Altimeter Products Research Articles

Effects of stencil width on surface ocean geostrophic velocity and vorticity estimation from gridded satellite altimeter data

This paper examines the effect of “stencil width” on surface ocean geostrophic velocity and vorticity estimated from differentiating gridded satellite altimeter sea surface height products. In oceanographic applications, the value of the first derivative at a central grid point is generally obtained by differencing the sea surface heights at adjacent grid points. This is called a “three‐point stencil centered difference”. Here the stencil width is increased from three to five, seven, and nine points, using well‐known formulae from the numerical analysis literature. The discrepancies between velocities computed with successive stencils decreases with increasing stencil width, suggesting that wide stencil results are more reliable. Significant speed‐dependent biases (up to 10–20%) are found between results computed from three‐point stencils versus those computed from wider stencils. The geostrophic velocity, and the variance of geostrophic velocity, are underestimated with thin stencils. Similar results are seen in geostrophic velocities computed from high‐resolution model output. In contrast to the case when three‐point stencils are used, wider stencils yield estimates of the anisotropy of velocity variance that are insensitive to the differences in grid spacing between two widely used altimeter products. Three‐point stencils yield incorrect anisotropies on the 1/4° anisotropic AVISO grid; we recommend the use of 7‐point stencils. Despite the demonstrated inadequacies of the three‐point stencils, the conclusions of earlier studies based on them, that the zonally averaged midlatitude eddy kinetic energy field is nearly isotropic, are found to pertain also with wider stencils. Finally, the paper also examines the strengths and limitations of applying noise‐suppressing differentiators, versus classic centered differences, to altimeter data.

New improved orbit solutions for the ERS-1 and ERS-2 satellites

Improved orbit solutions of the European Remote Sensing Satellites ERS-1 and ERS-2 have been computed in the ITRF2005 terrestrial reference frame using the recent models based mainly on IERS Conventions 2003. These solutions cover the periods 3 August 1991 to 8 July 1996 for ERS-1, and 3 May 1995 to 4 July 2003 for ERS-2. For each satellite, the final orbit solution is based on a combination of three separate orbit solutions independently computed at the Delft Institute of Earth Observation and Space Systems (DEOS) of the Delft University of Technology (The Netherlands), the Navigation Support Office of the European Space Operations Centre (ESOC, Germany) and the Helmholtz Centre Potsdam GFZ German Research Centre for Geosciences (Germany) using three different software packages for precise orbit determination, but using the same models in the same terrestrial reference frame within the European Space Agency (ESA) project ‘Reprocessing of Altimeter Products for ERS (REAPER)’. Validation using radar altimeter data indicates that the new combined orbits of ERS-1 and ERS-2 computed by us are significantly more accurate, approaching the 2–3cm level in radial direction, than previously available orbit solutions.

A geostrophic empirical mode based on altimetric sea surface height

Recent discovery of low-dimensional coherent structure in oceanic currents along with a new merged altimeter product called Absolute Dynamic Topography (ADT) makes it possible to derive subsurface ocean information from satellite remote sensing data. An altimetric geostrophic empirical mode (η-GEM) is developed in this study by projecting hydrographic transects onto the ADT sea surface height coordinate, based on which the four-dimensional thermohaline and velocity structures of oceanic currents are reconstructed from satellite surface observations. In the WOCE/SR3 area, the η-GEM fields capture more than 95% of the total thermal variance. The GEM-derived flow has equivalent-barotropic structure and represents the velocity profile better than traditional dynamic modes. Comparison with mooring observations also demonstrates that the η-GEM provides good estimates of the deep thermohaline fields.

