Geoscience Reference
In-Depth Information
The relatively high frequency of occurrence of high cloud structures at latitudes
greater than 60° for the GLAS and CALIOP instrument may be related to the tighten-
ing of the orbit footprints for these latitudes. Some caution must be exercised with the
interpretation of high clouds in polar latitudes. There may be over-representation of
the high cloud structures of greater horizontal expansion.
CROSS-COMPARISON BETWEEN LIDAR AND PASSIVE SPACEBORNE
INSTRUMENTS
In operational cloud retrievals from passive radiometric measurements, the assump-
tion is made that only one cloud layer is present in a satellite pixel (i.e., Platnick et al.,
2003; Stubenrauch et al., 2006b). In the case of multilayered cloud structures, this
assumption will introduce biases in the determination of CTH, especially for the case
of cirrus overlying a lower-level water cloud. Fortunately, spaceborne lidar can pro-
vide insight to the vertical distribution of clouds. Moreover, the determination of the
CTH is a direct measurement, and the multilayer cloud distribution can be assessed as
long as the lidar signal does not attenuate in the uppermost cloud structure.
We now compare the CTH distributions given by passive and active instruments.
The coherence between the approaches to retrieve the CTH from spaceborne lidar
measurements has been demonstrated. With the increased vertical resolution of the
LITE lidar and its better SNR, these data have been retained for the comparisons with
passive instruments.
From use of the surface echo for LITE measurements, we have separated the con-
tribution of high semitransparent clouds from that of optically thick clouds. The result
is presented in Table 2 for the latitude intervals that have been discussed earlier. We
note that the occurrence of semitransparent clouds approaches 70% for all latitude
bands.
Table 2.
Proportion of optically thin clouds compared to the total cover of high clouds for LITE,
MODIS (Chang and Li, 2005), and TOVS PATH-B (Stubenrauch et al., 2006b).
Instrument
All latitudes
[-60 -20]
[-20 +30]
[+30 +60]
LITE
70.6%
71.3%
72.4%
69.4%
ISCCP
87.2%
84.62%
87.6%
86.0%
MODIS
73.8%
72.2%
64%
80.5%
VS PATH-B
91.9%
90%
94.7%
89. 1%
Comparison to ISCCP Database
A first comparison between the CPDF of the CTH is given in Figure 4b. Only the
ISCCP measurements within the footprint of the LITE orbit have been considered. As
specified before, only the highest cloud structure has been considered. For clouds at
altitudes of less than 3 km (i.e., low level clouds), the ISCCP and the LITE CPDF are
very similar. The ISCCP CPDF shows a cumulative probability of about 47% of cloud
detected at up to 3 km height, which is in agreement with of the surface observation
analysis by Warren et al. (1985). For clouds over 3 km height, significant differences
