For several decades, routine observations of the global cloud distributions have been made by using passive radiometers on operational meteorological and earth observation satellites. (for example, Advanced Very High Resolution Radiometer (AVHRR) data from NOAA satellites, TIROS Operational Vertical Sounder (TOVS) aboard TIROS satellites, and the Moderate Resolution Imaging Spectroradiometer (MODIS) aboard the Terra (EOS AM) and Aqua (EOS PM) satellites). While providing continuous and widespread coverage of global cloud distribution, radiometer observations generally lack sensitivity to optically thin clouds and produce ambiguous results in discerning multiple cloud layers.

The Geoscience Laser Altimeter System (GLAS), launched into orbit in January, 2003 aboard the freeflying Ice, Cloud, and Land Elevation Satellite (ICESAT), provides a significant new observation to evaluate and improve global cloud observation. GLAS is a dual channel surface elevation altimeter and atmospheric lidar. Intended to provide continuous observations for a 3-5 year time span, technical problems have truncated useful lifetime and so, GLAS is only operated during discreet time intervals. Nevertheless, during its periods of operation, GLAS provides continuous and nearly pole to pole atmospheric lidar observations of the global cloud pattern. As a lidar, GLAS is sensitive to optically rarified atmospheric layers and is capable of detecting multiple layers if the higher layers are optically thin.

The routine GLAS product sets include cloud and aerosol layer heights and layer optical depths. We have used these products to garner statistical characterizations of global cloud and aerosol layer occurrence distributions, multiple layer distributions, layer altitudes, and layer optical depths. The data give accurate histograms of occurrences of clear skies and single and multiple layers on a global basis and as a function of region. From GLAS data products, we show global and regional mapping of average cloud top heights. We show results of statistical studies designed to address limits of sampling requirements for meaningful results. Comparisons with results from satellite radiometer studies are made. Similar statistics from different times and places are compared and contrasted . We demonstrate that observations from GLAS and other future spaceborne lidar significantly enhance knowledge of the global distribution atmospheric scattering layers.