Geoscience Reference
In-Depth Information
5.6 Coastal Effects
When the larger-scale winds blow from the land to the sea, then an internal
boundary layer (IBL) forms over the sea surface (see
Sect. 3.5
for an introduction
to internal boundary layers and the description of wind profiles in and above these
IBLs). If warmer air flows over colder water, then the thermal stratification is
stable and the internal boundary layer is growing slowly in depth and can persist
over distances of more than 50 kilometres. Aircraft measurements in a stable IBL
over the Irish Sea are presented in Rogers et al. (
1995
). These measurements show
profiles of mean quantities and spectra. The spatial development of stable IBLs is
described, e.g., by Mulhearn (
1981
) and Garratt (
1987
). They give for the height
h(x) of the stably stratified IBL:
r
DT
T
x
g
n
h
ð
x
Þ¼
cu
ð
5
:
20
Þ
where x is the distance to the coast, T is air temperature, DT is the surface air
temperature difference between land and sea and g is gravity. Mulhearn (
1981
) gives
c = 0.0146 and n =-0.47 while Garratt (
1987
) gives c = 0.014 and n =-0.5.
If colder air flows over warmer water, the internal boundary layer grows rapidly
in depth and is finally merged into the marine boundary layer after some tens of
kilometres. In such offshore flows in coastal regions we can observe the usual
diurnal changes in atmospheric temperature, stability and winds, which are well-
known from flow over land and which have been described in
Chap. 3
. Thus, the
statements in
Sect. 5.1.6
are not valid in coastal regions with offshore winds.
5.6.1 Land and Sea Winds
There are local wind systems which do not emerge from large-scale pressure
differences but from regional or local differences in thermal properties of the
Earth's surface. These local or regional wind systems often exhibit a large regu-
larity and have a sufficient depth so that they can be used for the energy generation
from the wind. See Atkinson (
1981
) for an overview on thermally induced
circulations.
Due to the different thermal inertia of land and sea surfaces, secondary circu-
lation systems—land-sea wind systems—can form at the shores of oceans and
larger lakes which modify the ABL structure. Under clear-sky conditions and low
to moderate winds, land surfaces become cooler than the adjacent water surface
due to long-wave emittance at night and they become warmer than the water
surface due to the absorption of short-wave irradiance during daytime. As a
consequence, rising motion occurs over the warmer and sinking motion over the
cooler surfaces. A flow from the cool surface towards the warm surface develops
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