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

Chapter 5. Cloud free statistics 55
The anomalies indicated by high power law exponents above the Sahara, at the west
coast of North and South America around the latitudes of 30 ◦ S and 30 ◦ N, at the Arabian
Peninsula and above the Chinese High Plateau are characterized by a high cloud free
fraction and large cloud gaps (see Figure 22c and d). In general all power law fits
exceeding σ ≥ 0.1 ( 27.4 % of all 15023 grid points) have a low amount of individual
cloud gaps and deviate from a power law fit (see Appendix for the corresponding σ and
x min for each grid point). Consequently the closest approach to the global power law
fit is reached only for grid points having more than 1000 individual cloud gaps (34.2 %
of all grid points), with a mean cloud gap length not exceeding 15 km and preferably a
high cloud free fraction. Regardless of the threshold of σ ≥ 0.1, the median power law
exponent for all grid points is 1.65. This is based on the irregular global distribution of
individual cloud gaps which causes a strong deviation from a power law fit.
With a global mean cloud free fraction of 40.5 % and a global mean cloud gap length of
only 7.41 km, the dominance of small cloud gaps becomes evident. Only for the probable
cloud free regions detected in Section 5.1 and especially for the Sahara, the mean cloud
gap length increases up to 230 km. The median cloud gap length (see Figure 24) is
related to the mean cloud free fraction. The median is the numerical value separating
the higher half of the data from the lower half, and with only a few grid points exceeding
5 km the global dominance of small cloud gaps is confirmed. Interestingly, three latitude
bands can be found above the oceans with rising medians to the equator. The first band
from ±82 ◦ to ±50 ◦ with a median of 666 m, is replaced by a band from ±50 ◦ to ±20 ◦
and a median of 1 km. The equatorial band from ±20 ◦ to 0 ◦ is dominated by medians
up to 2 km.
60°N
30°N
2.0
1.8
1.6
1.4
0°
30°S
60°S
180° 135°W 90°W 45°W 0° 45°E 90°E 135°E
180°
1.2
1.0
0.8
0.6
0.4
Figure 24: Median of cloud gap lengths for the entire CALIPSO data set of 2007
computed for 2 ◦ x 2 ◦ grid points.