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Gravity improvement of continental slope and shelf ocean modelling

Eric Jeansou(1), Georges Balmino(1), Chris Hughes(2), Isabel Andreu-Burillo(2), Florent Lyard(3), Muriel Lux(1), Laurent Roblou(1), Pierre De Mey(3), Enrique Alvarez-Fanjul(4), Guilhem Moreaux(1), John Huthnance(2), Marcos García Sotillo(4), Roger Proctor(2), Begoña Pérez(4), Helge Rebhan(5), Mark Drinkwater(5) and Roger Haagmans(5)

(1) NOVELTIS, 2 avenue de l’Europe, 31520 Ramonville Saint Agne, France
(2) Proudman Oceanographic Laboratory, 6 Brownlow Street, Liverpool, L3 5DA, United Kingdom
(3) Pôle d’Océanographie Côtière, 14 avenue Edouard Belin, 31400 Toulouse, France
(4) Puertos del Estado, Avda. del Partenón 10, 28042 Madrid, Spain
(5) ESA, P.O. Box 299, 2200 AG Noordwijk, Netherlands

Abstract

This ESA-funded study aims to assess the capabilities and the limits of the use of the GOCE geoid to improve modelling of shelf and coastal ocean low-frequency circulations. The approach consists in using techniques of data assimilation into hydrodynamic models to estimate the present and future benefit of altimetric data use. Assimilation allows control of dynamical simulations and estimates of model and data error covariances.

Components of the project are:

• Review of the state of the art (character of processes and circulation in this context, character and status of models, present knowledge of gravity fields);

• Deciding which models, domains, forcing, tide models, validation data and model performance metrics are to be used in the project;

• Runs of models over the respective domains without and with assimilation of altimetry;

• Analysis of model output, comparison with measurements, identification of scales of variability, effects of assimilating altimetry;

• Assessing expected impact of GOCE, with model runs and data to yield mean dynamic topography, filtering to GRACE and GOCE resolution, relating model skill to length scale;

• GOCE performance impact studies with runs of models over the respective domains, without and with assimilation of altimetry, without and with GOCE (“perfect” and degraded), including computation of the expected point-to-point covariance of the GOCE geoid height.

Conclusions from the review are that:

• Along-slope currents, tides, wind-driven circulation and storm surges, surface wave statistics and seasonal heating scale with the shelf or slope width and have time-scales f-1 or longer. Their surface elevation represents sub-surface pressure and near-surface flow.

• We expect improved knowledge of the geoid to be useful, for (i) absolute mean transports – models may be used to determine the spatial distribution of the transport in the vertical and with finer horizontal resolution than the geoid, (ii) open boundary conditions for limited-area models, especially the overall pressure difference applied across the extent of the model and thereby (iii) for model estimation of ocean-boundary and slope currents, associated secondary ocean-shelf exchanges and eddies, tides, wind-driven circulation, along-front and coastal currents.

• Good knowledge of tidal elevations is necessary to process “raw” altimetric data, and of tidal currents for good model friction and mixing. This knowledge is available.

• Well established models are effectively operational for the three proposed areas of study: Gulf of Lions, Iberian shelf and Biscay, North-west European shelf. However, there is little practice of assimilating altimetric data over such shelf-and-slope seas at present.

• R.m.s. geoid uncertainties (differences between models) over the study areas are presently O(0.2 m) which is larger than typical non-tidal surface elevations in shelf seas.

• The GOCINA project successfully used locally dense and precise gravity measurements to improve geoid model quality locally.

 

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