Abstract
AbstractA 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 observations 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.
Highlights
Sea ice extent in the Northern Hemisphere has been declining at an increasingly alarming rate for more than two decades (Parkinson & DiGirolamo, 2016)
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 observations along the altimeter's orbital track
We have introduced a method to determine the optimal interpolation of local sea level anomaly (SLA) at sea ice floes using all valid proximal lead observations both on the orbital track in focus and other adjacent tracks
Summary
Sea ice extent in the Northern Hemisphere has been declining at an increasingly alarming rate for more than two decades (Parkinson & DiGirolamo, 2016). Distances between an ice-covered sample and its closest lead can exceed 200 km along the track, in the compact pack ice of the Central Arctic Ocean (Wernecke & Kaleschke, 2015) In these cases, the SSH uncertainty is constrained only by the deviation of the interpolated sea surface from the local mean measured elevation and can reach 50 cm, varying considerably across the Arctic (Ricker et al, 2014). In regions with low lead density and high SSH uncertainty, derived freeboards can include long-distance spatially correlated biases (Xia & Xie, 2018) Such biases may either amplify or cancel each other out in different locations, for instance when estimating snow depth from centimeter-scale differences between radar and laser sea ice freeboards (Kwok et al, 2020).
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