Geoscience Reference
In-Depth Information
2000
1500
Height (m)
1000
1800
500
1530
1300
0930
1100
0
296
298
300
302
304
306
308
310
Potential temperature(K)
Fig. 2.8
Example of profiles of potential temperature in and above the atmospheric boundary layer. The
approximate heights of the inversion overlying the boundary layer are indicated by arrows. The
measurements were made over gently rolling terrain (
z
0
=
0
.
45 m,
d
0
=
8
.
9m; see Asanuma
et al.
,
2000) by means of radiosoundings at the indicated times (Central Daylight Savings Time) on June 13,
1992, in the Washita River Basin in Oklahoma.
(see below), it coincides approximately with the region where the direction of the wind
remains constant with height; this absence of “turning” confirms that it is indeed the
region where the effect of the rotation of the Earth is of little consequence. The surface
sublayer is also sometimes assumed to be the layer where the vertical turbulent fluxes
do not change appreciably from their value at the surface, say less than 10%. Although
the ASL occupies the lower part of the turbulent boundary layer, it does not extend all
the way down to the surface. As illustrated in Figure 2.6, the height of the lower limit
of the ASL can be assumed to be of the order of 30
u
∗
in the case of smooth flow, and
of the order of 3 to 5
h
0
in the case of rough flow;
h
0
is the characteristic height of the
roughness obstacles.
In general, under non-neutral conditions the air flow and the momentum transport
are greatly affected by the transport of sensible heat and, to a lesser extent, water vapor,
and vice versa. However, in the lower part of the atmospheric surface layer it is found
that sensible heat and water vapor may be considered as merely passive admixtures,
and that the effects of the density stratification resulting from temperature and humidity
gradients are negligible. This lower region of the atmospheric surface layer is referred
ν/
