Estimation of wintertime Arctic sea ice thickness with satellite scatterometers

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Accurate information on Arctic sea ice thickness has been historically limited both spatially and temporally to spare submarine sonar measurements until the advent of satellite altimeters such as ICEsat (operating from 2002 to 2008) and CryoSat-2 (operating from 2011 to present day). This study aims to use the historical record of normalized radar backscatter measurements from satellite Ku-band and C-band scatterometers (ERS, QuikSCAT and ASCAT), which have been continuously operating since 1992, to homogenize the satellite altimeter record and extend the record of Arctic sea ice thickness measurements backwards in time. This study is structured so as to first derive a set of empirical relationships between normalized backscatter measurements and wintertime sea ice thickness estimates in the Arctic using existing satellite altimeter records as a reference. Two separate scatterometer sea ice thickness models are produced using coincident scatterometer and altimeter observations, one for C-band sea ice thickness estimation using ASCAT and CryoSat-2 collocations, and another for Ku-band sea ice thickness estimation using QuikSCAT and ICESat collocations. Based on the agreement to the altimeter records, the estimation of wintertime sea ice thickness using the C-band and Ku-band scatterometer models is uncertain to within 0.5 m (1-sigma), that is, a precision similar to that of the original altimeter references. The homogenization of the satellite altimeter records cannot be done directly, because the ICESat and CryoSat-2 instruments operate in different periods, but it can be done indirectly by comparing the sea ice thickness estimates obtained from Ku-band (based on ICESat) and C-band (based on CryoSat-2) estimates during the years that the Ku-band and C-band scatterometers operate simultaneously. These overlap years have been used to verify the consistency between the C-band and Ku-band relationships, and to correct for a 0.55 m bias in the CryoSat-2 reference, having considered the earlier ICESat record as absolute standard. After removing this bias from the CryoSat-2 reference, the sea ice thickness estimates from C-band and Ku-band records agree to within 0.15 m (1-sigma). Moreover, the resulting scatterometer sea ice thickness models allow the introduction of new thickness thresholds from a previously existing backscatter-based classification of Arctic sea ice types, providing a thickness threshold of 1.54 m to define first year ice (FYI), and a thickness threshold of 2.25 m to separate second year ice (SYI) from older multiyear ice (old MYI). Ancillary datasets were used to investigate the correlations between backscatter-based sea ice thickness and physical variables, such as snow depth and surface deformation, in order to investigate possible sources of systematic error, which otherwise appear to be bound within 0.30 m (1-sigma). The maps of differences between scatterometer and altimeter sea ice thickness estimates were analysed in terms of collocated sea ice convergence, sea ice shear and snow depth parameters using a multiple regression model. The results show that both the Ku-band and C-band models underestimate ice thickness in areas of high convergence such as the Fram Strait, and overestimate ice thickness in areas with high shear such as the Beaufort Gyre. These correlations may be interpreted as led by increases in backscatter due to surface deformation with (case of convergence) or without (case of shear) associated increases in ice thickness. In addition, the Ku-band model is found sensitive to snow load, with overestimation interpreted as led by an increase in backscatter without associated ice growth, and the C-band model is sensitive to marginal rough ice. An unphysically large dependence on snow depth was found for the C-band estimate, which we conjecture is due to problems with the CryoSat-2 reference. Finally, a reconstruction of Arctic sea ice thickness in the wintertime has been made by combining the Ku-band and C-band sea ice thickness models with the normalized radar backscatter record of ERS, QuikSCAT and ASCAT from 1992 through 2017. These showed a decline average Arctic sea ice thickness of -0.28 m / decade, with a steady decline in second-year ice and high variability in the mean multi-year ice thickness.