Applications - Surface Based Remote Sensing of the Atmospheric Boundary Layer - page 103

presence of turbulent patches. Data from the high-resolution LIDAR HRDL have
proven useful in probing the behaviour of many of these kinds of atmospheric flow
features or events that produce turbulence. Breaking Kelvin-Helmholtz instabilities,
for example, were studied by Newsom and Banta ( 2003 ), Blumen et al. ( 2001 ), and
Poulos et al. ( 2002 ), and ducted gravity waves just above the LLJ maximum were
studied by Fritts et al. ( 2003 ). Other types of discontinuity, including those with
frontal or density current structure or larger-scale propagating gravity waves, were
investigated by Darby et al. ( 2002a ) and Sun et al. ( 2002 , 2004 ).
A possible solution for the problem of large measurement volumes due to conical
scanning with optical techniques may be the Dual-Doppler LIDAR technique. Here
two identical LIDARs look into the same air volume from two different positions.
Such an attempt using two 10.6 m LIDARs is described in Collier et al. ( 2005 ).

Newsom et al. ( 2005 ) used similar techniques to study boundary layer eddy struc-
ture near an urban centre. Both studies were directed at the improvement of urban
dispersion modelling.

μ

4.3.1.3 RADAR Windprofiler

UHF RADAR windprofilers operating at 915 or 1290 MHz can be used to monitor
boundary layer and lower troposphere wind profiles with a vertical resolution of
about 100 m. Boundary layer wind profile measurements together with extensive
discussion of this monitoring technology are, e.g. reported in Ecklund et al. ( 1988 )
or Carter et al. ( 1995 ).
Figure 4.18 shows a typical time-height cross section of the horizontal wind from
a UHF windprofiler (Caccia et al. 2004 ) for a layer between 400 and 3000 m above
ground for a three-day period. The windprofiler was situated at the Mediterranean
coast south of the French Alps. The figure displays an example for a mistral event
characterized by north-westerly to northerly winds at this site. The mistral blows
southward along the Rhone valley in Southern France through the gap between the
Massif Central and the Alps (see also Section 4.5.4 below). Maximum winds of
30 m s − 1 are observed on the first evening at 19.00 UTC. Wind profiles have been
obtained every 15 min, but only one out of eight profiles is shown in the figure.
Observations of the vertical component of atmospheric turbulence, i.e. the
variance of the vertical velocity component, have also been made with a newly
developed low-power X-band boundary layer RADAR operating at 9.42 GHz
(Al-Sakka et al. 2009 ).

4.3.2 Temperature

Air temperature is the most direct variable defining the static stability of the
atmosphere and indicating the presence of inversions. Boundary layer tempera-
ture profiles can be obtained with active measurement methods such as RASS and
Raman LIDAR. Additionally, passive methods using radiometer and interferometer
measurements are available. The following subchapters will show examples.