2D Wave Spectra

Wave Related Data Sets

The quantity that is actually computed at each grid point is the two-dimensional wave energy spectrum and the total surface stress for a given forcing by surface (10 metre) winds. The two-dimensional spectrum describes how the mean sea-surface elevation variance due to ocean waves is distributed as a function of frequency and propagation direction. The energy spectrum is discretised over a number of frequencies and directions.

The number of frequencies and directions is:

Operational model
- Global wave model
  - 26 January 2010 to present uses 36 frequencies and 36 directions
  - 20 November 2000 (time 1800 UTC) to 26 January 2010 used 30 frequencies and 24 directions
  - 29 June 1998 to 20 November 2000 (time 1200 UTC) used 25 frequencies and 12 directions
- Limited area wave model
  - 13 October 2009 to present uses 36 frequencies and 36 directions
  - 20 November 2000 (time 1800 UTC) to 13 October 2009 used 30 frequencies and 24 directions
  - 26 October 1998 to 20 November 2000 (time 1200 UTC) used 25 frequencies and 24 directions
ECMWF 40 year Re-analysis (ERA-40)
- Global wave model
  - used 25 frequencies and 12 directions
ECMWF Interim Re-analysis (ERA Interim)
- Global wave model
  - used 30 frequencies and 24 directions

The first direction is 5 degrees for 36 directions, 7.5 degrees for 24 directions and 15 degrees for 12 directions. A direction of 0 degrees means a wave propagating towards the North (Oceanographic convention). Note that this is the opposite of fields such as mean wave direction which follow the atmospheric convention.

The frequencies are geometrically distributed with a step of 1.1 (fr (n) = fr (n-1) * 1.1). The first frequency bin varies with the archive and date as given below:

Operational model
- Global model
  - From 21 November 2000 the first frequency was 0.0345 Hz
  - Prior to 20 November 2000 the first frequency was 0.0418 Hz
- Limited area wave model
  - From the 18 May 2011to present the first frequency has been 0.0345 Hz
  - From 3 June 2008 to 17 May 2011 the first frequency was 0.0418 Hz
  - From 20 November 2000 to 2 June 2008 was 0.0345 Hz
  - Prior to 20 November 2000 the first frequency was 0.0418 Hz
ECMWF 40 year Re-analysis (ERA-40)
- Global model
  - The first frequency was 0.0418 Hz
ECMWF Interim Re-analysis (ERA Interim)
- Global model
  - The first frequency was 0.0345 Hz

Note that information about the frequencies and directions is part of the GRIB header.

Data format

The logarithm (base 10) of the 2D spectra are encoded in GRIB 1, therefore once the values are decoded, they need to be transformed to their original values by using 10**(decoded values). Note that small values (below approximately 10**-4) were not encoded so the smallest values should be about -4. The values that were not encoded at a point are stored as missing data for that point. This does mean that missing data is used both for all land points and sea data with energy less than the chosen threshold.

Each spectral component (for a frequency and direction) is encoded as a separate field and hence at retrieval, one needs to request all frequencies and all directions and then re-assemble the different spectral components to get the actual 2D spectrum.

FREQUENCY=all, DIRECTION=all, can be used to retrieve all frequencies and directions without having to know in advance how many there are.

Interpolation

There is no `interpolation' for these fields. Instead if interpolation is requested then the values at any point are those from the nearest archived grid-point.

23.08.2011