To compute the directional standard deviation of the ocean waves using ERA5 data, please have a look at the following document (from Jean Bidlot, ECMWF):
directional_width_parameter_in_ECWAM.pdf
Moreover, to complement the mean direction parameters, ERA5 data also provides the wave spectral directional width parameters for the total sea, wind waves and total swell
For more details, see section 10.3.5, of part VII of the ifs documentation:
https://www.ecmwf.int/en/publications/ifs-documentation
5 Comments
JH Chow
Hi, the link to the document is dead. May I know if I could find it elsewhere?
directional_width_parameter_in_ECWAM.pdf
Michela Giusti
Hi,
could you please try again?
Thanks
Michela
Jean Bidlot
Dear Colleagues,
why is the link not visible to the outside world?
Jean
Mohamed Ouda
Dear Colleagues,
Is there a way to convert the wave spectral directional width to directional standard deviation?
Best regards,
Mohamed
Jean Bidlot
Dear Mohamed,
In the document
directional_spread_in_ECWAM.pdf
(1) is the square of the directional standard deviation, as defined as the one sided directional width (make sure to scroll down in the document to find the copy of Holthuijsen's book).
I then show that the parameter we provide for the wave spectral directional width is the square root of (6), which is nothing else than
the weighted average in frequency space of (1)
Note that besides the 2d spectrum, all other wave parameters are integral parameters,
namely, they no longer depend on frequency and/or direction, hence why we decided to provide the directional width parameter for the total sea as weighted average.
If you need the directional standard deviation for each frequency as given in (1), then you will need to get the 2d wave spectra and compute (1) for each frequency.
Note that, as explained in the IFS documentation, part VII, chapter 10, we decided that for the windsea and total swell, we would instead use the peak frequency to compute (1).