Altimetric Sea Surface Research Articles

Accuracy of the Preliminary GOCE Geoid Models from an Oceanographic Perspective

In the framework of the ESA High Processing Facility (HPF), eight gravity models have been computed from the GOCE data since the beginning of the mission in 2009. The accuracy of these models for oceanographic application is assessed and compared with other geoid models (i.e., ITG-GRACE 2010S, EGM2008, EIGEN6C). For that purpose, ocean Mean Dynamic Topography (MDT) estimates are computed by subtracting the different geoids from an altimetric Mean Sea Surface. The MDT assessments were carried out by analyzing their associated geostrophic surface currents at different maximum harmonic degrees and by comparing them with independent in situ oceanographic data. Compared with GRACE-only geoid models, GOCE-only geoid models are in better agreement with independent data at scales smaller than 150 km while they show similar performances at scales larger than 200 km. Regional comparisons show that at 125 km the HPF releases compare well to independent observations with significant improvement with the last releases that use more GOCE data. At 100 km the western boundary currents are well resolved by the last releases, but the satellite-only geoid models need to be further improved to resolve smaller scales and reach the targeted 100 km resolution with higher accuracy.

Rigorous fusion of gravity field, altimetry and stationary ocean models

Many characteristics of the ocean circulation are reflected in the mean dynamic topography (MDT). Therefore observing the MDT provides valuable information for evaluating or improving ocean models. Using this information is complicated by the inconsistent representation of MDT in observations and ocean models. This problem is addressed by a consistent treatment of satellite altimetry and geoid height information on an ocean model grid. The altimetric sea surface is expressed as a sum of geoid heights represented by spherical harmonic functions and the mean dynamic topography parameterized by a finite element method. Within this framework the inversion and smoothing processes are avoided that are necessary in step-by-step approaches, such that the normal equations of the MDT can be accumulated in a straightforward way. Conveniently, these normal equations are the appropriate weight matrices for model-data misfits in least-squares ocean model inversions. Two prototypes of these rigorously combined MDT models, with an associated complete error description including the omission error, are developed for the North Atlantic Ocean and assimilated into a 3D-inverse ocean model. The ocean model solutions provide evidence that satellite observations and oceanographic data are consistent within prior errors.

Geoid model in the Western Mediterranean Sea

In this work a geoid model is presented over the Western Mediterranean area. It has been computed using marine and terrestrial gravimetric data. Differences between results including several kinds of data are also studied. Altimetric data from a year of ERS-1 mission are used to test the precision of the results, overall close to the coastal line. A first approximation to the sea surface topography in the area is made with both results: altimetric mean sea surface and gravimetric geoid.

Analysis of global ERS‐1 altimetry by optimal Fourier transform interpolation

Optimal interpolation in the spatial domain is limited by computer storage requirements and the computational effort to invert the autocovariance matrix. The procedure can be simplified by use of the fast Fourier transform (FFT), where matrix inversion is replaced by a quotient of power spectral density (PSD) functions in the spectral domain. Such a methodology is developed and utilized to estimate an altimetric mean sea surface from a single 35‐day cycle of ERS‐1. ERS‐1 radial orbit error is first reduced by minimizing dual crossover residuals with TOPEXIPoseidon through solution for a sinusoidal correction for each ascending and descending arc along with the relative altimeter bias. A regular grid for ascending (descending) arcs is then formed by use of the equator‐crossing longitudes and the along‐track displacement. Gaps in the altimeter data set are filled by use of a reference model. PSDs derived from the data and reference surface enable computation of the least‐Squares estimator in the spectral do...

A synthetic geoid validation of Geosat mesoscale dynamic topography in the Gulf Stream region

A validation study is carried out for the Geosat altimeter in the Gulf Stream meander and ring region. A synthetic geoid, containing the medium scales of interest for mesoscale research, is constructed by subtracting an independent estimate of the oceanographic mean dynamic topography from the altimetric sea surface. The oceanographic mean dynamic topography which is concurrent with the altimetric mean is derived from a year of real‐time nowcasts and forecasts (Gulfcast) based on assimilating a composite data set into a dynamical model. Using the synthetic geoid, absolute dynamic topography from Geosat and Gulfcast for a year and 4 months in 1988 and 1989 compare very well in signal shape and feature locations. Sixteen cases of comparison of dynamic heights derived from AXBTs along Geosat tracks in 1987 are made with Geosat absolute topography. The rms difference is less than 10 cm. Thus the comparison of these two independent data sets validates the altimeter and the synthetic geoid within the noise and error levels of the system.