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

Chapter 6. Cloud flatness occurrence 61
discussed in Section 5.1. The lack of events in the equatorial latitude band from 20 ◦ N
to 20 ◦ S, especially over the ocean areas, arise not only from the general cloud free
fraction, but also from the general circulation of rising air masses due to the Hadley cell
circulation. Consequently several cloud flatness events located at the equator occur at
altitudes above 6 km. The few events in the Gulf of Mexico, the Caribbean Sea, the
Mediterranean Sea, the Indian Ocean and the equatorial part of the Pacific Ocean imply
a highly inhomogeneous cloud height distribution. Opposite to the elevated events at
the equator the accumulation of events at around 4 km above Antarctica and Greenland
result from the topographic elevation of approximately 2 km of the underlying land
masses. Whereas, the accumulation of events at around 4 km altitude above East China
might be originated in air masses being trapped and elevated due to the Chinese High
Plateau in the west. To derive seasonal variations (see Figure 27) the differences of
all four trimesters results were computed and analyzed (see Appendix A for detailed
results). Interestingly, seasonal differences only occur for events located above land,
while maritime events remain unaffected by seasonal effects.
April-September
October-March
Figure 27: A global relief world map with data from NOAA (http://www.ngdc.
noaa.gov/mgg/global/) and indicated seasonal cloud flatness events. Yellow boxes
indicate events occuring mainly between April to September, and red boxes indicate
events occuring mainly between October to March.
6.3 Low maritime clouds
As already mentioned, the fundamental cloud forming process over ocean regions differs
from cloud forming processes over continents. In general four cloud forming processes
can be distinguished. While the main process of cloud formation is always originated
in saturated air masses that are cooled to their dew or frost point this process can be