Geoscience Reference
In-Depth Information
To summarize the previous results, lidar-derived cloud statistics based on only the
uppermost cloud layer on each lidar shot from LITE, GLAS, and CALIOP are given
in Table 1. Cloud statistics are calculated for the latitude intervals defi ned by [60
°
S;
20
N]. These intervals were chosen from the two-
dimensional distributions of the CTH shown on Figure 6 and 7.
°
S], [20
°
S; 30
°
N], and [30
°
N; 60
°
Table 1.
Separation of lidar-based cloud rerievals into low, middle, and high cloud classes for LITE
(local method, September 10-19, 1994), GLAS (operational algorithm, last week of September and
first 2 weeks of October, 2003), and CALIOP (operational algorithm, September-October, 2006)
measurements as determined from the ISCCP classification against low, middle, and high clouds.
The bottom group gives the cloud cover ratio. The first column gives the global cloud statistics, and
the other columns the statistics on the three latitude intervals [
−
60°;
−
20
°
], [
−
20°; 30
°
], and [30°; 60
°
].
All latitude
[-60°; -20°]
[-20°; +30°]
[+30°; +60°]
LITE
52.4%
37.6%
57.4%
56.1%
High
GLAS
46.5%
47.5%
48.3%
42.0%
CALIOP
45.0%
33.9%
53.3%
48.6%
Middle
LITE
9.0%
10.2%
6.6%
12.4%
GLAS
18.8%
19.3%
18.3%
19.2%
CALIOP
13.9%
14.9%
10.9%
17.7%
Low
LITE
38.6%
52.2%
36.0%
3 1.5%
GLAS
34.7%
33.2%
33.4%
38.8%
CALIOP
41.1%
51.2%
35.8%
33.7%
Cover Ratio
LITE
69.8%
72.8%
72.5%
64.0%
GLAS
69.2%
76.5%
62.4%
70.5%
CALIOP
70.5%
78.6%
67.5%
64.52%
The global cloud fractions are similar for the three datasets with values of 69.8%,
69.2%, and 70.5% for LITE, GLAS, and CALIOP measurements, respectively. This
is somewhat surprising, as LITE should be able to detect more clouds due to its bet-
ter SNR. A strong occurrence of high altitude clouds is highlighted (52%, 46.5%, and
45% for LITE, GLAS, and CALIOP measurements, respectively) representing half
of the detected cloud structures. The LITE detects more high clouds than GLAS and
CALIPSO, probably due to its better sensitivity (SNR). Detected low cloud fraction is
comparable in the three datasets. However, middle-altitude clouds are more frequently
detected by GLAS and CALIPSO than by LITE (9.8 and 4.9%, respectively). Besides
a difference in atmospheric cloud structure due to inter-annual variability, this may be
also be attributed to some extent to the larger multiple scattering (MS) impact in LITE
measurements. The MS increases signal at altitudes below an upper cloud layer, and
may prevent the detection of a second lower cloud layer with our algorithm. Lower
level clouds tend to be composed of liquid water and are less perturbed by MS from
upper cloud layers due to their larger backscatter coeffi cient.
There is less cloud fraction variability between the tropical and middle latitudes.
However, we fi nd that the latitude interval between [60°S; 20°S] has a higher proportion
of low clouds that may exceed 50% although it is lower in the case of GLAS (~33%).
