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Figure 9 shows also the mean CTH of all the clouds retrieved for southern polar
latitudes (between 70ºS and 90ºS). The mean CTH signifi cantly evolves during the
year with the higher values between June and October (~7 km at 80
S) and smaller
values between February and April (~5 km) at 80ºS. July is an intermediate situation
with a mean CTH close to 6 km. In December and January the detection of cloud struc-
tures is not effi cient due to the duration of the day. The SNR in lidar measurements for
optically thin clouds is lower due to the infl uence of sky radiance at 532 nm.
There are very few instruments able to detect PSC and very few existing studies
about the climatology of this type of optically thin cloud. In a previous study, David et al.
(1998) showed a percentage of about 70% of detected PSCs between 11 and 26 km
height. Hopfner et al. (2007) used MIPAS instrument analyses to investigate the inter-
annual occurrence of PSC. They show that PSC mainly occur from May to the end of
October when the stratospheric temperature is very cold, with a maximal occurrence
retrieved between mid-July to the end of August.
Hence, the increase of the mean CTH observed between GLAS and CALIOP
measurement periods (Figure 6) is likely due to a more important presence of PSC
during year 2006 than during year 2003.
°
CONCLUSIONS
To investigate the statistical distribution of the CTH, we use the observations from
three spaceborne lidar missions: LITE, GLAS, and CALIPSO. We developed a
methodology to infer the CTH from the lidar measurements. This methodology was
compared with the operational algorithms of GLAS and CALIOP missions and proved
to be quite powerful. One way of comparing the CTH from these lidars is through the
CPDF. Optically thin cirrus clouds are better identified from LITE profiles because
these measurements have a higher SNR than with the other lidars. The better SNR
is mainly due to the altitude of the shuttle (~240 km) compared to the GLAS and
CALIOP satellites (~590 and ~705 km, respectively).
Important variations are noted from a comparison of the CTH statistics from LITE
and those from ISCCP, although these comparisons cover a very short time period.
These differences are noted especially for the high clouds but also on the mean CTH.
Low clouds are well identifi ed by the two types of instrument (i.e., active and pas-
sive remote sensing sensors). We note that a similar comparison using MODIS cloud
products based on the CO
2
slicing method led to results more similar to those deduced
from the lidar measurements.
Natural causes of variability can be related in the tropical areas to the position of
the ITCZ and in the southern polar regions with the monthly and inter-annual cycles
of the PSC. In particular, important differences can be recorded from 1 year to another
as between September, 2003 and September, 2006 when the mean CTH increases from
6 to 9 km, respectively. The fi rst year of measurements was obtained from GLAS and
the second by CALIOP. The observed differences are not thought to be caused by dif-
ferences in the instruments but from changes in the atmosphere itself.
Such a study has highlighted that the use of spaceborne lidar observations per-
formed at the global scale is a potentially powerful way of assessing critical parameters
