Glossary - Surface Based Remote Sensing of the Atmospheric Boundary Layer - page 159

wavelength is reduced) if the scattering air volume moves towards the instrument
and it is reduced if the air moves away.

Dual-Doppler technique Making wind speed measurements with two Doppler
RADARs or Doppler LIDARs within the same air volume from two different posi-
tions is called Dual-Doppler technique. While a single instrument delivers the radial
wind component along the line-of-sight only, two instruments allow for the determi-
nation of both horizontal wind components simultaneously without involving areal
averages.

Echogram Time-height cross-sections of the acoustic backscatter intensity mea-
sured from SODAR are often called echograms or facsimile plots.

Elastic scattering Elastic scattering is Mie or Rayleigh scattering. Apart from the
Doppler shift, the wavelength of the backscattered signal is equal to the emitted
wavelength. See also “inelastic scattering”.

Facsimile plot see “echogram”.

Fixed echoes Often, when the emitted beam is not well focused (side lobes are
present), backscatter from solid objects away from the path of the emitted beam can
disturb the analysis of the received signal. The term has been coined for acoustic
remote sensing. Fixed echoes are seen as enhanced backscatter intensities in certain
range intervals, which correspond to the distance of the object causing the fixed
echoes. If wind speeds are analyzed from acoustic backscatter, fixed echoes lead
to too low wind speeds. In RADAR meteorology the term “ground clutter” is used
frequently.

Fluorescence Fluorescence happens when incoming radiation of a certain fre-
quency excites an atom or molecule and brings it into a resonance state. When the
atom or molecule returns to its ground state, light of another specific frequency
is emitted. As fluorescence is obstructed by collisions of the air molecules among
each other, fluorescence methods can only be employed for the detection of trace
substances in the upper atmosphere above about 15 km, but not in the boundary
layer. Fluorescence is often much more intense than Raman scattering.
Frequency-modulated continuous-wave Continuous-wave RADARs cannot
determine the range from the travel time, i.e. the time delay between emitted
and received signal. Beam focusing is not applicable either. Therefore, frequency-
modulated continuous-wave RADARs (FMCW-RADAR) emit a signal with con-
stantly changing frequency. The range of the scattering object in the line of sight
can then be determined from the frequency difference between the emitted and the
received signal.

Groundclutter see “fixed echoes”.

Heterodyne see “coherent”.

Imaging Imaging is the process of measuring so as to build an extensive two-
dimensional description of the received properties; comparable to a classical