Fróðskaparrit - 01.01.2007, Page 133

SAMANBERING AV ALDUFORSØGNUM FRÁ ECMWF VIÐ ALDUMÁTINGAR
GJØRDAR Á FØROYALEIÐINI
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speed time series at F8 must be done with
caution as the effects of topography and
air/sea temperature gradients are expected
to be signifícant. The wind measured on
the K7 buoy is measured at 3 m height and
ought to be converted to 10 m height be-
fore it is compared to the modelled 10 m
wind. Inspection of the temperature gradi-
ent between air and sea reveals that the at-
mospheric stratification is predominantly
unstable in all the inspected events. In such
circumstances, it requires more informa-
tion than available to do a proper height
conversion. If the conditions had neutral
stability, the increase in the wind speed
from 3 m to 10 m height would be in the
region of 10-15% depending on the rough-
ness length of the sea. The K7 and F8 wind
speeds are therefore not used to derive sta-
tistical parameters from the performance
of the wind model, but only used for visual
inspection of possible general trends in the
predicted wind speed.
Events
The events chosen for this validation con-
tain the largest observed wave heights
recorded so far on the WVD-4 location,
and are long enough to give a general im-
pression of the EW4 model performance.
The meteorological circumstances leading
up to the diíferent stonns are linked to the
passages of one or several low pressures in
the NE Atlantic. The dynamics leading up
to the winter storms is quite complex and
the reader is referred to analyzed weather
charts (e.g. the NCEP reanalyzed weather
charts available at http://www.wetterzen-
trale.de/topkarten/fsreaeur.html). The rnain
observed characteristics of the peak storm
events are summarized in Table 3.
From Table 3 it is clear that the ob-
served wave fields at WVD-4 in the sum-
mer events (1 and 3) correspond quite well
to fully developed wind sea, whereas
swells play an important role in the winter
events (2 and 4).
Event 1 Event 2 Event 3 Event 4
Date 8/9 2000 2/2 2002 17/6 2002 15/1 2003
AP (hPa/hour) 36/50 at F8 29/36 at F8 12/6 at K7 28/12 at F8
AHmll (m/hour) 4.3-10.2/12 4.5-14.1/18 2.8-10.7/8 4.0-11.7/16
Max-wind (m/s) 19 (F8) 20 (F8), 27 (K7) 18 (K7) 8 (F8)
Dir-wind StoW W to S to SW SE to S SWtoN
Max-Hmo (m) 10.2 14.1 10.7 11.7
Max-Tt (s) 15.4 20.0 16.7 20.0
PM-T’{s) 16.0 18.8 16.4 17.1
PM-wind (m/s) 20.3 23.9 20.8 21.8
Table 3. Some general information related to the development of the peak storms in the four events. Date
gives the date of the largest observed wave height, AP gives the drop in atmospheric pressure, zl//m()gives the
increase in wave height from the beginning to the climax of the particular storm, Max-wind gives the
maximum observcd wind spced, Dir-wind gives the observed wind directions throughout thc event, Max-Hm0
and Max-T/t give the largest observed wave heights and peak periods at WVD-4, PM-T t and PM-wind give the
peak wave period and wind speed corresponding to the case where the observed wave height at WVD-4 is due
to fully developed wind sea.