Applications - Surface Based Remote Sensing of the Atmospheric Boundary Layer - page 95

Fig. 4.11 Vertical profiles of the Weibull scale factor in m s − 1 (which is proportional to the mean
wind speed) for flat terrain ( left ) and for a hill top ( right ). Here, SODAR measurements are com-
pared to power law profiles with different exponents ( thin full lines ), the wind resource assessment
model WAsP ( dashed double-dotted line , Troen and Petersen 1989 ) and a parameterization ( full
bold line ) suggested in Emeis ( 2001 )
varies with height on clear days. It is only for the lower heights below about 60-80 m
above ground that we have low wind speeds at night and slightly higher wind speeds
during daytime. Above 60-80 m above ground this behaviour is reversed: there are
lower wind speeds during daytime and higher wind speeds at night. This behaviour
is a consequence of the diurnal variation of the thermal stratification and the vertical
turbulent mixing during cloudless days, on which radiative fluxes are dominating.
Solar radiation is heating the ground during the day and thermal outgoing radia-
tion is cooling it again during the night, leading to stable thermal stratification at
night and neutral to unstable stratification during the day. In both cases, the changes
around sun rise and sun set start near the ground and then propagate into higher
layers of the boundary layer. This explains why the wind speed in the lowest height
(see Fig. 4.12 ) increases shortly after sun rise before decreasing again later in the
morning. In these hours the unstable well-mixed surface layer is only about 100-
150 m deep and the two wind speeds at 40 and 100 m mix to a nearly uniform value.
Later when the well-mixed convective surface layer grows deeper, the wind speed in
all heights is decelerated by the surface friction and is only slowly increasing during
the day due to the existing large-scale pressure gradient. Around sun set a new sta-
ble surface layer forms near the ground. The layers above nearly immediately loose
contact to the frictional forces at the ground and speed up. This is the very mecha-
nism which also leads to the formation of the nocturnal low-level jets (see Section
4.5.1 ).
A likewise diurnal variation can be observed for the atmospheric turbulence on
such days. Figure 4.13 displays the diurnal variation of the standard deviation of the
vertical wind component for the same case depicted in Fig. 4.12 . Lower turbulence
is present during nights in a stable nocturnal boundary layer and higher turbulence
during daytime hours in a convective boundary layer. Quite noticeable is the increase
of turbulence with height in the convective boundary layer during the day, which
follows the behaviour given in eq. (2.35) . The slight increase of turbulence with
height in the early morning hours is due to the wind shear at the lower side of the