Seasonal and interannual variability in the Gulf of Cadiz: Validation of gridded altimeter products

Nine years of gridded multi-mission altimeter products have been validated in the Gulf of Cadiz. The comparison was performed using two tide gauges deployed in the mouths of two estuaries: the Tinto–Odiel system and Guadalquivir River. The averaged seasonal cycle of the sea-level – corrected for atmospheric pressure effect – obtained from the two tide gauges and their closest altimeter points showed good agreement, correlation coefficients being about 0.95 (at both stations). The variability about the mean seasonal cycle observed in the altimeter data is about half that obtained from the tide gauges. The year-to-year analysis of the seasonal cycle confirmed the level of agreement between the two datasets, and indicated lower amplitude errors in the estimate of the annual harmonic (only significant at 95% confidence level in most of the years) in the altimeter data. The estimation of the annual component is affected by sporadic heavy river discharges only observed in the tide-gauge records. In those cases the river flow explained more than 50% of the variance associated with the daily mean sea-level.

Freshwater from the Bay of Biscay shelves in 2009

In April–November 2009, surface salinity data provide a good coverage of most of the south-east Bay of Biscay and nearby Aquitaine/Armorican shelves. By late April most of the shelf, in particular south of 46°N, is covered by a fresh surface layer amounting to a fresh water volume of 49·109m3. At that time, a moderate amount of fresh water has spread over the Landes Plateau. By mid-June, this shelf water penetrates over the Cape Ferret Canyon north of the Landes Plateau. By mid-July, it is found west of the Landes Plateau to at least 4°W, with an estimated fresh-water content of 11–14·109m3. Drifters deployed on June 17 in the Cape Ferret Canyon, or later on the shelves, confirm the spreading of shelf fresh-water over the deep ocean. Lagrangian tracking using altimetric products, also confirms the transport by a quasi-stationary circulation. Operational numerical simulations (PREVIMER, IBI, HYCOM) display this spread of the freshwater, but in different areas. In particular, all have some fresh water escaping westward near the coast in the Basque region, which is not observed. Later in the summer season, the fresh water spreads westward to south-westward and along the shelf break to at least 5.5°W in late September.

Using Objective Analysis of Scanning Radiometer Measurements to Compute the Water Vapor Path Delay for Altimetry

An objective analysis (OA) method is implemented to compute the water vapor path delay (PD) correction of the altimeter range using total precipitable water measurements from scanning microwave radiometers (Advanced Microwave Sounding Unit A, Advanced Microwave Scanning Radiometer-Earth Observing System, Tropical Rain Measuring Mission Microwave Imager, and Special Sensor Microwave Imager). The European Centre for Medium Range Weather Forecasts (ECMWF) model-derived water vapor PD correction given in the altimeter products is used as the first-guess field. The calculation of the statistical variables required by the OA is presented: These include the variance and correlation function of the radiometer observations minus its first guess, as well as the observation error variance. The performance of the OA-derived water vapor PD correction is assessed, using four months of Jason-1 altimeter data. It is shown that the OA-derived correction is more accurate than the ECMWF-derived correction but remains less accurate than the one derived from the Jason microwave radiometer.

Surface coastal circulation patterns by in-situ detection of Lagrangian coherent structures

Coastal transport and cross-shelf exchanges are important factors in controlling the dispersal of human and river discharged pollutants, as well as the advection of nutrients and larvae. Altimetry-based Lagrangian techniques provide accurate information on horizontal transport in the open ocean but are unreliable close to the coast. In order to circumvent this problem, during the Lagrangian Transport Experiment 2010 campaign (Latex10, 1-24 September 2010) transport structures in the western Gulf of Lion were investigated with an adaptive sampling strategy, combining satellite data, ship-based ADCP measurements, and iterative Lagrangian drifter releases. The sampling strategy was able to identify errors in the surface transport patterns derived from altimetry, and to track with in-situ observations attractive and repelling Lagrangian coherent structures for a period of 12 days. The structures maintained a corridor ˜10 km-wide, roughly parallel to the coast, along which waters from the continental shelf leave the gulf. This is confirmed by high-resolution SST imagery. The use of this sampling strategy to explore surface transport structures may provide important information for the environmental management of coastal regions, and may serve for validating future coastal altimetric products.

