Applications - Surface Based Remote Sensing of the Atmospheric Boundary Layer - page 134

Doppler LIDARs, including HRDL and other systems with greater range, have
proven capabilities to reveal the relationship between wind flow variability and ter-
rain. For example, localized nocturnal flows emanating from valleys or canyons
emptying onto a plain were studied in Colorado (Banta et al. 1995 , 1996 ;Levinson
and Banta 1995 ; Darby et al. 1999 ) and near Salt Lake City, Utah (Banta et al. 2004 ;
Darby et al. 2006 ); the structure of flow within Alpine valleys was documented
by Drobinski et al. ( 2001 ; 2003a , b ); windstorm-type flow structure was noted in
Colorado (Clark et al. 1994 ) and to the north of the Alps in Austria (Flamant et al.
2002 ; Weissmann et al. 2004 ); and a recurrent nocturnal LLJ structure was found
in the valley of the Great Salt Lake, Utah (Banta et al. 2004 ). The use of Doppler
LIDAR scan data, e.g. for wind energy for site assessment can replace extensive
arrays of surface tower networks.
A typical phenomenon of flows over hills is the speed-up of the flow over the
hill crest and a decrease of the flow speed on the downwind side of the hill. Under
appropriate thermal conditions the flow can even detach from the downstream side
of a hill and re-circulations can form there. SODAR measurements in Fig. 4.11
(right) demonstrate that the major part of the wind speed increase is limited to lower
heights (about 50 m above the hill top). Above this height, the wind speed increase
is much lower than over flat terrain (Fig. 4.11 left).
Larger north-south oriented valleys in east-west oriented mountain chains such as
the European Alps are often influenced by foehn flows when the large-scale synoptic
wind direction is nearly perpendicular to the mountain chain. On the northern side of
the Alps, the Rhine valley between Chur (Switzerland) and the Rhine's estuary into
Lake Constance near Bregenz (Austria) is such a valley. The interaction of the foehn
flow with the usual valley wind systems is a complicated phenomenon. In order to
get deeper insight into this interaction, the project FORM (Foehn in the Rhine Valley
during MAP) was conducted in this valley in 1999. MAP was the Mesoscale Alpine
Programme. An overview of FORM and some Doppler wind LIDAR measurements
from this valley are given in Drobinski et al. ( 2007 ). Aerosol layers in the valley
air have been monitored continuously with almost no interruption during the entire
field phase of FORM with a backscatter LIDAR (Frioud et al. 2004 ). Vertical wind
and temperature profiles were observed from five Doppler SODARs and two wind
profiling RADARs (one equipped with a radio acoustic sounding system, RASS).
They contributed to validate high-resolution numerical simulations of the com-
plex interaction between the foehn and the valley flow systems (Vogt and Jaubert
2004 ).
4.5.5 Drainage and Katabatic Flows
Drainage and katabatic flows are purely thermally generated orographic flow fea-
tures in a mountain boundary layer. Long-wave radiative loss to space leads to
cooling of land, snow, and ice surfaces and a compensating downward sensible heat
flux, which cools the atmospheric surface layer and forms a temperature inversion.
In the presence of slopes this induces a horizontal temperature gradient equivalent