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).

• 10 m wind direction (“wind rose”) is divided into eight main directions or octants, each covering 45° (N, NE, E, SE etc,  i.e. 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 and the exact probability of each octant is indicated by shading, using a continuous colour scale from light to dark blue given by the colour scale in the upper right corner.   The probability is taken as the proportion of ENS member forecasts verifying that day at 00UTC, 06UTC, 12UTC and 18UTC and falling in each octant.  Note:
• The size of the wind rose does not refer to wind speed, but only to the probability of wind in that direction regardless of its strength.
• 10 m wind speed is the instantaneous mean speed in m/s. Note:
• In cases of strong small-scale wind systems, the maximum wind can sometimes be considerably stronger in HRES than in the ENS.
• The peaks of the whiskers should not be interpreted as wind gusts.  The special ENS products related to gusts, such as gust probability charts, should be used.
• Significant wave height is an instantaneous forecast value in metres.  It is estimate of the mean height of the highest one-third of the waves. This corresponds with international conventions.  Note:
• The peaks of the whiskers should not be interpreted as peak wave heights.  ENS products for maximum wave height and the corresponding wave period should be used ("maximum individual wave height" is available from MARS).
• 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).  Note:
• Directions are shown in accordance with oceanographic convention which is the direction towards which waves are propagating (eg wave direction of 180° means waves are propagating towards the south).  This is opposite to the way in which wind direction is displayed (eg 180° means the wind is blowing from the south).
• Significant wave heights are shown by the colouring of each octant according to the adjacent scale and the probability of the significant wave height is shown by the area of each colour (See Fig8.1.6.2).
• Mean wave period is an instantaneous forecast value in seconds.  The mean period presented corresponds to the “energy period”.  The key point for users to note is that more weight is given to low frequency waves containing swell than to high frequency waves.

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

• Wave data are always selected from the nearest sea point.  But coastlines are represented differently in the ENS and HRES land-sea masks due to the difference in their resolutions and a sea point in one may be a land point in the other.  Consequently, wave forecasts may differ between models.
• During the forecast period, NEMO and ENS alter the location and amount of sea ice allowing or hindering wave propagation.  HRES is not coupled at present and so maintains the analysed distribution of ice through the forecast period.  Consequently, wave forecasts may differ between models.
• Small islands may be unrepresented by a land grid point and WAM wave energy will pass the location undisturbed, rather than being partly blocked by the island as it ought to be.
• Constructive interference of waves can occur giving much higher energy or wave heights but is not evaluated by WAM.
• Much larger waves than shown by the model can occur locally in coastal waters with a shelving sea bed, or where there is a contrary sea current.

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:

• swell period 25-29sec (centre, in blue) with swell height well over 1m;
• swell period 21-25sec (right, in green) with swell height well over 5m.
• other swell periods are not shown.

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