Open Ocean Observation of Sea Surface Height with ICESat-2

ICESat-2 is unique among ocean altimeters, especially as to the range of spatial scales it samples. Its Advanced Topographic Laser Altimeter System (ATLAS) is a photon counting lidar pulsing at 10kHz. Every 70-cm of along-track distance, ATLAS illuminates an area about 10-m across. From the time of flight of individual surface reflected photons, we calculate the height of the ocean surface (ICESat-2 data in ATL03). The footprint of each height measurement is essentially infinitesimal. For the typically low reflectance of the ocean surface, we get about one surface reflected photon per laser pulse, so our observations are point measurements of height positioned with an order 10-m position uncertainty every 70-cm along-track. Consequently, rather than thinking in terms of measuring surface height over a large footprint by retracking a waveform, we assemble the photon heights to make histograms of sea surface height(SSH),which we then correct for the uncertainty due to instrument impulse response, and for which we derive the first four moments of SSH. In parallel, we use 10-m along-track bin averages of photon heights as the high-resolution representation of the wave-covered surface and correlate this with photon return rate to make a priori estimates of the electromagnetic (EM) sea state bias. This presentation will explore the range of ICESat-2 open ocean observations including the resolution of surface waves, the inherent uncertainty measuring average sea surface height over a wave covered surface, subsurface photon returns, and comparisons with CryoSat-2 and sea ice ICESat-2 measurements.