ICESat-2 ATL12, Release 4: Ocean Altimetry From 10-m to Global Scales

Abstract:

The Ice, Cloud, and Land Elevation Satellite-2 (ICESat-2) provides satellite ocean altimetry that is unique in measuring sea surface height (SSH) at scales from global down to 10-m. To achieve high resolution, it uses the Advanced Topographic Laser Altimeter System (ATLAS), a photon-counting, multi-beam lidar pulsing at 10 kHz illuminating for each of 6 beams an order 10-m patch (13-m 2-sigma diameter) of the surface every 0.7 m of along-track distance. The lidar determines the height of the surface for each surface reflected photon along with the apparent height of a lower density of noise photons. In our earliest releases of the ICESat-2 ATL12 ocean height product, we focused on producing the first four moments of the ocean surface height over up to 7-km long along-track ocean segments, corrected for the instrument impulse response, and including an estimate of EM sea state bias. However, EM bias is essentially the correlation of photon return rate and surface height over ocean waves, and computing this involves computing photon rate and height in 10-m along-track bin averages. Further, characterizing uncertainty in the ocean segment average sea surface height requires determination of the horizontal correlation length scale from these same 10-m bin averages, and improved identification of surface reflected photons uses departures from the 10-m running averages of the height of a priori ATL03 high-confidence signal photons. Consequently, it has been natural to add to the ATL12 Release 3 and new (Sept. 2020) Release 4, a suite of higher resolution SSH products including: complete height histograms, 10-m along-track bin averaged height, and harmonic components of height characterizing surface waves. Here, we will discuss dynamic ocean topography (SSH-geoid) measured globally with ICESat-2 and explore these new high-resolution products looking at surface waves, lidar sea state bias and the dependence of reflectance on surface waves, and the critical importance of sea state in determining the uncertainty in measurement of average SSH.