Icesat-2 Atmospheric Channel Description, Data Processing And First Results

The Advanced Topographic Laser Altimeter System (ATLAS) was launched aboard the Ice Cloud and land-Elevation Satellite-2 (ICESat-2) satellite in September 2018. ATLAS is a single wavelength (532 nm) lidar system designed to acquire high resolution measurements of the earth's surface while also obtaining atmospheric backscatter from molecules, clouds, and aerosols. Because ATLAS is optimized for altimetry, the atmospheric data acquired is unique in many respects and requires non-standard analysis techniques. For example, the high repetition rate laser limits the vertical extent of the profiles to just 14 km and causes atmospheric scattering from above 15 km to be added to the scattering in the lower 0-14 km profile. In addition, the limited vertical range of the acquired profiles renders it difficult to compute the magnitude of the solar background and hinders the application of standard calibration techniques. Despite these limitations, methods have been developed to successfully produce data products that have value to the atmospheric community for cloud and aerosol research and are currently available at the National Snow and Ice Data Center (NSIDC). In this paper we describe the ICESat-2 atmospheric channel and the methods used to process the ATLAS raw photon count data to obtain calibrated backscatter and higher level products such as layer heights and type, blowing snow, column optical depth and apparent surface reflectance.Plain Language Summary ICESat-2 is a polar orbiting satellite equipped with a high repetition rate laser that fires green pulses of light to earth 10,000 times per second. The main objective of ICESat-2 is the high-resolution measurement of the height of the earth's surface, with emphasis on the change in elevation of ice sheets that cover most of Greenland and Antarctica. In addition to surface elevation, ICESat-2 also obtains information on the vertical structure of the atmosphere including the height and thickness of clouds and aerosols. The atmospheric measurements are important for climate studies and because they extend the data record begun by other earth-orbiting satellite lidars like Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observations (CALIPSO) which has been acquiring atmospheric data since 2006 and is nearing the end of its life. The creation of a long record of cloud and aerosol observations is very important for detecting changes that may be occurring due to humanity's influence on the climate system.