Altimeter Near Real Time Wind and Wave Products: Random Error Estimation

Triple collocation technique is used to estimate the errors in near-real time altimeter wind and wave products of Jason-2, Envisat and Jason-1. Significant wave height (SWH) error is estimated by collocating altimeter products with buoys and model hindcasts over the period August 1, 2009 to July 31, 2010. It is estimated that Jason-2 absolute SWH error is about 0.13 m, or about 5.4% relative to the mean SWH value. The SWH errors of Envisat, Jason-1, buoys and the stand-alone model hindcast are about 6.2%, 7.8%, 8.6% and 9.7%, respectively. The wind speed error is estimated by collocating altimeter products with buoys and ECMWF model 1-day forecast over the same period. Jason-2 wind speed absolute error is about 1.0 m.s−1 (∼11.9% of the mean). For the collocation data sets used, it is found that wind speed errors of Envisat, Jason-1, the buoys and the model 1-day forecast are, respectively, 0.9 m.s−1, 1.0 m.s−1, 1.2 m.s−1 and 1.0 m.s−1.

Stream-coordinate structure of oceanic jets based on merged altimeter data

The jet structure of the Southern Ocean front south of Australia is studied in stream-coordinate with a new altimeter product—Absolute Dynamic Topography (ADT) from AVISO. The accuracy of the ADT data is validated with the mooring data from a two-year subantarctic-front experiment. It is demonstrated that the ADT is consistent with in-situ measurements and captures the meso-scale activity of the Antarctic Circumpolar Current (ACC). Stream-coordinate analysis of ADT surface geostrophic flows finds that ACC jets exhibit large spatio-temporal variability and do not correspond to particular streamfunction values. In the circumpolar scope ACC jets display a transient fragmented pattern controlled by topographic features. The poleward shift of jet in streamfunction space, as revealed by a streamwise correlation method, indicates the presence of meridional fluxes of zonal momentum. Such cross-stream eddy fluxes concentrate the broad ACC baroclinic flow into narrow jets. Combined with a recent discovery of gravest empirical mode (GEM) in the thermohaline fields, the study clarifies the interrelationship among front, jet and streamfunction in the Southern Ocean.

Characterizing the Navidad current interannual variability using coastal altimetry

In this study, the Navidad current, which flows along the northern coast of Spain in winter, is observed and characterized using coastal altimetry data over the period 1992–2002. This coastal current, marked by a strong interannual variability, is associated with eastward transport of warm waters along the shelf slope. Specific data editing and processing strategies have been applied to the along-track altimeter data, which allows us to retrieve altimetric sea level anomalies closer to the coast, with a better spatial coverage and improved quality when compared with standard altimetric products. The current variability observed upstream by in situ time series after November 1996 is well reproduced by the satellite across-track surface geostrophic current anomalies up until September 1999; this agreement degrades later in time. The combined use of satellite-derived current anomalies and sea-surface temperature anomalies allows us to develop indices of Navidad occurrences, in the first long-term, systematic survey of that current based on a multi-sensor approach. The satellite analyses confirm the previously identified Navidad occurrences in winter of 1995–1996, 1997–1998, and 2000–2001. Furthermore, a weak Navidad event was identified in winter 1996–1997. These four winters are associated with a negative North Atlantic Oscillation index in the previous fall, but the intensity of the Navidad is not correlated to the amplitude of that index.

Ocean eddy momentum fluxes at the latitudes of the Gulf Stream and the Kuroshio extensions as revealed by satellite data

Eddy momentum fluxes, i.e. Reynold stresses, are computed for the latitude bands of the Gulf Stream and Kuroshio extensions using 13 years of data from the merged satellite altimeter product of Le Traon et al. The spatial pattern and amplitude of the fluxes is remarkably similar to that found by Ducet and Le Traon using the 5 years of data that were available to them. In addition to updating the work of Ducet and Le Traon, we provide new insight into the role played by the underlying variable bottom topography, both for determining the structure of the eddy momentum fluxes seen in the satellite data and for influencing the way these fluxes feedback on the mean flow. While there is no clear evidence that eddies locally flux momentum into the eastward jets of the Gulf Stream and Kuroshio extensions, a clearer picture emerges after zonally integrating across each of the North Atlantic and North Pacific basins. We argue that the eddy momentum fluxes do indeed drive significant transport, a conclusion supported by preliminary results from a 3-D model calculation. We also present evidence that in the North Pacific, the Reynolds stresses are important for driving the recirculation gyres associated with the Kuroshio extension, taking advantage of new data from both observations and high-resolution model simulations.

