Page History

...

Fig2.2-3: Wave parameter charts available on ecCharts (see Fig22.A and Fig22.B Fig2.2-1and Fig2.2-2 above) and may be displayed by clicking on the desired icon.

...

Fig2.2-5: Menu to select wave parameter charts from Open Charts (See Fig22Fig2.A 2-1 above).  Select "Range" (here medium and extended ranges); Select "Ocean Waves".

...

Fig2.2-7:  Wavegram on Opencharts associated with Fig22Fig2.F 2-6 at location near 52N 27W.

...

Fig2.2-9: Seasonal forecast charts for Tropical Storm, Hurricane, Typhoons frequencies are available on Open Charts by selecting Long option in the menu (See Fig22Fig2.E2-5)  and then may be displayed by clicking on the desired icon.

...

When wind-sea and swell move in similar directions the wave heights can give information on the likely sea state as one is superimposed on the other, particularly where both have a significant and comparable wave height.  On occasion the swell and wind-sea may be moving in opposite directions (an opposing sea) and wave heights give information on the likely rougher sea state to be expected.  Often the wind-sea and swell are at right-angles (a cross sea).  Where the wind-sea and swell heights are similar the sea can be very disturbed and difficult for shipping.   An illustration is given in Figs22Figs2.S1-5.2-14(a to e) 

Fig2.2-14(a): ecChart of mean wave direction (wave height is indicated by the length of the arrow).  On ecCharts, wave height may be shown by use of the probe tool or more graphically by superimposing mean wave heights).  This chart gives an overview of wave conditions.  Northwesterly waves (i.e. moving towards the northwest) are indicated near point A.  Easterly waves i.e. moving towards the east) are indicated near point C.   However, it is important to investigate the contributions to the mean wave directions and heights from inspection of the wind-sea and swell at this time.

...

Fig2.2-14(e):The forecast mean wave directions derived from the wind-sea and mean swell (as shown in Fig22.S1Fig2.2-14(a)) superimposed on the previous chart (Fig22.S4 leftFig2.2-14(d)).  This illustrates the important additional information that is gained from consideration of the wind-sea and mean swell forecasts together.  The mean wave directions (Fig22.S1) give no indication of that a sea passage to the west of Portugal is likely to be through confused rough seasFig2.2-14(a)) give no indication of that a sea passage to the west of Portugal is likely to be through confused rough seas.

Waves and swell with a long period

Large swell waves with a long period breaking on a beach slope tend to have a large swash with water washed well up the beach.  This is often unexpected and takes people by surprise and can cause damage and casualties.  Users should note when large waves with a long period are forecast to run onto an exposed coast.  Extreme forecast index products for waves can alert users to the potential problem. 

Image Added

Fig2.2-15: Wave energy forecast VT 09UTC 16 Sep 2023, DT 12UTC 15 Sep 2023.   The chart shows extreme wave energy flux (~1300KW/m) being driven towards the exposed southern coast of South Africa.

Image Added

Fig2.2-16: Wave energy forecast VT 09UTC 16 Sep 2023, DT 12UTC 15 Sep 2023.  The chart shows the extreme forecast index for significant wave height at 0.9 to 1.0 alerting to the significant nature of the ocean swell.

Image Added

Fig2.2-17: Wave ENSgram DT DT 00UTC 16 Sep 2023. Significant wave heights were forecast to exceed 9 m with wave period above 15 s.  These values were also observed at a nearby buoy.  

Very large waves and swell were induced by a deep depression in southern Atlantic associated with very strong winds. The waves became larger as they approached the coasts and coincided with spring tides. Significant wave heights exceeded 8 m in many places suggesting maximum waves were significantly higher.  There was considerable coastal damage and some loss of life.

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.

...

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. 

Fig22Fig2.T2-18: An example chart of wind-wave and swell. Some shoaling is possible towards the French and British coasts as the sea becomes less deep but forecast values cannot be absolutely relied upon. See Fig22Fig2.U 2-19 for detail around the Azores.  No parameters are shown on coasts nor where ice cover >30% (i.e. where some of the grid points used in interpolation of wave data for display are on land or ice. Users should identify whether the ice areas or coastal zones are the cause. See section regarding wave parameters near sea ice) 

 Fig22Fig2.U2-19: The same example chart of wind-wave and swell as in Fig22Fig2.T2-18, 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.

...

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.

Fig22Fig2.V2-20: Illustration of the importance of distinguishing between ice cover and shallow water when an area of wave parameters is missing.

Fig22Fig2.W2-21: 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.

...