Applications - Surface Based Remote Sensing of the Atmospheric Boundary Layer - page 91

Fig. 4.9 Boundary layer height and layering from combined ceilometer and SODAR measure-
ments over Budapest, Hungary, on July 9, 2003. Black squares are from eq. ( 4.1 ), red asterisks are
from eq. ( 4.2 )and( 4.3 ), blue triangles are from eq. ( 4.10 ) (From Emeis and Schäfer 2006 )
with a depth of a few hundred metres can be detected underneath the black squares
derived from the SODAR soundings. The convective boundary layer during day-
time fills the full depth of the boundary layer. This combination offers the same
advantages as the combination of SODAR and wind profiler presented in Beyrich
and Görsdorf ( 1995 ). First results from a combined deployment of a RASS and a
ceilometer are given in Emeis et al. ( 2009 ).
In such combined remote sensing measurements a SODAR better detects the
near-surface features such as nocturnal stable layers (a RASS instead of a SODAR
directly delivers the near-surface temperature profile) while the ceilometer is able
to follow the diurnal variation of the daytime convective boundary layer and the top
of the whole boundary layer. The residual layer then becomes visible as the layer
between the new nocturnal surface layer below and the top of the boundary layer
above.
4.2.3 Clouds and Cloud Base Height
The detection of the base height of clouds at the top of the atmospheric boundary
layer is one of the simplest remote-sensing tasks to be executed. The oldest method,
a cloud searchlight, does not involve sounding but is based only on geometrical
calculations. For this bistatic method a strong searchlight produces a light fleck on
the lower edge of the cloud by a vertically directed light beam. An observer, who is
located in a known distance from the position of the searchlight, measures the angle