Cloud Cover and Aerosol in the Arctic and Antarctic from the Geoscience Laser Altimeter System

Starting in 2003 the Geoscience Laser Altimeter System (GLAS) provides the first global measurements of clouds and aerosol by satellite lidar. From the GLAS data the presence and height of all clouds and significant haze are detected, and in the case of transmissive layers, the thickness is also found. Since bright surfaces and low surface temperature limit the study of polar cloud cover and haze by passive techniques, GLAS measurements of cloud and aerosol layer presence, height and thickness are an advance in capability and accuracy. Initial analysis of results for 2003 data show there are two basic polar cloud types profiled: stratus below 3 km and cirrus form clouds with cloud top altitude and thickness tending at 12 km and 1.3 km. For Antarctica zonal average cloud fraction varies from over 93% for ocean and coastal regions to a consistent average of 40% over the East Antarctic plateau and 60-90% over West Antarctica. Comparisons have been made between GLAS data products and MODIS. Differences between the zonal average GLAS and MODIS cloud fractions are as much as 40% over Antarctica and in the Arctic. MODIS and other passive data show increased aerosol loading over the ocean north of Antarctica and a similar increase is seen in GLAS data. For GLAS surface ice sheet measurements, polar clouds and aerosol are an important factor for the accuracy of the altitude measurements of ice sheet elevation change. Multiple scattering through even the thinnest clouds introduces an effective pulse spreading and altitude bias. Pulse spreading up to 10's 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. Operational problems of the instrument however will severely limit the quantity of quality data. The current status of accuracy recovery due to correction of cloud scattering effects is addressed.