The following new wave parameters have been introduced:
1) Atmospheric and ocean forcing
U-component of atmospheric surface momentum flux (UTAUA)
V-component of atmospheric surface momentum flux (VTAUA)
U-component of surface momentum flux into ocean (UTAUO)
V-component of surface momentum flux into ocean (VTAUO)
Wave turbulent energy flux into ocean (WPHIO)
These parameters are not entirely new, however, before they had to be derived/approximated from existing parameter.
The wave model is forced by stress equivalent 10m neutral winds. This forcing is archived as magnitude (WIND) and direction (DWI).
In turns, the wave model computes the sea state dependent drag coefficient, which is archived (CDWW).
The wave model also needs the surface air density (RHOAO) to determine the strength of the atmospheric momentum flux (atmospheric surface stress) that is used to generate the waves and drive the ocean circulation:
Namely, the magnitude of the surface stress is RHOAO*CDWW*WIND^2 and its direction is given by DWI (following the meteorological convention),
or 4 parameters to derive 2 (U- and V-component of atmospheric surface momentum flux (UTAUA, VTAUA) ) hence the reason for the new parameters (
Moreover, the wave model is able to determine the fraction of the momentum flux that is actually used to generate waves, and therefore is not available to drive the ocean circulation,
The model is also able to estimate the momentum flux that is released back into the oceans when the waves are breaking.
These two effects are combined to estimate the momentum flux that is actually contributing to driving the ocean circulation.
Until now, the ratio of the magnitude of the stress driving the ocean circulation to the atmospheric surface stress was output,
parameter "Normalized stress into ocean " (TAUOC),
but, in order to get the actual stress going into the ocean, one has to multiply TAUOC by the magnitude of the surface stress (RHOAO*CDWW*WIND^2)
and assume that its direction is given by the direction of the stress equivalent 10m neutral winds (DWI). This is an approximation and it is much better to direction output the
U- and V- components of surface momentum flux into ocean. (UTAUO, VTAUO)
Finally, the wave model also estimates the amount the turbulent energy that is dumped into the ocean as waves breaks. The quantity has been output (PHIOC) normalised by
RHOAO*u*^3, where u*^2 = CDWW*WIND^2
Again, it is much better to provide this quantity directly : Wave turbulent energy flux into ocean (WPHIO)
Note: whereas PHIOC is negative (i.e. leaving the waves), we decided to define WPHIO as positive (going into the ocean, W/m^2)). See plot below
For info, here is the corresponding sea state
2) Extension of the freak waves parameters
Wave induced mean sea level correction (WETA)
Ratio of wave angular and frequency width (WRAF)
Number of events in freak waves statistics (WNSLC)
Based on the recent development on Peter Janssen 's work on extending and revising ECMWF freak wave warning system
https://www.ecmwf.int/en/elibrary/18063-shallow-water-version-freak-wave-warning-system
and
Janssen, Peter A.E.M. and Augustus J.E.M. Janssen, 2019: Asymptotics for the long-time evolution of kurtosis of narrow-band ocean waves. J. Fluid Mech., 859, 790-818. doi:10.1017/jfm.2018.844
three new parameters were introduced. More details will be published in the IFS documentation (part VII)
When estimating the potential occurrence of larger than averaged waves, one needs to determine an estimate of Number of events in freak waves statistics, hence WNSLC.
Because, there is a subtle balance between how much the waves in a wave field are distributed in frequency and in direction, the ratio of wave angular and frequency width (WRAF) was introduced. It enters into the revised calculation of quantities such as the skewness and kurtosis of the sea surface elevation and ultimately into the estimation value of the largest single wave in the field (HMAX)
Finally, the non-linear wave-wave interactions that lead to freak waves also have an impact on the mean sea surface elevation, the new parameter "Wave induced mean sea level correction (WETA)" was introduced to quantify the impact on the sea surface elevation of those wave-wave interaction.