Wave EPSgrams

The data are based on the resolution of the EPS wave model and on the higher resolution deterministic wave model. All ensemble members use the unperturbed wave analysis as the initial condition. The divergence between the ensemble members with respect to ocean waves is, therefore, due only to different wind forcing, if the coupled atmospheric ensemble members develop in different directions (see Figure 49).

10 m wind direction (“wind rose”) is divided into eight main directions or octants, each covering 45° (N, NE, E, SE etc) e.g. the northerly octant is between 337.5° and 22.5°. The length of the radius of an octant is proportional to the probability of that wind direction (i.e. to the proportion of forecasts falling in that octant). The exact probability of each octant is indicated by shading, obtained using a continuous colour scale from light to dark blue (see colour scale in the upper right corner).

10 m wind speed (m/s) is given as the mean of the instantaneous-forecast wind speed in m/s. The length of the whiskers should not be interpreted as likely wind gusts.

Significant wave height is given as an instantaneous-forecast value in metres. It is an estimate of the mean height of the highest 1/3 of the waves, corresponding with international conventions.

Mean wave direction is the mean direction of propagation of the waves, based on a weighted average of the wave spectrum. Distribution roses for wave direction are created similarly to those for wind direction (see above). Directions are shown in accordance with oceanographic convention, i.e. the direction towards which waves are propagating, the opposite to the way in which wind direction is displayed: e.g. zero means waves propagating towards the north, wind blowing from the north.

The instantaneous distribution is shown for the first 12 hours into the forecast and then for all subsequent 24 hour intervals. In order to relate wave height to direction, each octant is coloured, based on the distribution of the significant wave height associated with each mean wave direction. The straight red and blue lines are the mean direction of the control and deterministic forecasts.

Mean wave period is given as an instantaneous value in seconds. The mean period presented corresponds to the “energy period”. The key point for users is that more weight is given to low frequency waves containing swell than to high frequency waves.

Note also that waves might appear unrealistic near small islands that are not represented by at least one land point: in this case, the model’s wave energy will pass the location undisturbed rather than, as in reality, being partly blocked by the island. Similarly, coastlines are represented differently in the EPS and in the deterministic system, owing to the difference in their resolutions. Moreover, wave data are always selected from the closest sea point in the relevant grid. For these reasons, care should be taken when using wave EPSgrams for points very near complicated coastlines.

Figure 49:  An example of a wave EPSgram for the North Sea 18 July 2011, 12 UTC. A north-westerly wind is forecast to increase over the location, giving rise to amplified waves with increasing periods. The mean wave direction indicates that during the weekend the waves travel mainly to the south with a higher proportion of large waves.