Environmental Engineering Reference
In-Depth Information
Global air circulation systems can therefore only occur there. Below this free at-
mosphere is the planetary boundary layer that ends at the surface of the earth.
Even within this boundary layer, and thus close to the ground, air currents are
generated that are called local winds. Thermal up and down currents, land and sea
winds, mountain and valley winds belong to this category. Such air movements
are mostly generated in the same way. Ascending air masses are found on areas
heating up quickly due to the incident solar radiation, i.e. areas having a low heat
capacity (e.g. land) and descending air masses, however, are above the neighbour-
ing areas with a larger heat capacity (e.g. sea). During the day the wind blows
from the latter zones to the former zones (e.g. sea wind) and during the night this
process is reversed (e.g. land wind) /2-15/.
These local air circulation systems with a different force occur practically eve-
rywhere on earth. Due to their closeness to the surface of the earth they can be
utilised for wind power generation. Examples of the use of such land and sea
winds are some of the large wind parks in California (USA).
Free atmosphere
Free atmosphere
2,000
2,000
Free atmosphere
Free atmosphere
Free atmosphere
Free atmosphere
1,000
1,000
4.5
4.5
500
500
490
490
v Wi ~ h 0.4
v Wi ~ h 0.4
400
400
4.5
4.5
Planetary
boundary
layer
Planetary
boundary
layer
370
370
v Wi ~ h 0.28
v Wi ~ h 0.28
300
300
4.5
4.5
240
240
v Wi ~ h 0.16
v Wi ~ h 0.16
200
200
100
100
1.4
1.4
2.2
2.2
3.1
3.1
35
35
0
0
Large buildings
Large buildings
Trees
Trees
Houses
Houses
Large bodies of water and plains
Large bodies of water and plains
Fig. 2.25 Dependence of wind velocity v Wi on altitude h (see /2-7/)
Due to friction with the (rough) surface of the earth, the geostrophic wind, or
the air circulation due to local effects, is reduced within the planetary boundary
layer (therefore often also called friction layer) almost to a point of standstill at
direct proximity to the ground. The resulting vertical mean wind velocity profile
for selected surface conditions is described in Fig. 2.25. The vertical course of the
wind, and thus the altitude of the planetary boundary layer depend on the weather
conditions, the roughness of the soil and the character of the topography. The
thickness of the boundary layer varies between approximately 500 and 2,000 m
above ground.
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