Geoscience Reference
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Fig. 3.14 Balance of forces
(black arrows) in the daytime
convective boundary layer
(top) and above the nocturnal
stable boundary layer
(below). The disappearance
of the frictional force leads to
an increase in wind speed and
a turning in wind direction
(red arrows). ''L'' and ''H''
indicate minima and maxima
in surface pressure and thin
lines are surface isobars
non-vanishing horizontal synoptic pressure gradients. Therefore, nocturnal low-
level jets usually appear at the edges of high-pressure systems (see shaded area in
Fig. 3.15 ).
In the spatial domain this corresponds to a sudden transition of the flow from a
surface which is warmer than the air temperature to a smooth surface which is
colder than the air temperature. This may happen when the flow crosses the coast
line from warm land to a colder ocean surface or from bare land to snow or ice-
covered surfaces.
3.4.2.2 Frequency of Low Level Jets
It was mentioned in the preceding section that the occurrence of nocturnal low-
level jets depends on certain synoptic weather conditions. Therefore, it can be
expected that the frequency of occurrence is linked to the appearance of certain
weather or circulation types. For Central Europe the ''Grosswetterlagen'' (large-
scale weather types) have proven to give a good classification of the weather
situation (Gerstengarbe et al. 1999 ). Figure 3.16 shows the frequency of occur-
rence of low-level jets over Northern Germany as function of these 29 large-scale
weather types. The two most relevant types (the two left-most columns in
Fig. 3.16 ) are a high-pressure bridge over Central Europe (type ''BM'') and a high-
pressure area over the British Isles (type ''HB''). All in all a low-level jet appeared
in 23 % of all nights.
Figure 3.16 showed the frequency of occurrence of a low-level jet as function
of the weather type. The relevance of a certain weather type for the formation of a
low-level jet can be assessed when comparing the frequency of low-level jet
occurrence with the overall frequency of occurrence of the respective weather
type.
Figure 3.17
has
been
produced
by
dividing
the
frequencies
shown in
 
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