Cloud Statistics from Calipso Lidar Data for the ... - espace-tum.de

Chapter 3. Methodology behind cloud free and cloud gap statistics 30
5km
MERGING
ALGORITHM
+8.22 km
1 km
2 km
7 km
0 0 0 0 0 0 1 1 1 0 0 0 0 0 0 0 0 0 1 1 1 1 1 1 0 0 0
0 0 0 0 0 0 2 2 2 1 1 1 1 1 1 1 1 1 2 2 2 1 1 1 0 0 0
Ground
Number Layers Found in 333 m
Number Layers Found in merged data set
Figure 10: The 333 m Number Layers Found array gets larger if a cloud layer is found
with z t ≥ 8.22 km and if the corresponding τ col ≥ 0.5.
The threshold was chosen in a way to guarantee an acceptable measurement precision
of the future MERLIN mission [Kiemle et al., 2011]. In addition, the cloud detection
algorithm works only correct for clouds with τ ≥ 0.45 [Karlsson and Johansson, 2013].
Therefore, optically thin aerosol layers, even though they can have τ > 0.5 are neglected
as their corret detection is questionable for a horizontal resolution of 333 m. However if
a single shot feature with τ > 0.5 has been detected it is most likely classified as a cloud
and therefore included in the merged data set. The same holds for optically thin clouds
below 8.2 km with 0.5 < τ < 1.0. If this condition is met, the previously cloud free
(Number Layers Found) entry in the 333 m product is raised by the amount of layers
found and from there on appears as cloudy (see Figure 10). The Column Optical Depth
Cloud 532 parameter is the sum of all detected optical depths of each feature detected in
the complete 5 km column. Due to the limited resolution of 5 km horizontally and 180
m vertically as well as to a different cloud retrieval methodology, the merging of both
data sets cannot consider any cloud gaps smaller than 5 km for columns where a cloud
above 8.2 km was added. This effect can bias the occurrence frequency of cloud gaps
as multiples of 5 km seem to appear more often (see Figure 21). But the benefit of the
single shot resolution up to an altitude of 8.22 km circumvents this drawback, especially
for the cloud flatness analysis and high resolution statistics. Analyzing the merged data
set (see Table 4) for a limited test period of one month reveals the composition of the
merged product. While the overall cloud free fraction diminishes from 52.1 % to 40.6
%, 11.5 % clouds are added from the 5 km product, and 3.7 % of the 5 km columns
previously described as being cloud free are now set to cloudy. The less transmissive
layers of the 5 km product occur both over land and ocean, whereas the previously cloud
free 5 km profiles occur mainly over land (e.g., Sahara, South China).