Developing a Global Gridded Vegetation Product for ICESat-2

While the Earth's climate is often represented by changes in average temperature, there are many feedback processes that exhibit strong influences on the full climate system, including coverage of ice at the poles, oceanic circulation, biomass distribution, cloud coverage and other atmospheric phenomena. Because the poles are the least regularly observed region on the globe yet are vital for understanding the current and future climate, a new satellite known as the Ice, Cloud, and Land-Elevation Satellite-2 (ICESat-2) is scheduled for launch in September 2018 with the goal of providing frequent high-resolution observations of polar ice and its changes over various timescales. Several of the benefits of this satellite is that it also provides information regarding atmospheric cloud and aerosol layers, ocean wave conditions, and vegetation distribution.

In this study, we focus on the estimation of vegetation coverage and height, and the development of a global gridded product of these characteristics. Because the gridding process requires at least one year of ICESat-2 data to develop, we will primarily provide a description of the methodology and preliminary assessments of the gridding process. Specifically, we examine the effects of cloud and aerosol layers on the retrieval of surface returns and the impact of the off-nadir pointing of the onboard lidar. The region near Sonoma County, CA was chosen as a preliminary study area because of an existing high-resolution lidar dataset that can be used to test changes to grid size, data quality, and effects of cloud and aerosol layers. With respect to the latter, cloud and aerosol data collected by the Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observation (CALIPSO) satellite between June 2006 and March 2018 were used to determine the annual and seasonal cloud effect on the ability to retrieve information at the surface. Examining CALIPSO's estimation of the optical depth (τ), regions closer to the coast are significantly more impacted by cloud cover than locations further from the coast, mainly due to the existence of persistent low-level stratiform clouds that form over the cold ocean waters. Based on these initial results, a discussion of the uncertainty limitations, the expected reasonable grid size, and potential uses of the gridded dataset are provided.