Geoscience Reference
In-Depth Information
Only the GLAS and CALIOP measurements provide data at latitudes higher than 60°
because the inclination of the LITE orbit is only 57°.
Between 60° and 82
N, the mean CTH tends to decrease linearly from 6 to 4 km
(5-4.5 km) height from CALIPSO (GLAS) measurements. There is no signifi cant dif-
ference between the two spaceborne lidar observations. On the contrary, between 60°
and 82
°
S, the mean CTH tends to increase from 4 to 6.3 km height for GLAS measure-
ments, and from 5.1 to 8.6 km height for CALIPSO measurements. Such a difference
may be due to the presence of PSC. It is interesting to note that Hopfner et al. (2007)
showed a stronger occurrence of PSC in 2006 compared to 2003 using MIPAS/Envisat
(Michelson Interferometer for Passive Atmospheric Sounding) instrument (Burkert et
al., 1983).
Winker et al. (2007) demonstrated the capabilities of CALIOP measurements to
provide high resolution vertical profi les to a latitude of 82°S. Winker et al. (2007)
showed that clouds observed over the East Antarctic plateau in the middle of the
Antarctic Winter were relatively tenuous PSC extending up to ~25 km. Pitts et al.
(2007) analyzed such PSC structures from the CALIPSO database and document the
occurrence of extensive PSC over large regions of Antarctica throughout the 2006
Austral winter. They show that the 2006 season is very similar to the cold 1987 sea-
son, with a higher probability of occurrence under 16 km in September. Furthermore,
in a personal communication, C. David, PI of the LOANA (Lidar Ozone and Aerosol
for NDSC in Antarctica) ozone lidar, based at the French NDSC Station of Dumont
d'Urville, confi rms that there was a high occurrence of PSCs in 2006. The infl uence of
PSC on the CTH will be discussed again in a later.
°
Distributions as Functions of Latitude and CTH
Figure 7 shows the two-dimensional histograms as a function of latitude and altitude
of the CTH. The occurrences are given for the LITE, GLAS, and CALIPSO mis-
sions, respectively. They show strong similarities but with a noisier pattern for the
LITE-derived CTH. However, this variability may be caused by a sampling issue,
with LITE providing a lower number of observations compared to GLAS and CALIOP
measurements.
In the following discussion, low-level clouds are defi ned as having cloud-top pres-
sures ranging from 1,000 to 680 hPa; middle-level and high-level clouds ranging re-
spectively from 440 to 680 hPa and 440 to 50 hPa.
For the three cases, a low cloud pattern is observed between 0 and 2 km with a
maximum in the frequency of occurrence at a height close to 1.5 km. A minimum in
the frequency of low cloud occurrences is observed near the location of the ITCZ
due to the presence of high optically dense clouds that mask the potential presence of
low-and mid-level cloud structures. For all cases a strong occurrence of high clouds is
clearly highlighted between 13 and 18 km height in the ITCZ region, that is between
[10
N], corresponding to regions of deep convection, and the occurrence of cu-
mulonimbus and cirrus clouds. Generally, one fi nds higher occurrences of cloud near
the tropopause, in particular for the northern middle latitudes and the polar latitudes
(>60°). The altitude of the maximum cloud occurrence decreases with increasing latitude.
°
S; 20
°