On the Quality of Real-Time Altimeter Gridded Fields: Comparison with In Situ Data

Abstract The timeliness of satellite altimeter measurements has a significant effect on their value for operational oceanography. In this paper, an Observing System Experiment (OSE) approach is used to assess the quality of real-time altimeter products, a key issue for robust monitoring and forecasting of the ocean state. In addition, the effect of two improved geophysical corrections and the number of missions that are combined in the altimeter products are also analyzed. The improved tidal and atmospheric corrections have a significant effect in coastal areas (0–100 km from the shore), and a comparison with tide gauge observations shows a slightly better agreement with the gridded delayed-time sea level anomalies (SLAs) with two altimeters [Jason-1 and European Remote Sensing Satellite-2 (ERS-2)/Envisat] using the new geophysical corrections (mean square differences in percent of tide gauge variance of 35.3%) than those with four missions [Jason-1, ERS/Envisat, Ocean Topography Experiment (TOPEX)/Poseidoninterlaced, and Geosat Follow-On] but using the old corrections (36.7%). In the deep ocean, however, the correction improvements have little influence. The performance of fast delivery products versus delayed-time data is compared using independent in situ data (tide gauge and drifter data). It clearly highlights the degradation of real-time SLA maps versus the delayed-time SLA maps: four altimeters are needed in real time to get the similar quality performance as two altimeters in delayed time (sea level error misfit around 36%, and zonal and meridional velocity estimation errors of 27% and 33%, respectively). This study proves that the continuous improvement of geophysical corrections is very important, and that it is essential to stay above a minimum threshold of four available altimetric missions to capture the main space and time oceanic scales in fast delivery products.

Comparison between Eulerian diagnostics and finite-size Lyapunov exponents computed from altimetry in the Algerian basin

Transport and mixing properties of surface currents can be detected from altimetric data by both Eulerian and Lagrangian diagnostics. In contrast with Eulerian diagnostics, Lagrangian tools like the local Lyapunov exponents have the advantage of exploiting both spatial and temporal variability of the velocity field and are in principle able to unveil subgrid filaments generated by chaotic stirring. However, one may wonder whether this theoretical advantage is of practical interest in real-data, mesoscale and sub-mesoscale analysis, because of the uncertainties and resolution of altimetric products, and the non-passive nature of biogeochemical tracers. Here we compare the ability of standard Eulerian diagnostics and the finite-size Lyapunov exponent in detecting instantaneous and climatological transport and mixing properties in the south-western Mediterranean. By comparing with sea-surface temperature patterns, we find that the two approaches provide similar results for slowly evolving eddies like the first Alboran gyre. However, the Lyapunov exponent is also able to predict the (sub-)mesoscale filamentary processes occurring along the Algerian current and above the Balearic Abyssal Plain. Such filaments are also observed, with some mismatch, in sea-surface temperature patterns. Climatologies of Lyapunov exponents do not show any compact relation with other Eulerian diagnostics, unveiling a different structure even at the basin scale. We conclude that filamentation dynamics can be detected by reprocessing available altimetric data with Lagrangian tools, giving insight into (sub-)mesoscale stirring processes relevant to tracer observations and complementing traditional Eulerian diagnostics.

Global Analysis of Envisat RA-2 Burst Mode Echo Sequences

The Envisat RA-2 burst echoes are being gathered throughout the mission; however, these data are only now being made generally available by the European space agency. Considerable work has been necessary to turn these engineering-level data into a useable altimeter product. This paper documents the processing steps undertaken to generate usable data and presents the first extensive analysis of this unique dataset, using over 75 000 burst sequences with a global distribution. The results show that the burst echo data from Envisat are of extremely low noise, which is particularly evident over non-ocean surfaces, and contain a wealth of detailed information from both land and ocean surfaces. Examples illustrate the complexity of surface response from targets such as inland water and rough terrain. These unique data clearly have the potential to inform future instrument design as well as to improve the understanding of existing altimeter datasets.

