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This page provides an explanation of the main topics related to wave forecast that at a first sight may appear tough to the majority of users.

What does UTC mean?

UTC stands for Coordinated Universal Time. For weather and ocean forecast purposes, we can say that it is equivalent to the Greenwich Mean Time (GMT). Time zones around the world are expressed using offsets from UTC. This offset must be added to the UTC to obtain the local time of a time zone. For instance, to obtain local time in Rome, we must add one hour in winter and two hours in summer (due to the daylight saving time) to the UTC.

What does the Iribarren number represent?

The Iribarren number $\xi_0$ is one of the most important parameters in coastal oceanography. It relates the geometrical characteristics of the profile to those of the incoming waves. It can be thought as the slope of the dimensionless profile. The dimensionless profile can be obtained by diving the vertical and horizontal profile distances $z$ and $x$ by characteristic lengths of the wave field. The characteristic vertical length of the wave field is represented by $H_s$ . The characteristic horizontal length is $\sqrt{H_sL_0}$ . In mathematical terms, we have the slope of the profile $\beta$ : $\beta=\frac{z}{x},$ and the slope of the dimensionless profile $\xi_0$ : $\xi_0=\frac{z/H_s}{x/\sqrt{H_sL_0}}=\frac{\beta}{\sqrt{H_s/L_0}}.$ The Iribarren number reminds us that everything is relative in this world. In particular it tells us that long waves on mild beaches behave in the same way as short waves on steep beaches. A widely used wave breaking classification operates with three breaker types that are defined based on Iribarren number ranges:

Spilling breaking occurs for $\xi_b$ <0.4
Plunging breaking occurs for 0.4< $\xi_b$ <1.5
Surging breaking occurs for $\xi_b$ >1.5

where the subscript $b$ indicates that the significant wave height at the breaking point is used in the Iribarren number computation.

How does WW3 compute the significant wave height?

WaveWatch3 is a phase-averaged model and is thus unable to compute the significant wave height $H_s$ from individual waves. What WaveWatch3 actually computes is the zeroth-order moment wave height $H_{m0}$ : $H_{m0}=4\sqrt{\int \int S(f,\theta) \, df \, d\theta,}$ where $S(f,\theta)$ is the directional wave spectrum, $f$ is the frequency and $\theta$ is the propagation direction. Under the assumptions of narrow banded wave spectrum and that the statistical distribution of wave heights is well described by a Rayleigh distribution, it follows that the discrepancy between the significant wave height and the zeroth-order moment wave height is negligible: $H_s \sim H_{m0}.$ We are not going too much into details here but, for the sake of simplicity, we can say that the more a sea state is represented by a clean swell outside the wave generation area, the more the two mentioned assumptions are valid.