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return flow emerges in the opposite direction aloft in order to keep the mass bal-
ance. This leads to the well-known sea breeze during daytime and in the evening
and the land breeze at night and in the early morning (Fig.
2.9
). This common fea-
ture of a land breeze is the reason why in former times sailing ships left harbours
in the early morning and tended to return in the afternoon. This thermally driven
secondary motion system can be several hundreds of metres deep. The sea breeze
front propagates inland during the day and is - if enough moisture is available in the
air - often marked by a chain of cumulus clouds.
2.6 Mountainous Terrain
The presence of hills and mountains lead to much larger horizontal inhomogeneities
in the ABL than all that has been presented so far in this chapter. Larger mountains
can even have a larger vertical extension than the depth of the ABL (Fig.
2.10
).
Thus, the applicability of eqs (
2.1
)to(
2.43
) can only be expected in small limited
parts of the mountain boundary layer over smaller hills or in wide valleys with flat
floor. The differences to the boundary layers introduced above come due to both
mechanical and thermal forcing. While the mechanical forcing such as channelling
of flows in valleys and large-scale blocking by mountain chains are quite obvious,
the thermal forcing is more difficult to understand. The thermal forcing is a mixture
of the presence of elevated heating (or cooling at night) surfaces and the reduced
Fig. 2.10
Some features of the atmospheric boundary layer in mountainous terrain
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