Waves and Wave Meteograms

Wave Meteograms

The ECWAM runs as an ensemble and produces a wide range of output in a similar manner to the ENS atmospheric model.   The forecast data are based on the resolutions of the WAM, HRES and ENS.  All ensemble members use the unperturbed wave analysis as the initial condition. 

Fig8.1.6.1: To view Wave Meteograms:

  1. On Charts Catalogue page, click on Meteograms.  Display of product (in this case default is Prague).
  2. Select Wavegram from drop-down menu.
  3. Select location. Display of product (in this case a location in the southeast of the North Sea).

The divergence between the ensemble members with respect to ocean waves is, therefore, due only to different wind forcing.  This can be attributable only to the coupled atmospheric ensemble members evolving in different ways, and indeed starting out from T+0 with (slightly) different fields of 10m winds (and other atmospheric parameters).   The Ensemble wavegram provides a probabilistic interpretation of the WAM ensemble for specific locations.  It displays the time evolution of the distribution of several marine parameters from the ensemble at each forecast range by a box and whisker plot and information on wind and wave movement by a polar diagram (See Fig8.1.6.2).

Care should be taken when using wave Ensemble Meteograms for points very near to complicated coastlines, islands, or sea ice edges.  Main considerations are:


 

 Fig8.1.6.2: An example of a wave Ensemble Meteogram for the southern Denmark Strait (near 64N 35W) base time 00UTC 11 May 2017.  A strengthening NE wind is forecast over the location giving rise to amplified waves with increasing periods.  The mean wave direction indicates that on Saturday 13 May the waves become more directed towards the SSW with a higher proportion of ENS members showing very large waves (exceeding 9m significant wave height). Note wind directions indicate where the wind blows from; wave directions indicate the direction the waves travel towards. The dotted red and continuous blue lines are the values of the control and deterministic WAM forecasts respectively.

The significant wave height corresponds to the average wave height of the one third highest waves (H1/3).   This means that there will be some waves of greater trough-to-crest height than indicated by the significant wave height.  Interference of wave trains, a shelving sea bed, or a contrary sea current not represented in the WAM and so may also induce much larger waves than shown.  The mean wave direction is the spectrally averaged propagation direction of the waves (weighted by amplitude). 

 

Example Wave Charts


 

 Fig8.1.6.3: An example of Significant wave height / Mean wave direction and height. Thursday 11 May, 00 UTC T+60 Valid: Saturday 13 May, 12 UTC.  Note: Dark blue generally indicates areas in open water where waves are forecast to be very small (e.g. among the islands off western Scotland),  but the area of dark blue off the Greenland coast is in effect denoting sea ice.  The ice extent is modified during the forecast by information passed from NEMO to WAM.  However, HRES is not coupled at present and so uses the initial extent of sea ice.  This may affect the development of synoptic scale developments in the atmosphere with consequent influence over the forecast winds.


Fig8.1.6.4: An example of Significant wave height probability greater than 4m.  Thursday 11 May, 00 UTC T+60 Valid: Saturday 13 May, 12 UTC. 

 

Fig8.1.6.5: An example of wave energy flux magnitude (KW/m). Thursday 11 May, 00 UTC T+60 Valid: Saturday 13 May, 12UTC.  Colour thresholds are at 40KW/m intervals - lightest pink represents <40KW/m.

The energy carried by swell and wind waves is important to aid assessment of potential damage to shorelines or fixed marine installations (e.g. oil rigs) and also as a guide to potential output of marine wave energy generation installations.   Charts of wave energy flux magnitude (the Integral over all frequencies and directions of the product of the group speed and the two-dimensional energy wave spectrum) are available on ecCharts.


Long Swell Forecasts


 Fig8.1.6.6:  Long swell forecasts.  WAM base time 00UTC 4 Jan 2014.  In the mid-Atlantic, significant wave height (left) is very high at about 20m.  This consists of swell at several periods.  The wave energy in terms of swell height for:

Note the wavelengths differ but are about 1km.  Wind waves have much shorter wavelengths. 

Additional Sources of Information

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