Comparison of cloud detection methods for the Geoscience Laser Altimeter System

Cloud measurements are one of the major components of the Ice, Cloud, and land Elevation Satellite (ICESat) mission. The Geoscience Laser Altimeter System (GLAS) on board ICESat is equipped with two laser channels, the 532 nm channel and the 1064 nm channel, both of which can be used for cloud measurements. GLAS also measures the energy that comes back to the sensor as the result of surface reflection and hence gives apparent surface reflectance. For a given surface type, when the atmospheric column is clear, GLAS retrieves higher apparent reflectance; when it is cloudy, however, the reflectance becomes lower. Therefore, apparent surface reflectance can also be used as a cloud detection test. In this study we compare cloud detection results using the three different methods: cloud detection with the 532 nm channel, with the 1064 nm channel, and with surface apparent reflectance. We will present the performance of each method over different types of surface and for clouds with different optical depth. The pros and cons of each method will be discussed.