Applications - Surface Based Remote Sensing of the Atmospheric Boundary Layer - page 136

shape of the low-level katabatic jet with wind speed. On average, stronger kata-
batic flows (wind speed maxima of typically 8-10 m s − 1 ) are deeper with a jet
maximum at 20-60 m altitude; moderate katabatic flows (wind speed maxima of

typically 4-8 m/s) are shallower with a jet maximum more often between 3 and 30
m. The strongest katabatic flows have a greater cross-slope component, consistent
with a greater Coriolis force and frictional drag. For all katabatic wind profiles, there
is a backing in wind direction with height over the lowest

∼

100 m, consistent with a
frictional forcing at the surface. During summer there is a clear diurnal signature in
the katabatic flow at all heights. It is more pronounced at 20 m and above, where the
flow often ceases entirely, whereas at the surface there always appears to be some
weak (2-4 m/s) katabatic flow. Such cessation events may lead to katabatic jumps,
analogous to hydraulic jumps, as the flow changes from supercritical to subcritical
down the slope (Renfrew and Anderson 2006 ).
Earlier observations of katabatic winds in Antarctica and Greenland with
SODARs had already been made by Argentini and Mastrantonio ( 1994 ), Meesters
et al. ( 1997 ) who also used a RASS, and Bromwich and Liu ( 1996 ). Wave struc-
tures in Antarctic katabatic flows have been analyzed from SODAR echograms by
Kouznetsov ( 2009 ).
SODAR instruments can also be used to determine the seeing, a variable impor-
tant for optical astronomers (Bonner et al. 2008 ). For such a purpose a SNODAR
(see Chapter 3 ) has been deployed to Antarctica to monitor the height and variability
of the Antarctic ABL up to heights of about 100 m above ground with 5 m vertical
resolution.

∼

4.6 Conclusions on the Applicability of Ground-Based Remote
Sensing for ABL Research and Monitoring

The examples for probing the atmospheric boundary layer by ground-based remote
sensing gathered in this chapter have shown the impressive progress which has been
made since the last review by Wilczak et al. ( 1996 ). This progress is at least as
substantial as the progress stated by Wilczak et al. ( 1996 ) in comparison to the
earlier and probably first review of this subject by Derr and Little ( 1970 ). The short
outlook in the next chapter will show that the development of ground-based remote
sensing is still ongoing.
Acoustic remote sensing with SODAR, which had been - together with
RADAR - the pioneering technology for boundary layer profiling, is still frequently
used for the detection of wind and turbulence profiles and for monitoring mixing
layer heights. The limited height range of the SODAR technology (which neverthe-
less is nearly always sufficient for the detection of shallow nocturnal mixing layers),
together with possible acoustic disturbances to near-by residents due to the audible
signals, are the two largest deficiencies of this otherwise cheap and robust sounding
method.