Geoscience Reference
In-Depth Information
h
day
~1 km
ABL
ASL
0
0
h
night
ABL
~100 m
ASL
0
0
Wind speed
Potential
temperature
Turbulent
heat flux
Figure 3.1
Sketch of proiles of mean wind speed, mean potential temperature and
the turbulent sensible heat lux in the atmospheric boundary layer (ABL) with depth
h
. The atmospheric surface layer (ASL) constitutes the lower 10% of the ABL. Note
the order of magnitude difference in boundary-layer depth between the day time and
night time cases.
and the nonturbulent free troposphere. During the day the ABL is heated from below
and convection causes strong turbulent mixing, leading to more or less uniform pro-
iles of, for example, wind and potential temperature (see
Figure 3.1
showing ideal-
ized proiles and
Figure 3.2
showing a vertical cross section of the turbulent ields of
temperature, humidity and vertical wind speed). On the other hand, during night time
surface cooling stabilizes the ABL, leading to weak turbulence and large gradients.
The fact that turbulence is restricted to the ABL can be deduced from the fact that
the turbulent luxes decrease to zero at a certain level near the top of the ABL (see
Figure 3.1
). During daytime, convection in the ABL can be so strong that penetrat-
ing thermals cause an exchange of air between the free troposphere and the ABL
(entrainment). This entrainment is visible in
Figure 3.1
as a negative sensible heat
lux, which is due to the downward transport of warm air from the free troposphere.
In
Figure 3.2
the entrainment is visible as the inclusion of patches of warm and dry
air from the free troposphere into the ABL (e.g., at horizontal location 2 km). Above
the ABL, the atmosphere is mostly ignorant of the time of day (e.g., if the temperature
at 5 km height is -20 °C at night, it will remain so during daytime, unless large-scale
processes such as advection affect the temperature).
Although the entire ABL is linked to processes at Earth's surface, this chapter is
restricted to a description of processes close to the surface, roughly in the lower 10%
of the ABL, loosely named the atmospheric surface layer (ASL; see
Section 3.4.3
for
a more thorough discussion). This part of the ABL is characterized by large gradients
in temperature and wind speed. Furthermore, the turbulent luxes do not deviate much
from their values at the surface.
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