Improved Arctic Sea Ice Freeboard Retrieval from Satellite Altimetry Using Optimized Sea Surface Decorrelation Scales

Jack C. Landy, Jerome Bouffard, Chris Wilson, Stefanie Rynders, Yevgeny Aksenov, Michel Tsamados
2021 Journal of Geophysical Research - Oceans  
A growing number of studies are concluding that the resilience of the Arctic sea ice cover in a warming climate is essentially controlled by its thickness. Satellite radar and laser altimeters have allowed us to routinely monitor sea ice thickness across most of the Arctic Ocean for several decades. However, a key uncertainty remaining in the sea ice thickness retrieval is the error on the sea surface height (SSH) which is conventionally interpolated at ice floes from a limited number of lead
more » ... servations along the altimeter's orbital track. Here, we use an objective mapping approach to determine sea surface height from all proximal lead samples located on the orbital track and from adjacent tracks within a neighborhood of 30-220 (mean 105) km. The patterns of the SSH signal's zonal, meridional, and temporal decorrelation length scales are obtained by analyzing the covariance of historic CryoSat-2 Arctic lead observations, which match the scales obtained from an equivalent analysis of high-resolution sea ice-ocean model fields. We use these length scales to determine an optimal SSH and error estimate for each sea ice floe location. By exploiting leads from adjacent tracks, we can increase the sea ice radar freeboard precision estimated at orbital crossovers by up to 20%. In regions of high SSH uncertainty, biases in CryoSat-2 radar freeboard can be reduced by 25% with respect to coincident airborne validation data. The new method is not restricted to a particular sensor or mode, so it can be generalized to all present and historic polar altimetry missions. Plain Language Summary Arctic Ocean sea ice thickness has been estimated with satellite altimeters for several decades by stitching together observations of the sea level at open water leads or 'cracks' in the ice. The height difference between the sea ice surface and sea level, known as the freeboard, can then be converted to an estimate for the ice thickness. However, open water lead observations can be hundreds of kilometers apart along the satellite's orbit, so here we apply a method that also uses leads on nearby orbits to improve the sea level estimate at ice-covered locations. This requires us to understand how rapidly the Arctic sea level varies over space and time, which we do use ESA's CryoSat-2 satellite radar altimeter. With an optimal processing method that exploits 10-100s of times more observations than normal, we can improve the precision of the sea level estimated 'under' sea ice. Up to 25% improvement in sea ice freeboard indicates that the new method could upgrade current and historic altimetry-derived Arctic sea ice thickness records.
doi:10.1029/2021jc017466 fatcat:ybcsh57pznbjxhy3uieb3w2zse