Global and polar cloud cover from the Geoscience Laser Altimeter System, observations and implications

The Geoscience Laser Altimeter System (GLAS) on board the Ice, Cloud and Land Elevation Satellite provides space-borne laser observations of atmospheric layers. The GLAS laser profiling permits unambiguous detection of clouds with high sensitivity and direct height profiling. Climatologies of clouds in the high latitudes in particular are compiled to more accurately than passive satellite observations where clouds and snow surfaces appear similar at both infrared and visible wavelengths. Since its launch in early 2003, GLAS has obtained a substantial sample of cloud observations. In addition to the detection of the coverage and height distribution of clouds, the GLAS data algorithms also derive the optical thickness and scattering cross sections for transmissive clouds. A substantial fraction of clouds are sufficiently transmissive such that a laser pulse from surface reflection is obtained. The accurate results for cloud coverage and distribution obtained from GLAS may be contrasted with previous climatologies and other satellite observations. Overall GLAS has detected about 70% average global cloud cover with 45% of the total being single layered. Results for many parameters agree well with MODIS and other passive cloud retrievals but other, especially night time and polar clouds, show significant discrepancies. Influence of clouds and aerosol is important to the accuracy of altitude measurements of polar ice sheet elevation change. Multiple scattering through even the thinnest clouds introduces an altitude bias due to pulse spreading. Pulse spreading up to 10 of meters is observed in data. The GLAS mission was designed so that cloud clearing and corrections would permit centimeter accuracies for surface elevation change. Results showing the adequacy the cloud detection and clearing for surface measurements are presented.