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Table 5.3 Mean of the scale parameter of the Weibull distribution A in m/s for all seasons at 90
and 40 m and their vertical difference in m/s
Seasons
90 m
40 m
Difference
SON (autumn)
10.68
10.12
0.56
DJF (winter)
12.36
11.57
0.79
MAM (spring)
10.27
9.23
1.04
JJA (summer)
8.55
8.03
0.52
Fig. 5.33 Schematic
diagram of the seasonal
variation of the correlation of
the two Weibull parameters
A and k in a marine boundary
layer. The colouring is the
same as in Fig. 5.32 . (From
Bilstein and Emeis 2010 )
parameter 0 : 01\ oA = oz \0 : 02 s -1 (see Table 5.3 ) is not as big as in a similar
height over land 0 : 02\ oA = oz \0 : 04 s -1 , (see, e.g., Emeis 2001 ). The offshore
shape parameter profile does not show a maximum as the onshore shape parameter
profile does, but decreases monotonically with height.
Furthermore, the two Weibull parameters show a clear seasonal dependence.
Smaller parameters are detected in summer, higher parameters in winter, while
spring and autumn are between both extremes. This is explained by the annual
variation of thermal stability in the marine boundary layer. Due to the enhanced
heat capacity of water, the stability of the marine atmosphere is out of phase by
about 3 months compared to the stability of the atmosphere over land (Coelingh
et al. 1996 ). Consequently the atmosphere is stable in spring, neutral/stable in
summer, unstable in autumn and neutral/unstable in winter.
On the whole those seasons, which are stable or neutral/stable have parameters
smaller in magnitude compared with unstable seasons. It is noted, that shape
parameters with larger scale parameters (autumn and winter) have a higher vari-
ability compared with ones with smaller scale parameters (spring and summer).
Figure 5.33 shows a schematic diagram of this correlation, where intervals for
each season and parameter are presented.
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