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Vendredi 1 decembre - X. Xu - Coastal sea level by Jason-2

Par SEMSOU Dernière modification 27/11/2017 13:13
Quand ? Le 01/12/2017,
de 14:00 à 15:00
Où ? Salle Clairaut
Participants Xi Yu Xu, Associate Professor, National Space Science Center, Chinese Academy of Sciences
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Xi Yu Xu,

Associate Professor, Key Laboratory of Microwave Remote Sensing,

National Space Science Center,

Chinese Academy of Sciences (CAS),

Beijing, China

 

Title : Evaluation of Coastal Sea Level of Jason-2 Altimetry Offshore Hong Kong

 

Abstract :

Satellite altimetry provides precise sea surface height measurements up to about 10 to 20 km from the coastlines. In the coastal zones, radar echoes reflected to the satellite become much more difficult to interpret because of land contamination. In parallel, the quality of geophysical corrections we need to apply to altimeter range measurements also degrades significantly. This leads to an important decrease in the number of valid data when approaching the coast. In the recent years, a number of groups have developed different approaches to overcome those difficulties, either by improving the altimeter geophysical corrections (e.g. tropospheric correction, ocean tide models, etc.) or by developing specific methodologies (called retracking) to extract accurate range measurements (distance between satellite and sea surface) from non-standard radar echoes (e.g., PISTACH and ALES retrackings). Thanks to these efforts, a number of experimental reprocessed coastal altimetry sea level data sets are now available. In this paper, we evaluate those available for the Jason-2 satellite mission. We focus on the Hong-Kong region and consider high-resolution (20-Hz) along-track data for a specific Jason-2 pass that crosses the coast nearby the Hong-Kong tide gauge. Six years and a half of data are analyzed, from July 2008 to December 2014 (orbital cycles 1-238). We compare sea surface height measurements obtained using the ALES retracker and the different retrackers of the PISTACH products with the official standard data (called Geophysical Data Records –GRD-) distributed by space agencies. We compare each near-coastal sea level estimate with data from the Hong Kong tide gauge (located 10km away). We also compute sea surface height bias and noise over both open ocean (>10km away from coast) and coastal zone (within 10km or 5km coast-ward). Finally, coastal sea level trends from the different retrackers are estimated over the 2008-2014 time span. The results show that, after outlier removal, in the coastal band, ALES performs better than the other retrackers considered in this study, both in terms of noise level and trend uncertainty. An interesting, but still preliminary result is that over the 6.5-year time span considered here, the altimetry-derived sea level trend is significantly larger within 5 km of the coastline than at larger distances from the coast.

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