Marc Naeije

The Radar Altimeter Database System project RADS

The Radar Altimeter Database System project is part of the Netherlands Earth Observation NETwork (NEONET), an Internet facility, funded by the Dutch government, for exploitation of remote-sensing expertise and data (http://www.neonet.nl). In the frame of RADS, The Delft Institute for Earth-Oriented Space Research DEOS is building a data base that contains validated and verified altimeter data products that are consistent in accuracy, format, correction and reference system parameters. The availability of such a data base will attract users with less altimetry expertise like advisory councils, water management authorities and even high schools. Much effort has been put in calibrating and validating the raw data, i.e. harmonization of geophysical corrections, of secondary data, and of the measurements themselves. The validation includes editing, tide experiments, radiometer-model collocation, and Rossby and Kelvin waves propagation analysis. This paper introduces RADS, and deals with cal/val aspects and how to use/access the data.

GEOSAT and ERS-1 radar altimetry over the North Atlantic

Abstract ERS-1 (35-day repeat) and GEOSAT (17-day repeat) altimetric measurements over the North Atlantic have been processed to study the mean sea surface and the sea surface currents. This paper presents some details on the processing techniques applied and on the results obtained. The large-scale semi-permanent circulation is determined from a least squares parameter adjustment method in which a number of gravity field coefficients and a low order and -degree dynamic ocean topography model are solved for simultaneously. After radial orbit error reduction by crossover difference residuals minimization, the mesoscale eddy circulation is obtained from analyzing the temporal variation of the local sea surface. In addition, mean wavenumber spectra have been computed for two areas in the North Atlantic, and some Gulfstream eddy characteristics, such as translation and rotation velocity, have been determined.

Quality of ocean variability results from satellite altimetry

Abstract One year of Geosat altimeter data is used for a comparison among three different processing techniques for the determination of mesoscale ocean currents. The observations, like sea surface variability and sea surface anomaly time series, yield a valuable contribution to all kinds of oceanographic studies. The article presents an a‐priori estimate of the feasible spatial and temporal resolutions of the observations by validating the different processing techniques and comparing the results. It shows an excellent correlation between the different sea surface anomaly field solutions. For the Agulhas Retroflection area it was found that the standard deviation of the obtained relative sea level may amount to up to 4 cm and that the average correlation is better than 0.90 (±0.04). The overall quality of the results obtained was shown to depend on the mesoscale signal strength and therefore differs for each region. This nonisotropic behavior of the correlation between the sea surface anomaly fields is s...

Seasonal cycle and inter-annual variability of the Kuroshio/Oyashio current system using multi-channel sea surface temperature and altimetry sea level data

Abstract Altimeter data from ERS-2 and TOPEX/POSEIDON are analyzed in conjunction with infrared radiometer data from LANDSAT to study the seasonal cycle and inter-annual variability of the sea surface height and temperature in the northwestern pacific waters, more specific the Kuroshio/Oyashio current system. This entails direct comparisons, frequency and trend analyses, and empirical orthogonal functions analyses. The latter we use for extracting the dominant patterns. Most of the variability propagates westward in correspondence with Rossby wave theory. Deviations are caused by eddies shed from the Kuroshio, and the interaction of current variability with bottom topography.

MANICORAL (Multimedia and network in cooperative research and learning)

The fast development of new IT-tools and communication technologies provides new opportunities for computer mediated collaboration in geographically distributed groups. One of the fields suitable for application of these technologies is collaboration among scientists. This opportunity is especially important to many of the smaller European countries where it may be difficult to maintain competent scientific research within highly specialized fields. The new tele-communication technologies provide scientists in smaller countries with opportunities to maintain close collaboration with colleagues in foreign countries in spite of dispersion in time and geographical location, and combined with multimedia to enhance visual representations of scientific data they open up radically new dimensions in research.

A global sea level variability study from almost a decade of altimetry

Altimeter data of the GEOSAT, ERS-1 and TOPEX/POSEIDON (T/P) missions have been used to study large-scale seasonal and long-periodic changes in the sea level. The radial orbit error is reduced by crossover minimization. The authors computed large-scale global dynamic topographies with a time resolution of 7 days and a space resolution of about 5/spl deg/. EOF and DFT analyses show a dominant annual cycle for most regions. Time-longitude sections reveal ENSO warm and cold events accompanied by Kelvin and Rossby waves. A secular change of 4.9 mm/yr is evident both from the ERS-1 and T/P data.

Seasonal changes in dynamic heights based on altimetric data

One year of TOPEX/POSEIDON altimeter data have been evaluated to study seasonal changes in the ocean dynamic heights. The authors modeled these heights by a spherical harmonics expansion up to degree 30 for each 10-day cycle covering the year 1993. The global mean sea level rate is estimated at -1.3 mm/yr. The seasonal sea height variability shows the anticipated behavior of most of the western boundary currents; relatively high levels in spring/fall, and low in winter/summer. The Agulhas Retroflection Front, however, shows the opposite. It also shifts to the west in spring and fall. Local harmonic analyses (FFT) of the dynamic heights reveal a dominant annual cycle for most areas, whereas the semi-annual cycle is confined to the western boundary currents. Dominant cycles of about 100 days are found in highly instable areas like the Brazil-Malvinas Confluence and the Agulhas Retroflection. A longitude-time diagram of a section in the Pacific along the equator gives evidence of equatorial Kelvin waves.<<ETX>>