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VII Waves

6.6 ECMWF Post-Processing

Although, the ECMWF version of WAM is basically following the same structure as the original version, there are also important differences to be noted. In particular, our version takes full advantage of grib coding and decoding both for the integrated parameters and the two dimensional spectrum. The advantages of grib coding are that the fields are archived in a platform independent form and that the size of the fields reduces by a considerable factor. For example the size of an integrated parameter field reduces by a factor of 3, while the size of a spectral fields reduces by a factor of 9. The large reduction in the size of spectral fields is accomplished by archiving the logarithm of the spectrum, thereby reducing the range of the values considerably. Furthermore, rather than archiving one spectrum per grid point, which would result in spectral fields of a large size, ECMWF archives a particular frequency-direction bin as one global field. Thus, the global spectral field is splitted up in fields, where is the number of frequencies and is the number of directions of the spectrum.

Table 6.1 Archived parameters of the ECMWF wave forecasting system.

Code figure	MARS abbreviation	Field	Units
221	MP1	Mean wave period from 1st moment	s
221	MP2	Mean wave period from 2nd moment	s
222	WDW	Wave spectral directional width	-
223	P1WW	Mean wave period from 1st moment of wind waves	s
224	P2WW	Mean wave period from 2nd moment of wind waves	s
225	DWWW	Wave spectral directional width of wind waves	-
226	P1PS	Mean wave period from 1st moment of swell	s
227	P2PS	Mean wave period from 2nd moment of swell	s
228	DWPS	Wave spectral directional width of swell	-
229	SWH	Significan wave height	m
230	MWD	Mean wave direction	º
231	PP1D	Peak period of 1d spectra	s
232	MWP	Mean wave period	s
233	CDWW	Coefficient of drag with waves	-
234	SHWW	Significant heightof wind waves	m
235	MDWW	Mean direction of wind waves	º
236	MPWW	Mean period of wind waves	s
237	SHPS	Significant height of swell	m
238	MDPS	Mean direction of swell	º
239	MPPS	Mean period of swell	s
244	MSQS	Mean square slope	-
245	WIND	10 m wind speed modified by wave model	m/s
246	AWH	Gridded altimeter wave height	m
247	ACWH	Gridded corrected altimeter wave height	m
248	ARRC	Gridded altimeter range relative correction	m
251	2DFD	2-D wave spectra	m2s/rad

Because IO is relatively slow it is advantageous to minimise the amount of IO. This is accomplished at initial time by transferring grib coded information from disk to one of the PE's and by transfering the initial data to one of the other PE's where it is decoded. Next the decoded data is is distributed over all the other PE's. Since spectral data have been splitted up, the reading of the initial conditions may be performed in a balanced manner. To that end the spectral file is read on PE 1, who distributes the fields per frequency and direction to all other PE's where it is decoded. Writing output is accomplished in a balanced manner by collecting on the first PE the data for the first field from all other PE's, by coding it and by transfering it to disk, while at the same time the second PE is doing the same task for the second field etc.

Finally, the information written to disk is temporarily stored in a sophisticated Fields Data Base (FDB), where it is picked up by archiving tasks that store the information in the MARS archive. The full list of products that is being archived is given in Table 6.1. Post-processing may now be accomplished in various manners. One way is by running programs that read and plot analysed and forecast wave parameters. A more popular method nowadays is to do post-processing in interactive mode using METVIEW.