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Figure 6. Mean cloud top height against the altitude of the highest structure for LITE (local method
applied on September, 1994), GLAS (operational algorithm applied on the last week of September
and the first 2 weeks of October, 2003), and CALIOP (operational algorithm applied on September,
2006). The mean cloud top height given for both the global coverage of ISCCP data and the ISCCP
data under the LITE footprint on September, 1994 is also given.
Latitudinal Cloud Top Height Distributions
Mean Cloud Top Height
Figure 6 gives the mean CTH for the highest-level structure as a function of the lati-
tude for LITE, GLAS, and CALIOP. The acquisition periods are different. Ten days
are considered for the LITE mission on September, 1994. A full month (September,
2006) of CALIOP is analyzed, and for GLAS the time period encompasses the last
week of September and the first 2 weeks of October, 2003. The reduced number of
days for LITE mission may contribute to the greater variability of the data. No signifi-
cant difference in the shape was observed on GLAS data when splitting the data into
weekly sequences.
Between 60
N, there is good agreement between the observations of the
various missions, which is fortunate considering difference in the acquisition periods
and the potential for interannual variability to infl uence the results. The northern maxi-
mum near 10
°
S and 60
°
N corresponds to the position of the ITCZ (Inter Tropical Convergence
Zone) in September (Waliser et al., 1993). The ITCZ moves seasonally, following
the solar heating and the warmest surface temperatures: it moves toward the Southern
Hemisphere from September through February and reverses from March through
August.
Mean CTHs are calculated for clouds ranging between 6 and 11.5 km in the tropi-
cal latitudes. Minima are found near 20
°
N, corresponding to the de-
scending circulation of the Hadley cell. In the latitude range [−60°; −20
°
S and near 30
°
°
] and [30°;
60
°
] corresponding to the mid-latitudes, the mean CTH varies between 4.5 and 6.5 km.
 
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