Comparison of CryoSat-2 and ENVISAT freeboard height over Arctic sea ice: Toward an improved Envisat freeboard height retrieval
Other literature type
(issn: 1994-0424, eissn: 1994-0424)
During the past decade, sea ice freeboard height has been monitored with various satellite altimetric missions with the aim of producing long-term time series of ice thickness. To achieve this goal, it is essential to analyze potential inter-mission biases and to produce freeboard height datasets as free of instrumental error as possible. In the present study, we compare Envisat and CryoSat-2 freeboard height during the common flight period (2010–2012). Our results show that Envisat freeboard height is always thinner (−14 cm in average) when compared to CryoSat-2 (3 cm in average). In addition, Envisat freeboard height displays an unrealistive negative growth from November to April (−2.4 to −3.7 cm) while CryoSat-2 dispalys a positive and coherent winter growth (2.4 to 2.7 cm). The discrepancy between the two datasets is found to be related to a dissimilar impact of ice roughness and snow volume scattering on SAR (CryoSat-2) and pulse-limited (Envisat) altimetry. Following this result, we show that the freeboard height difference between the two datasets can be expressed as a function of the waveform pulse-peakiness. Based on the relation between the Envisat pulse-peakiness and the freeboard height difference, we produce a monthly CryoSat-2-like version of Envisat freeboard height that reduces the average RMSD with CryoSat-2 from ~ 16 cm to ~ 2 cm and improves the freeboard height growth cycle (2–3 cm). The comparison of the altimetric datasets with in situ ice draft measurements during the common flight period shows that the corrected Envisat dataset (RMSE = 16–29 cm) is as accurate as CryoSat-2 (RMSE = 13–25 cm) and highly more accurate than the uncorrected Envisat dataset (RMSE = 108–132 cm). The comparison of the improved Envisat freeboard height dataset is then extended to the rest of the Envisat mission to demonstrate the validity of the improved Envisat dataset out of the calibration period. As a result, we find a good agreement between the Envisat and the in situ ice draft datasets (RMSE = 14–30 cm), which demonstrates the potential of the pulse-peakiness-correction to produce accurate freeboard height estimates over the entire Envisat mission. Finally, we show the averaged-circumpolar ice thickness variations from 2002 to 2015 by combining CryoSat-2 and Envisat datasets.