Applications - Surface Based Remote Sensing of the Atmospheric Boundary Layer - page 113

for all range gates in one selected minute from measurements in April 1998. The
spectral peak in the lower range gates is at about 6 m s − 1 . At heights above 1100 m
(the melting level at this time), the spectral peak is shifted to 2 m s − 1 and becomes
fairly narrow which is typical for snowfall. The corresponding profiles of RADAR
reflectivity, mean fall velocity, and rain rate are shown in Fig. 4.26b -d. The melt-
ing level appears in these profiles as enhanced reflectivity, as step in fall velocity,
and as apparently enhanced rain rate, respectively. The last feature is an artefact, as
the retrieval is only applicable for the liquid phase. Nevertheless, it can be used as
sensitive indicator for the melting level, even when it is not detectable in the reflec-
tivity profile. These profiles, showing the melting level, may be used to identify the
state of precipitation, rain, or snow. This information could be helpful also for the
interpretation of weather RADAR data (Peters et al. 2002 ).
By assuming a relationship between drop size and the vertical velocity, the
drop size distribution can be derived. A detailed error analysis has been per-
formed by Peters et al. ( 2005 ) on a theoretical basis and by comparison with
surface disdrometers. It was found that the deviations of the micro rain RADAR
drop size distributions from the in situ drop size distributions are in the same
range as the mutual deviations of the in situ drop size distributions (Crewell et al.
2008 ).
4.3.4 Trace Substances
Remote sensing of other trace substances than water vapour can be done along ver-
tically and horizontally oriented paths in the atmospheric boundary layer. Only
optical techniques are available for these gases and aerosols. Usually, LIDARs
such as DIAL, Raman LIDARs, and ceilometers are operated in a vertical profil-
ing mode while FTIR and DOAS are employed for horizontally path-averaged trace
gas measurements.
Remote sensing of trace substances may be based on three methods, either the
detection of radiation absorption, emitted radiation, or by scattered radiation. The
wavelength-dependent absorption of radiation by trace gases may be utilized to mea-
sure the concentration of trace gases in the atmosphere. Depending on the physical
properties of the gas molecules (i.e. the typical energy amounts these molecules can
absorb by electronic transitions, excitations, and increased vibrational and/or rota-
tional energy), different spectral ranges must be analyzed to find these gases. For
the detection of trace gases, analyses are made either at one single absorption line
or over a wider range of spectra. For investigations concentrating on one absorp-
tion line, interference from other trace gases can become important and should be
known. On the other hand, the spectral signature or fingerprint of a trace gas in a
wider spectral range is rather unique. For spectral methods splitting of incoming
radiation can be achieved either by a grating or prism spectrometer, or by in an
interferometer.
In the UV range, we find, e.g. the absorption bands of ozone molecules (Hartley
bands between 200 and 300 nm, a Huggins band at 340 nm, see Malicet et al. 1995 ).