Eddy length scales in the North Atlantic Ocean

Eddy length scales are calculated from satellite altimeter products and in an eddy‐resolving model of the North Atlantic Ocean. Four different measures for eddy length scales are derived from kinetic energy densities in wave number space and spatial decorrelation scales. Observational estimates and model simulation agree well in all these measures near the surface. As found in previous studies, all length scales are, in general, decreasing with latitude. They are isotropic and proportional to the local first baroclinic Rossby radius (Lr) north of about 30°N, while south of 30°N (or for Lr > 30 km), zonal length scales tend to be larger than meridional ones, and (scalar) length scales show no clear relation to Lr anymore. Instead, they appear to be related to the local Rhines scale. In agreement with a recent theoretical prediction by Theiss [2004], the observed and simulated pattern of eddy length scales appears to be indicative of two different dynamical regimes in the North Atlantic: anisotropic turbulence in the subtropics and isotropic turbulence in the subpolar North Atlantic. Both regions can be roughly characterized by the ration between Lr and the Rhines scales (LR), with LR > Lr in the isotropic region and LR < Lr in the anisotropic region. The critical latitude that separates both regions, i.e., where LR = Lr, is about 30°N.

Improved satellite altimetry in coastal systems: Case study of the Corsica Channel (Mediterranean Sea)

The fidelity of corrections and processing are critical for a realistic use of official altimetric products close to the coast. A new processing strategy, which starts from the TOPEX/Poseidon GDRs with the addition of improved corrective terms, is proposed and evaluated in the area of the Corsica Channel. Sea level anomalies agree with the coincident sea truth (bottom pressure and tide gauge) within 2–3 cm rms for seasonal and longer time scales. Analysis for almost ten years of coincident mooring and altimetric velocities shows that a substantial reduction of uncertainty to ∼4 cm s−1 may be possible after reasonable filtering of the noise introduced by more variable coastal sea surface states. The conclusion is that the altimetry success is still limited to seasonal time scales, and provided that the oceanographic signal ensures an adequate signature to be isolated from background noise.

The use of radar altimeter data in combination with other satellite sensors for routine monitoring of the ocean: Case study of the northern Arabian Sea and Gulf of Oman

Merged radar altimeter products from TOPEX/POSEIDON and European remote sensing satellite (ERS) have been used to monitor mesoscale ocean features (eddies and fronts) in the Gulf of Oman and northern Arabian Sea. The altimeter data have been compared with coincident thermal infrared and colour images from along-track scanning radiometer (ATSR), advanced very high resolution radiometers (AVHRR), and sea-viewing wide field sensor (SeaWiFS), which provide finer scale detail on fronts and eddies seen in the altimeter data. Time series of data from all of these sources, collected over the period July 1998 to June 1999, reveal that the combination of altimeter and ocean colour data allows tracking of these features in an operationally realistic way. The all-weather altimeter data complement the cloud-limited ocean colour and thermal infrared data. Ocean colour data are especially useful for supporting the indications of mesoscale features provided by the altimeter data. Eddies confirmed using combined data can be tracked during cloudy periods using altimeter data alone. The altimeter is found to perform better in the northern Arabian Sea, where features are larger and less dynamic than in the Gulf of Oman, where ocean features tend to be small, short-lived, and quickly moving.

The D-PAF ERS-1 Geocoding System

Abstract The German Remote Sensing Data Centre (DFD) located at DLR in OberpfafTenhofen will be one of the primary European data centres for ERS-1 products. The two major systems of I his German Processing and Archiving Facility (D-PAF) comprise the generation of Altimeter and Orbit products and the SAR data product generation chain. The latter comprise the ISAR data processor and the SAR Geocoding System (GEOS). A brief description of the algorithms and subsystems of GEOS is given.