of wave observations are assimilated by Cy50r1.
• 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:

• Ensemble Control Forecast-WAM at 00UTC and 12UTC data times giving forecasts to Day 15  and 06UTC and 18UTC data times giving forecasts to Day 6.  Global coverage at 9km resolution (Ocean wave model ensemble forecast (Set IV ENS-WAM).
• Sub-seasonal ENS-WAM daily giving sub-seasonal range forecasts from Day 16 to Day 46 associated with the sub-seasonal range forecasts based on 00UTC data times.  Global coverage 36km resolution (Atmospheric Model Ensemble sub-seasonal forecast (Set VI - ENS sub-seasonal), 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

Fig2.3-1: ECMWF forecast entry page.

Wave output on ecCharts.

Fig2.3-2: 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).

Fig2.3-3: Wave parameter charts available on ecCharts (see Fig2.3-1 and Fig2.3-2 above) and may be displayed by clicking on the desired icon.

Wavegram output on ecCharts

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

Fig2.3-4: 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 Opencharts

Open Charts.

Fig2.3-5: Menu to select wave parameter charts from Open Charts (See Fig2.3-1 above).  Select "Range" (here medium and sub-seasonal ranges); Select "Ocean Waves".

Fig2.3-6: Sample wave chart. Click on any point to produce a meteogram and/or wavegram.

Fig2.3-7:  Wavegram on Opencharts associated with Fig2.3-6 at location near 52N 27W.

Fig2.3-8: Wave parameter charts available on Open Charts.  These may be displayed by clicking on the desired icon (the above ensemble products are available at post-processing steps, 12-hourly from T+0h to T+168h).

Fig2.3-9: Seasonal forecast charts for Tropical Storm, Hurricane, Typhoons frequencies are available on Open Charts by selecting Seasonal (Long) option in the menu (See Fig2.3-5)  and then may be displayed by clicking on the desired icon.

Fig2.3-10: Sequence of ocean wave forecasts.  Significant wave height forecast (colours) and 10m wind (arrows) from data time 00UTC 4 January 2014, step 12 hours.  Wave heights at 1.25m intervals as scale.

Swell propagates outwards well away from the source.  Increased swell (e.g. reaching a coast) can give forewarning of a storm system well before any indication in the atmosphere.  Fishermen have long used the arrival of long-period swell as an indication of an approaching storm even if the sky is clear.  Surfers often benefit from significantly large swell in calm conditions well away from the swell source region.

Fig2.3-11:  180h forecast for significant wave height (contours) for all waves with periods between 21 and 25 seconds (shading), initialised at 00UTC 2 December 2016.  The highest significant wave heights (contours) are still confined to the storm location in the Atlantic south of Iceland, while long waves from that storm are already affecting coastlines from Iberia to South Greenland (coloured). 

Waves with different periods propagate with different speeds - longer periods travel fastest.  These can be tracked through the forecast period and areas where different wave trains potentially interact can be identified.

Fig2.3-12:  Chart showing forecast significant wave heights for several ranges of wave periods (Blue,10-12s; Green,12-14s; Yellow,14-17s; Red,17-21s).  Forecast data based on data time 00UTC 25 October 2017.  The faster southward propagation of the long period waves over the shorter period waves from their source off NW Africa is clear.

The Extreme Forecast Index (EFI) can be used to indicate the significance of forecast significant wave heights when compared with the range of wave heights that might usually be expected as defined by the wave M-climate.

Fig2.3-13: 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).

Additional Sources of Information

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

• Definitive information on the ECMWF WAVE MODEL products and availability can be found:
  • for Ensemble Control Forecast WAM in the Catalogue of ECMWF real-time products (Set II).
  • for Ensemble WAM in the Catalogue of ECMWF real-time products (Set IV)
• See IFS documentation part VII for extensive information on ECWAM model.
• Read more on ocean wave modelling and model output parameters.
• Read an in depth view of the structure of ECWAM which gives a description of the theory behind the ECWAM model.  Old but still relevant paper.
• Fact sheet - Ocean wave forecasting.
• ECMWF - Lead centre for wave forecast verification and information on national models and wave verification results. 
• Read about techniques regarding satellite measurement of wave height.
• Watch a comprehensive lecture on ocean wave forecasting at ECMWF or air-sea Interaction and earth system modelling.

(FUG associated with Cy50r1)