Applications - Surface Based Remote Sensing of the Atmospheric Boundary Layer - page 93

Table 4.2 Overview of methods to derive vertical profiles of atmospheric variables using ground-
based remote sensing (see rightmost column for section number)

Variable Detection method Section

Wind speed, direction, turbulence SODAR 4.3.1.1

Wind speed, direction, turbulence Wind LIDAR 4.3.1.2

Wind speed, direction, turbulence RADAR windprofiler 4.3.1.3

Temperature RASS 4.3.2.1

Temperature Raman LIDAR 4.3.2.2

Temperature Radiometer 4.3.2.3

Temperature Interferometer 4.3.2.4

Humidity DIAL 4.3.3.1

Humidity Raman LIDAR 4.3.3.2

Humidity Radiometer 4.3.3.3

Humidity Interferometer 4.3.3.4

Rain Micro rain RADAR 4.3.3.5

Trace gases Vertical profiling 4.3.4.1

Trace gases Horizontal path-averaging 4.3.4.2

4.3.1 Wind and Turbulence

Initially, remote sensing of the wind in the atmospheric boundary layer was mainly
done with acoustic methods (SODAR), because the speed of sound which is only
one to two orders of magnitude larger than the wind speed leads to relatively large
Doppler shifts that are easily detectable (Neff and Coulter 1986 ). Additionally, this
method offers a high vertical resolution (5-10 m) and a small lowest range gate
(10-30 m), which makes it suitable for investigating wind profiles in shallower
boundary layers as well. The technical and electronic obstacles to overcome for
the application of the acoustic method are considerably lower and cheaper than for
the respective optical and electromagnetic methods. The advantages are neutralized
by the limited data availability of the acoustic method. In fact, none of the remote-
sensing methods (SODAR, RADAR windprofiler, wind-LIDAR) can guarantee a
one hundred percent data availability for a given height due to the backscattering
properties of the atmosphere. The strongest limitation by atmospheric conditions
with respect to wind measurements is found with the weather RADAR technique
that delivers reasonable data only in case of rain, although under limited conditions
signals from insects or index-of-refraction turbulence have been used in clear-air
boundary-layer studies. A SODAR fails in situations with missing turbulence and
low temperature gradients. These situations lead quite often to a low range of
a SODAR around dawn and dusk (Maughan et al. 1982 ; Emeis et al. 2004 ). A
RADAR windprofiler usually suffers from very dry air masses only, and the LIDAR
technique is hampered by strong rain, fog, and clouds.
If all three components of the wind have to be measured by sounding methods,
then successive velocity-component profiles have to be made in at least three differ-
ent directions shortly after each other. The wind components can subsequently be
computed from the radial velocities by applying trigonometric relations. SODARs