Operational DMI-WAM resolves the wave spectrum into 25 frequencies:

2.43 - 23.9 sec

This is in use since 1999. Now, with more computing power available, it is time to examine the effect of extending the high-frequency spectral range by adding 7 more frequencies:

1.25 - 23.9 sec

This model is designated Development DMI-WAM. The added frequencies facilitates model wave growth, especially in regions where the sea develops quickly from a state of relative calm. Until now, the high-frequency spectral tail (1.25-2.43 sec) has been parameterised (the freq**(-4) tail) and wave-wave interaction perhaps not adequately represented in this spectral range.

Below are shown maps of wind speed and significant wave height for the North Atlantic, at a randomly selected time. Development DMI-WAM has been run for a 10-day period in order to remove spin-up effects. Apart form that, the two model set-ups are identical.

South of Greenland we find a region with relatively strong wind (16-18 m/sec), and comparatively low wave height. Other regions with strong wind (west of Irland, Denmark Strait btw. Iceland and Greenland, northern North Sea) have a local maximum of significant wave height at the same time, but such a maximum is not present south of Greenland. A wind speed of 16-18 m/sec corresponds to a fully developed sea of 6-7 m, so in this region, the sea is still developing.

The figure below shows the Operational - Development DMI-WAM difference at the same instant. When more high frequencies are included, the model wave height increases by up to approx. 1 meter. This effect has a very limited spatial and temporal range.

We zoom in on the region in question (50-60N, 30-40W). showing Operational + Development DMI-WAM (left), and the difference (right). The model wave height increases in particular where the wave height gradient is large. We get fine structure (or instability) in this region.

The time series for a single position shows the persistence of this difference. Below is shown Operationel minus Development DMI-WAM, at 59.5N, 38W. The model wave height increases at this position during a 3 hour period, by up to 0.75 meter. During the rest of this 60 hour simulation, the two predictions are identical within plus/minus 0.1 meter.

Conclusion: Extending the high-frequency spectral range gives almost identical model wave height in most of the time/space domain, with changes of the order of 0.1 meter. However, in regions with quickly developing sea, ther predicted wave height increases intermittently by up to about 1 meter.