Basics of Renewable Energy Supply - Renewable Energy: Technology, Economics and Environment - page 54

The roughness of the soil is determined by vegetation and land development.
Above surfaces with a low level of roughness (e.g. water surfaces) the wind ve-
locity increases very quickly with increasing altitude in the lower 10 % of the
planetary boundary layer. Under these conditions the thickness of the planetary
boundary layer shows low values. In contrast, above areas with a high level of
roughness (e.g. settlements) the wind velocity of the free atmosphere is reached at
higher altitudes; the vertical increase of the wind velocity above ground is slower
in this case. Thus the roughness of the ground is a measure for the rate of increase
of the wind velocity in the vertical direction above ground. It is generally de-
scribed by the term roughness length. Table 2.2 shows some typical values.

Table 2.2 Exemplary roughness lengths of different surfaces /2-16/
Type of ground cover Roughness length in cm

Smooth surface
Snowy surface
Sandy surface
Grassland (depending on vegetation)
Corn fields
Forests and cities

0.002
0.01 - 0.1
0.1 - 1.0
0.1 -10
5 - 50
50 - 300

Besides the roughness length, the thermal stratification also has an impact on
the vertical change in wind velocity in the planetary boundary layer. If, for exam-
ple, the vertical temperature decrease is in the range of 0.98 K/100 m, this is
called a (dry-)adiabatic temperature gradient. The atmosphere is then layered neu-
trally. In this case the thermal stratification of the atmosphere has no influence on
the vertical wind profile. If, however, the vertical temperature gradient is smaller
than the adiabatic gradient, stratification is stable. The wind velocity increases
more quickly with increasing altitude above ground under these conditions. In the
case of unstable stratification (i.e. a larger vertical temperature gradient compared
to the adiabatic case) the wind velocity increase is smaller with increasing alti-
tudes.
The stability of thermal stratification varies with the approaching air mass, but
also, within such an air mass, during the course of a day. Above the sea, however,
no diurnal course of layer stability worth mentioning can be observed, as the high
specific heat capacity of water in connection with the turbulent heat transfer
within the water lead to very little change of the water surface temperature during
the course of a day. In the course of a year, on the other hand, due to the delayed
change in the water surface temperature, in spring a tendency towards stable, and
in late autumn a tendency towards unstable conditions can be observed. In com-
parison, above land areas, the diurnal course of the stratification stability is much
more pronounced at times of strong incident solar radiation.
However, at higher wind speeds a neutral stratification can generally be as-
sumed, as deviations from the neutral condition are less pronounced due to the
turbulent mixing of atmospheric layers. In the lower velocity segment including