Forum for Science, Industry and Business

Sponsored by:     3M 
Search our Site:

 

Envisat making first direct measurements of ocean surface velocities

28.03.2006


For more than a decade space-based radar instruments have been routinely observing ocean surface phenomena including wind, waves, oil slicks, even the eyes of hurricanes. Now – employing the same principle as police speed guns – satellite radar has also begun to enable direct measurements of the speed of the moving ocean surface itself.



The oceans that cover 71% of the Earth’s surface are constantly in motion. Ocean surface currents can lead to strong interaction with wind and waves, and through the transfer of heat influence the weather. Their dynamic range in the mesoscale is broad, with features such as current eddies and fronts, occurrences of filaments and jets, and wind-driven coastal upwelling or downwelling.

High-resolution surface current observations on a large scale are very difficult, although satellite altimeters permit monitoring of sea level anomalies that are related to surface currents. Sea surface temperature and ocean colour measurements also throw light on ocean circulation pattern and seasonal variations.


However Synthetic Aperture Radar (SAR) instruments such as the ASAR on ESA’s Envisat have now been demonstrated to potentially offer a more direct and therefore very valuable source of high-resolution information for oceanographers.

SAR sensors record microwave radar backscatter in order to identify patterns of surface roughness. Indeed, over the ocean these patterns are linked to varying surface winds and currents – in particular current shear and varying convergence zones that can focus wave energy, resulting in increased wave steepening and enhanced radar-detectable roughness changes. Unfortunately, interpreting radar images to accurately identify and quantify such signatures has been easier in theory than in practice.

"Due to a general lack of sufficient high-quality in-situ observations, the understanding of how these dynamic features contribute to the complicated surface roughness modulation pattern often manifested in SAR images is incomplete," explained Johnny Johannessen of the Nansen Environmental and Remote Sensing Center (NERSC) in Bergen, Norway, who presented results at ESA’s SeaSAR 2006 workshop.

But a new light is being shed on these analyses by using neglected extra information embedded in the radar signal: the Doppler shift in electromagnetic waves reflected from the water surface. This shift occurs due to changing relative velocities, experienced in everyday life in the way the pitch of a siren on a passing ambulance goes up as it approaches, then goes down as the vehicle recedes away. This ‘Doppler Effect’ also serves as the basis of police speed-measuring radar guns.

In this instance, the shift is introduced by the relative motion between the satellite platform, the rotation of the Earth and the velocity of the particular facets of the sea surface from which the SAR signal scatters back to orbit. The initial two values are well known – particularly for Envisat, with its very stable satellite orbit and attitude – and can be simply subtracted to extract the useful sea surface velocity information.

"This surface velocity, in turn, is composed of contributions from the wind-wave induced motion, and the background surface current," Johannessen added. "Following the Doppler shift equation an estimate of the surface current can be obtained, providing the contributions from wave- and wind-induced motion are first quantified and removed. It is here that the Radar Imaging Model (RIM) comes in."

The RIM is a numerical model that combines outputs from specialised high-resolution ocean models with fundamental equations for SAR imaging of ocean current features and simulate appropriate radar cross section signatures. RIM outputs provide both wave- and wind-induced ocean surface motion for subtraction.

"Our ability to interpret and quantify surface current features imaged by SAR has not been adequately developed," Johannessen added. "By applying this combined Doppler and Radar Imaging Model technique – DopRIM for short – things are now looking much more promising."

This new analysis is still limited to surface velocity measurements in the line-of-sight of the radar instrument. But this new source of information already provides a useful complement to the standard roughness pattern analysis.

Initial tests have been carried out on tidal currents off Brest and Cherbourg in France, with simulations compared to SAR images of the same sites and tidal surface currents derived from the combined results. Impressive new views of the ocean from radar measurements are now available to open up new scientific investigations and applications.

The results so far were presented at SeaSAR 2006, which took place at ESRIN, ESA’s European Centre for Earth Observation in Frascati, Italy, at the end of January. More than 100 researchers from 20 countries participated in the four-day workshop.

