International Polar Year 2007–2008 - WMO

of the aerosol component in the atmosphere over the
Atlantic from 60° N to the Antarctic coast (Kotlyakov
et al., 2010). A variable, called a spectral aerosol
optical thickness (AOT) of the atmosphere, is used
to characterize attenuation of the solar radiation by
the aerosol particles within the whole air column.
Magnitudes of the aerosol attenuation of the solar
radiation measured in the Antarctic were the lowest
values on the Earth, and they did not exceed limits
of their natural variability. This is again the evidence
of the fact that still to the present time the Antarctic
atmosphere is not polluted by any aerosol of the
anthropogenic origin.
ORACLE-O3 (no. 99)
LOLITA-PSC and MATCH-PSC campaigns
As part of the ORACLE-O3 (“Ozone layer and UV RAdiation
in a changing CLimate Evaluated during IPY”)
global project, LOLITA-PSC (“Lagrangian Observations
with Lidar Investigations and Trajectories in Antarctica,
of PSC”) is devoted to Polar Stratospheric Clouds
(PSC) studies. Understanding the formation and evolution
of PSC particles is an important issue to quantify
the impact of climate changes on their frequency of
formation and, further, on chlorine activation and subsequent
ozone depletion. Statistical studies on PSC
and temperature over the Dumont D’Urville in Antarctica
have been updated (David et al., 2009) and a study
based on the “Match” method, developed initially for
ozonesondes, has been applied, for the first time, to
lidar observations of PSC acquired during campaigns.
These campaigns took place in Antarctica during winters
2006, 2007 and 2008, involving the three PSC lidar
deployed in Antarctica, at Dumont d’Urville (66.67°S,
140.01°E), Davis (68.00°S, 78.50°E) and McMurdo
(77.86°S, 166.48°E) and CALIPSO space-borne lidar observations.
Observations were performed at each lidar
station when the weather conditions permitted. Tendays
forward trajectories calculations from any station
are performed each time a PSC is detected at the station.
We consider a match when a trajectory issued
from a station passes less than 200 km of another lidar
station during a PSC observation period and when potential
vorticity variations remain less than 40% along
the trajectory. From the ground-based lidars, the evolution
of scattering ratio can be drawn along the trajectories,
completed with the CALIPSO values selected
with a maximum time difference of 2.5 minutes and a
maximum time distance of 200 km from the trajectories.
As expected, a clear correlation appears between
high scattering ratio values and the coldest temperatures,
close or below the ice formation temperature
[see Fig. 2.1-5, pers. comm. Nadège Montoux, LATMOS
(Laboratoire atmosphères, Milieux Observations Spatiales),
DNRS, France].
The impact of the model for trajectory and of the
initialisation fields on the match determination was
explored (Montoux et al., 2009 and publication in
preparation). For cold temperatures, of interest for
PSC formation, the pressure and altitude discrepancies
are not significant. Time difference could occasionally
impact, but do not seem to affect greatly,
the lidar scattering ratios extracted. Yet, when close
to PSC temperature thresholds, the temperature differences
are a key issue and more realistic values for
nitric acid and water vapour mixing ratios are needed
to determine these thresholds (using, for instance,
the Microwave Limb Sounder onboard the AURA satellite).
The current step of the analysis is the modelling
of PSC formation along the trajectories using the
Danish Meteorological Institute microphysical box
model (Larsen et al., 2000). The model includes microphysical
Mie and T-Matrix modules, together with
optical modules, and is able to simulate the size dis-
Fig. 2.1-5. Evolution
of the temperature
(left) and of the lidar
scattering ratio at
532 nm (right) along
different trajectories
(color code) started
from Davis station at
0300 UTC 2 August
2007 to 0300 UTC 12
August 2007.
(Courtesy: Nadège Montoux)
s C I e n C e P r o g r a m 143