Abstract
Over the Arctic regions, current conventional altimetry products suffer from a lack of coverage or from degraded performance due to the inadequacy of the standard processing applied in the ground segments. This paper presents a set of dedicated algorithms able to process consistently returns from open ocean and from sea-ice leads in the Arctic Ocean (detection of water surfaces and derivation of water levels using returns from these surfaces). This processing extends the area over which a precise sea level can be computed. In the frame of the European Space Agency Sea Level Climate Change Initiative ( http://cci.esa.int ), we have first developed a new surface identification method combining two complementary solutions, one using a multiple-criteria approach (in particular the backscattering coefficient and the peakiness coefficient of the waveforms) and one based on a supervised neural network approach. Then, a new physical model has been developed (modified from the Brown model to include anisotropy in the scattering from calm protected water surfaces) and has been implemented in a maximum likelihood estimation retracker. This allows us to process both sea-ice lead waveforms (characterized by their peaky shapes) and ocean waveforms (more diffuse returns), guaranteeing, by construction, continuity between open ocean and ice-covered regions. This new processing has been used to produce maps of Arctic sea level anomaly from 18-Hz ENVIronment SATellite/RA-2 data.
Highlights
T HE Arctic is an important component of the climate system whose exact influence on the global oceanic and atmospheric circulation is still not well known
An algorithm for sea-ice detection has been developed for moderate-resolution imaging spectroradiometer (MODIS) [55], but here we choose to use MERIS for two main reasons: 1) in its fine resolution mode, it provides data at 300-m resolution commensurate with the waveform spacing for the RA-2 altimeter and 2) it is on the same satellite platform (ENVISAT), which means that the two sets of observations are simultaneous
Radar altimetry is a mature discipline with sea level records provided on a near-global basis for both climate science and operational applications
Summary
T HE Arctic is an important component of the climate system whose exact influence on the global oceanic and atmospheric circulation is still not well known. Studying and understanding the dynamic circulation of the Arctic necessitate the development of an SSH retrieval system that operates consistently through the changes between open ocean and floes with leads This paper constructs such a product using nearly 10 years of ENVIronment SATellite (ENVISAT) altimetry data. A very few have been focused on sea level determination with the constraint to ensure continuity of the observations between deep ocean and sea-ice regions, which is based on the computation of geographical bias between sea level estimates in the two areas [8], [25], [45] It is the objective of this paper to present the processing allowing the provision of accurate sea level measurements over the majority of the Arctic Ocean. POISSON et al.: DEVELOPMENT OF AN ENVISAT ALTIMETRY PROCESSOR PROVIDING SEA LEVEL CONTINUITY
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