Fróðskaparrit - 01.01.2007, Page 145

SAMANBERING AV ALDUFORSØGNUM FRÁ ECMWF VIÐ ALDUMÁTINGAR
GJØRDAR Á FØROYALEIÐINI
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lution was increased so it now is roughly
equivalent to 25 km horizontal resolution,
and the associated coupled global wave
model resolution was increased to 40 km.
This new model configuration has been
found to be better able at predicting intense
storms (Untch et al., 2006). Moreover, ad-
ditional wave observations fronr the radar
altimeter onboard Jason-1 and from ASAR
on board ENVISAT were added to the
wave model data assimilation resulting in
better wave model analysis and short range
forecasts (Abdalla et al., 2005). Overall, it
should therefore be expected that the pres-
ent implementation of the EW4 model
would generate analysis data that relate
even better to buoy data, than the analysis
values used in this investigation.
Planned wave model changes at
ECMWF in the near fiiture are, increased
model resolution in the limited area model,
and inclusion of more shallow water
physics (Jean-Raymond Bidlot at
ECMWF, pers. com., 2007).
Conclusions
Analysis values from four one-month peri-
ods, of the limited area wave model are
found to give satisfactory results when
compared to wave data from the Faroe
area. There is a general under-prediction,
especially of the wave height, at the peak
of all the extreme stonns. A seasonal varia-
tion is observed with larger negative bias
in the modelled wave height in the winter
months compared to the summer months.
This seasonal bias seems to be connected
to a similar bias in the forcing wind model.
The shape of the modelled spectra cor-
relates well with the observed spectra, but
the behaviour in the lower frequencies in
the model spectra are found to be slightly
smoother than the observed, and with ex-
amples of missing swell events.
It is observed that the predicted wave
periods have a consistent positive bias
compared to the measurements and that the
EW4 model has a tendency to under-pre-
dict the energy content in the high fre-
quency range of the spectrum.
Overall the model perfonnance is found
suitable to force a nested high resolution
wave model for the Faroese area. Due to
recent improvements in the EW4 model
formulation and wind forcing, it is ex-
pected that the model performance, espe-
cially in average wave period, would be
even better with the recent implementation
of the model.
In this investigation, only analysis val-
ues and not forecasted values are used for
the model validation, but a comparative
global test of five operational models (Bid-
lot et al., 2002) finds the ECMWF model
to have the lowest forecast error in wind
and waves. The ECMWF model is there-
fore not only a suitable candidate to force
high resolution local hindcasts, using
analysis data, but also a suited candidate to
force high resolution forecasts for the area.
Acknowledgements
This papcr is writtcn as a part of a PhD project, which
is funded by the Statoil-group. Special thanks to those
who have provided data: Landsverk (WV-1, WV-2,
WV-3 and F8), Sp/f Data Quality (WVD-4) and Met
Officc UK (K7). Commcnts and suggestions to this
papcr from Gerbrant van Vledder (Alkyon Hydraulic
Consultancy and Research) and Jean-Raymond Bidlot
(ECMWF) arc grcatly apprcciatcd.