"More evaluations are certainly needed to consolidate and validate this DopRIM method," Johannessen said. "We believe however that the preliminary but promising results will open new opportunities for more systematic studies of mesoscale ocean variability based on SAR in the coming years. Provided the method is found to be reliable it will make a significant contribution to operational oceanography."

These measurements would be very useful for determination of surface drift, important for oil dispersion, wave-current interaction and pollution transport. The method could complement the use of additional information sources, such as 3-D ocean models, atmospheric forcing fields and in-situ devices such as high-frequency radar stations, as well as differing satellite sensors, wavelength and polarisation characteristics.

This includes future experimental SAR satellites, such as TerraSAR-X and Cosmo-Skymed, planned to join the currently operating Envisat, ERS-2, Canada’s Radarsat and Japan’s just-launched Advanced Land Observing Satellite (ALOS). ESA is also planning a series of operational satellites known as Sentinels - designed to support the Global Monitoring for Environment and Security (GMES) initiative of the Agency and the European Union – with one being dedicated to ocean applications.

Envisat ASAR wave data sharpening Meteo France ocean forecasts

Also presented at SeaSAR 2006 was the latest in an ambitious project to assimilate another Envisat ASAR product – Wave Mode data, which yields wind speed and wave spectra over the open ocean – into the sea state prediction models of French national weather forecaster Meteo France. The forecaster already assimilates altimetry data from Envisat sensor, the Radar Altimeter-2 (RA-2), and has been working on ASAR assimilation since 2003.

"The work is still part of our research and development activities but hopefully we will step forward into operational mode soon," said Lofti Aouf of Meteo France. "The impact of the assimilation of ASAR wave spectra processed with an upgraded algorithm on wave parameters – wave height, period and direction – stays efficient for more than three days into the forecast period, following the end of the assimilation.

"We developed an assimilation system which conjointly uses RA-2 significant wave height and ASAR directional wave height. The results indicated a significant impact on mean wave parameters, results being validated with independent wave data, such as wave height from the French-US Jason-1 satellite altimeter.

"During a one-month test run the results showed a correction of root mean square (RMS) error of 20%, with the impact duration induced by the combined assimilation being further enhanced. This task has important consequences on related research topics such as air-sea interaction processes, including heat fluxes transfers and current circulation."

Harald Johnsen of Norwegian company NORUT IT explained how an upgraded ASAR Wave Mode algorithm had been developed following detailed comparisons of ASAR coverage across the world’s oceans with in-situ data from buoys and platforms, as well as numerical ocean and atmospheric models, to arrive at a database of co-located spectra. The cross-comparison revealed a number of problems and ambiguities in terms of swell estimation which the processing algorithm was then upgraded to remove.

Validation of the results clearly shows an increase in meaningful spectra – as opposed to ‘non-physical spectra’ previously observed – and an improved flagging of ambiguous data, as well as a decreased average deviation between the data and wave forecasting system of the European Centre for Medium-Range Forecasting (ECMWF). Reprocessing of the entire ASAR Wave Mode archive is now being undertaken at the Centre ERS d’Archivage et de Traitement (CERSAT) operated by the French Research Institute for Exploitation of the Sea (IFREMER).

Envisat ASAR wave data analysis over coastal areas

ASAR measurements are also providing a unique ability to estimate the wave climate over coastal areas. The processing of SAR images in general – and Envisat’s ASAR in particular – offers potential for imaging large areas with a high resolution capability to provide fine information about the spatial variations of the wave field.

As presented during SeaSAR, ASAR measurements revealed details of the swell transformation that can be related to the underlying bathymetry and/or coastal currents. Encouraging results have been obtained to produce SAR-derived maps of significant wave heights at a resolution of about 1 km, to be combined with simple ray-tracing models.

