Geoscience Reference
In-Depth Information
occur in the number of detected CTH. The LITE CTH tends to place clouds at higher
altitudes than ISCCP. The CPDF retrieved from LITE profiles reaches 95% at an alti-
tude close to 16 km whereas that from ISCCP reaches the same value at an altitude
close to 11 km. It seems that detection of optically thin high cloud layers is problem-
atic for ISCCP. This leads to notable differences when comparing mean CTH retrieved
from ISCCP and LITE datasets. The mean altitude of the cloud layers is much lower
for ISCCP, more than 30% on average, as shown on Figure 6 and 7. The difference is
less marked in the southern hemisphere but is about 2 km under 20
°
S and increases
toward the northern latitudes.
A different way of comparing LITE and ISCCP CTH is given in Figure 8, which
divides the CTH into the cloud classes defi ned by ISCCP: high (H), middle (M), and
low (L) clouds. For the LITE CTH, each class is sub-divided considering the potential
overlap from the other cloud layers. Our goal is to use data from active instruments to
highlight the occurrence of cloud overlap.
Figure 8.
Distribution of each cloud class determined from LITE measurements following the
classification of ISCCP in term of high (H), middle (M), and low (L) clouds (left figure). For each class
the percentage corresponding to the multilayered clouds by the other cloud classes is given. The
bottom percentage gives the occurrence of the clear sky. The ISCCP-DX classification is given on
the right figure.
The largest difference between LITE and ISCCP statistics (14%) is observed in the
occurrence of the high clouds. Such a difference may result from two causes. First,
ISCCP may not detect all the high semitransparent clouds. In particular, situations
with optically thin cirrus clouds may be classifi ed by ISCCP as being clear sky. This
