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Use of oceanographic in-situ measurements and altimetry to assess the accuracy of present (GRACE) and future (GOCE) geoid models. Impact for the estimation of the ocean Mean Dynamic Topography

Marie-Hélène Rio(1), Philippe Schaeffer(1), Jean-Michel Lemoine(2), Gilles Larnicol(1) and Fabrice Hernandez(3)

(1) CLS, 8-10 rue Hermes, 31256 Ramonville Saint Agne, France
(2) GRGS, 14 avenue Edouard Belin, 31400 toulouse, France
(3) IRD-MERCATOR, 8-10 rue Hermes, 31256 Ramonville saint agne, France

Abstract

The accurate knowledge of the geoid at all spatial scales (and thus, by subtraction from an altimetric Mean Sea Surface, of the ocean Mean Dynamic Topography) is a crucial issue for the correct assimilation of altimetric measurement into operational forecasting systems. At the time the first results of the GOCE mission will be available, the French system MERCATOR will be operating routinely the PSY3v2 protoype, providing on a global ¼° grid forecasts and analysis of the ocean state through the joint assimilation of altimetric data and in-situ measurements. The integration in the system of the information brought by GOCE will be highly valuable provided an accurate knowledge of the error level of all information entering the system (MDT, altimetry data, in-situ measurements) is accurately known and overall consistent. A method has been developed to compare the MDT obtained by direct subtraction between a geoid model and an altimetric Mean Sea Surface (‘direct’ MDT) to independent synthetic estimates of the MDT. Synthetic estimates are obtained from the combined use of altimetric Sea Level Anomalies and in-situ measurements of the ocean absolute dynamic topography (or related geostrophic circulation). Both the direct and synthetic MDTs are filtered to various spatial resolutions (from 133 km to 1000 km) and a thorough analysis of the different error contribution (including the comission and omission errors of the geoid model, as well as the altimetric and in-situ data measurement and processing errors) is done to make sure that all signal are consistent in the range of their error bar. The method is applied on the latest geoid models available based on GRACE data. The models are found to be accurate for oceanographic use at scales longer than 400 km. The better accuracy of combined solution compared to satellite-only solutions is also characterized as well as the strong impact of using two years of GRACE data compared to one year only for the geoid model computation. The shortest scales contained in the synthetic estimates are finally combined to the largest scales of the GRACE direct MDT to obtain a full resolution MDT estimate.

 

Workshop poster

Keywords: ESA European Space Agency - Agence spatiale europeenne, observation de la terre, earth observation, satellite remote sensing, teledetection, geophysique, altimetrie, radar, chimique atmospherique, geophysics, altimetry, radar, atmospheric chemistry