Mariangela D’Acunto | alfa
Further information:
http://www.esa.int/esaEO/SEMKZSMVGJE_economy_0.html

More articles from Ecology, The Environment and Conservation:

nachricht Conservationists are sounding the alarm: parrots much more threatened than assumed
15.09.2017 | Justus-Liebig-Universität Gießen

nachricht A new indicator for marine ecosystem changes: the diatom/dinoflagellate index
21.08.2017 | Leibniz-Institut für Ostseeforschung Warnemünde

All articles from Ecology, The Environment and Conservation >>>

The most recent press releases about innovation >>>

Die letzten 5 Focus-News des innovations-reports im Überblick:

Im Focus: Highly precise wiring in the Cerebral Cortex

Our brains house extremely complex neuronal circuits, whose detailed structures are still largely unknown. This is especially true for the so-called cerebral cortex of mammals, where among other things vision, thoughts or spatial orientation are being computed. Here the rules by which nerve cells are connected to each other are only partly understood. A team of scientists around Moritz Helmstaedter at the Frankfiurt Max Planck Institute for Brain Research and Helene Schmidt (Humboldt University in Berlin) have now discovered a surprisingly precise nerve cell connectivity pattern in the part of the cerebral cortex that is responsible for orienting the individual animal or human in space.

The researchers report online in Nature (Schmidt et al., 2017. Axonal synapse sorting in medial entorhinal cortex, DOI: 10.1038/nature24005) that synapses in...

Im Focus: Tiny lasers from a gallery of whispers

New technique promises tunable laser devices

Whispering gallery mode (WGM) resonators are used to make tiny micro-lasers, sensors, switches, routers and other devices. These tiny structures rely on a...

Im Focus: Ultrafast snapshots of relaxing electrons in solids

Using ultrafast flashes of laser and x-ray radiation, scientists at the Max Planck Institute of Quantum Optics (Garching, Germany) took snapshots of the briefest electron motion inside a solid material to date. The electron motion lasted only 750 billionths of the billionth of a second before it fainted, setting a new record of human capability to capture ultrafast processes inside solids!

When x-rays shine onto solid materials or large molecules, an electron is pushed away from its original place near the nucleus of the atom, leaving a hole...

Im Focus: Quantum Sensors Decipher Magnetic Ordering in a New Semiconducting Material

For the first time, physicists have successfully imaged spiral magnetic ordering in a multiferroic material. These materials are considered highly promising candidates for future data storage media. The researchers were able to prove their findings using unique quantum sensors that were developed at Basel University and that can analyze electromagnetic fields on the nanometer scale. The results – obtained by scientists from the University of Basel’s Department of Physics, the Swiss Nanoscience Institute, the University of Montpellier and several laboratories from University Paris-Saclay – were recently published in the journal Nature.

Multiferroics are materials that simultaneously react to electric and magnetic fields. These two properties are rarely found together, and their combined...

Im Focus: Fast, convenient & standardized: New lab innovation for automated tissue engineering & drug

MBM ScienceBridge GmbH successfully negotiated a license agreement between University Medical Center Göttingen (UMG) and the biotech company Tissue Systems Holding GmbH about commercial use of a multi-well tissue plate for automated and reliable tissue engineering & drug testing.

MBM ScienceBridge GmbH successfully negotiated a license agreement between University Medical Center Göttingen (UMG) and the biotech company Tissue Systems...

All Focus news of the innovation-report >>>

Anzeige

Anzeige

Event News

“Lasers in Composites Symposium” in Aachen – from Science to Application

19.09.2017 | Event News

I-ESA 2018 – Call for Papers

12.09.2017 | Event News

EMBO at Basel Life, a new conference on current and emerging life science research

06.09.2017 | Event News

 
Latest News

Comet or asteroid? Hubble discovers that a unique object is a binary

21.09.2017 | Physics and Astronomy

Cnidarians remotely control bacteria

21.09.2017 | Life Sciences

Monitoring the heart's mitochondria to predict cardiac arrest?

21.09.2017 | Health and Medicine

VideoLinks
B2B-VideoLinks
More VideoLinks >>>