Model Simulation ofTemporal Variability of Suspended Matter in the North Sea
57
station in Fig. IS (stations marked as crosses) and (ii) at each of the 477 horizontal grid cells covering the model domain of the southern North Sea in Fig. 16.
The difference between the two figures is: in Fig. 15 the std. dev. factors outside
the stations must be inter- or extrapolated in order to obtain a contour plot,
while in Fig. 16 there is already one computed std. dev. factor for each of the 477
horizontal grid cells, inter- or extrapolation is not necessary.
The comparison of Figs. 15 and 16 shows that two areas with high std. dev.
factors are not realized by the NSP network of stations: (i) along the Dutch and
the German coast and (ii) in the central southern North Sea above the Oyster
Grounds. The recording of the near-coast "details" was beyond the scope of the
NERC North Sea Project. The non-consideration of the Oyster Grounds patch
with std. dev. factors> 2.5 in the central southern North Sea, however, is a problem. It leads to the conclusion that, with regard to the temporal variation of SPM
concentrations, the spatial arrangement of NSP stations does not yield a representative picture for the whole southern North Sea. It is "bad luck" that the patch
is located in just that area where there is a "hole" in the network of NSP stations.
It must be kept in mind that the std. dev. factor is a measure for the relative variation of the SPM concentration. The high standard deviations of the Oyster Grounds
patch are due to (i) a low basic level of SPM concentrations during calm weather and
(ii) a comparatively intense increase of the SPM concentration during storms because of the high mud content in the Oyster Grounds sediment (see Fig. 2).
5.3
Conclusion for the Measured Data
The above considerations and conclusions are valid for the computed std. dev.
factors. Regarding the approximate agreement of the measured and the computed distributions of std. dev. factors shown in Figs. 13 and 14, the above conclusions are assumed to hold also for the measured (i.e. the true) std. dev. factors.
An additional remark concerning the std. dev. factors shown in Fig. 16: the
SPM concentration used for the determination of this plotted distribution has a
time frequency of 18 values per day (time step: 80 min). The spatial mean of the
std. dev. factors in Fig. 16 is 2.37. If the 18 SPM concentrations per day are averaged, and the std. dev. factors are determined from these daily averages, the horizontal mean is 2.29. The small difference of the two horizontal means shows that
the SPM variation within a day (i.e. due to tidal currents and due to the random
number of particles within a horizontal grid cell) plays an insignificant role. The
dominant reason for the temporal variability of SPM concentrations is the succession of storm events and calm periods.
6
Meteorological Conditions During the NSP Period
This final section answers the question after the representativity of weather conditions during the single months of the NSP period August 1988-0ctober 1989:
57
station in Fig. IS (stations marked as crosses) and (ii) at each of the 477 horizontal grid cells covering the model domain of the southern North Sea in Fig. 16.
The difference between the two figures is: in Fig. 15 the std. dev. factors outside
the stations must be inter- or extrapolated in order to obtain a contour plot,
while in Fig. 16 there is already one computed std. dev. factor for each of the 477
horizontal grid cells, inter- or extrapolation is not necessary.
The comparison of Figs. 15 and 16 shows that two areas with high std. dev.
factors are not realized by the NSP network of stations: (i) along the Dutch and
the German coast and (ii) in the central southern North Sea above the Oyster
Grounds. The recording of the near-coast "details" was beyond the scope of the
NERC North Sea Project. The non-consideration of the Oyster Grounds patch
with std. dev. factors> 2.5 in the central southern North Sea, however, is a problem. It leads to the conclusion that, with regard to the temporal variation of SPM
concentrations, the spatial arrangement of NSP stations does not yield a representative picture for the whole southern North Sea. It is "bad luck" that the patch
is located in just that area where there is a "hole" in the network of NSP stations.
It must be kept in mind that the std. dev. factor is a measure for the relative variation of the SPM concentration. The high standard deviations of the Oyster Grounds
patch are due to (i) a low basic level of SPM concentrations during calm weather and
(ii) a comparatively intense increase of the SPM concentration during storms because of the high mud content in the Oyster Grounds sediment (see Fig. 2).
5.3
Conclusion for the Measured Data
The above considerations and conclusions are valid for the computed std. dev.
factors. Regarding the approximate agreement of the measured and the computed distributions of std. dev. factors shown in Figs. 13 and 14, the above conclusions are assumed to hold also for the measured (i.e. the true) std. dev. factors.
An additional remark concerning the std. dev. factors shown in Fig. 16: the
SPM concentration used for the determination of this plotted distribution has a
time frequency of 18 values per day (time step: 80 min). The spatial mean of the
std. dev. factors in Fig. 16 is 2.37. If the 18 SPM concentrations per day are averaged, and the std. dev. factors are determined from these daily averages, the horizontal mean is 2.29. The small difference of the two horizontal means shows that
the SPM variation within a day (i.e. due to tidal currents and due to the random
number of particles within a horizontal grid cell) plays an insignificant role. The
dominant reason for the temporal variability of SPM concentrations is the succession of storm events and calm periods.
6
Meteorological Conditions During the NSP Period
This final section answers the question after the representativity of weather conditions during the single months of the NSP period August 1988-0ctober 1989: