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Scientific Report 2007-2009
Geophysics
G4. Atmospheric optical remote sensing in Polar regions
With climate problems becoming a critical issue, there
is an increasing need for better monitoring the real
changes affecting the atmosphere. In particular the Polar
zones are known to be among the most sensitive to
global changes.
The Group of Atmospheric Physics runs a lidar
system placed at Thule, Greenland, which is
part of the international Network for the Detection
of Atmospheric Composition Changes (NDACC)
(http://www.ndsc.ncep.noaa.gov/). The network is
composed of more than 70 high-quality, remote-sensing
research stations for observing and understanding the
physical and chemical state of the stratosphere and upper
troposphere and for assessing the impact of stratosphere
changes on the underlying troposphere and on
global climate. The lidar (LIDAR, LIght Detection And
Ranging), a radar-like instrument using a laser as radiation
source and an optical telescope as receiver, is able
to detect aerosol (by Mie scattering), minor constituents
like water vapor (by Raman scattering), and temperature
(by Rayleigh scattering) profiles from the ground
up to the mesosphere (approximately 70 km).
Figure 2: Atmospheric temperature profile at Thule measured
by Rayleigh scattering lidar. Blue broken line: climatological
temperature profile for January at latitude 75N.
Red: radiosounding.
Figure 1: Lidar system at Thule. The black case contains
the 800mm telescope pointing vertically; the Nd:YAG laser
(red case) is placed in the lower part of the structure.
The lidar system was constructed at the University of
Rome and installed in 1990 at Thule within a collaboration
with the Danish Meteorological Institute (Figure
1). The system uses several receiving channels, which
can be used to obtain vertical profiles of backscatter
cross-section and depolarization of atmospheric particulate
at two wavelengths (λ = 532nm and λ = 355nm).
The depolarization provides information on the physical
phase (solid or liquid) of the aerosol. In the absence of
aerosol and clouds, the lidar can provide temperature
profiles up to the mesosphere by molecular Rayleigh
scatter (Figure 2). In the last twenty years the system
has had the possibility to gather a wide statistics by
operating in very different atmospheric conditions,
such as during the stratospheric aerosol enhancement
after the Pinatubo eruption in 1991, during conditions
of extremely low temperatures and during Sudden
Stratospheric Warming conditions. Moreover due to the
high variability of the Polar vortex, Thule often passes
from within to without the vortex and vice-versa with
large temperature changes allowing the monitoring of
peculiar polar thermo-dynamical phenomena like the
ozone laminae [1]. Narrow band interference filters permit
the operation also in high background illumination
although only for aerosol profiles. Both aerosol and
temperature data are continuously downloaded into the
data base of NDACC. Presently the Thule station is run
in collaboration with ENEA and INGV researchers who
provide support and other complementary instrumentation.
References
1. G. Muscari et al., J. Geophys. Res., 112, D14304, (2007).
Authors
G. Fiocco, D. Fuà, M. Cacciani, A. di Sarra 8 , G. Muscari,
T. Di Iorio, L. Di Liberto, F. Angelini, O. Lanciano, V.
Ciardini, C. Tirelli, G. Casasanta, C. Di Biagio.
http://g24ux.phys.uniroma1.it/
Sapienza Università di Roma 169 Dipartimento di Fisica