Waspie_Dwarf
May 17 2007, 12:47 AM
Huge waves that hit Reunion Island tracked from space
16 May 2007
Animation of observed swell generated by a storm south of Cape Town, South Africa, on 9 May 2007 propagating across the Indian Ocean. Colour correlates to the wavelength of the waves tracked by the Envisat ASAR Wave mode, with red being the longest.
Credits: IFREMER - BOOST Technologies
The origin and movement of waves reaching up to 11 metres that devastated France’s Reunion Island in the Indian Ocean on Saturday evening have been detected with ESA’s Envisat satellite.
The waves that thrashed the southern port of Saint Pierre, leaving two fishermen missing, causing several piers to collapse and flooding several homes and businesses, originated south of Cape Town, South Africa, and travelled northeast for nearly 4000 km over a period of three days before slamming into Reunion Island.
Dr Bertrand Chapron of IFREMER, the French Research Institute for Exploitation of the Sea, and Dr Fabrice Collard of France's BOOST Technologies in Brest located and tracked the swells using standard processed Synthetic Aperture Radar (SAR) ESA products.
"Swells are still surprise factors, which can unfortunately be deadly," Chapron said. "The SAR Wave Mode product allows us to locate and systematically track swells globally. In the near future we anticipate using SAR wave data to predict their arrival time and intensity."
Although waves were forecast to hit Reunion Island, their intensity was predicted to be only a couple of metres, Collard explained.
Satellite observation of the storm winds at the origin of the huge waves that thrashed Reunion Island. The left panel shows the satellite wind speed in colour, and the right panel shows the corresponding atmospheric model wind speed at the same location and time.
Credits: ECMWF, NASA Quikscat
"Because strong swells are preceded by calm water, it is impossible to detect their arrival from shore," Collard said. "SAR is the swell instrument and can typically observe swell periods in the range of 12 to 25 seconds."
A larger wave period correlates to a more extreme wind event. The one that hit Saint Pierre, Reunion Island, had a 19-second range and initially originated from very intense storm winds on 8 May.
Approaching the coastline, the wave system slows down and individual waves increase to reach at least two times the mean average of their initial wave height easily. For instance, a 5-metre significant wave height system can hit the coast with the height of 10 metres.
Chapron and Collard are working on a project that will make data for global swells available to scientists and users by the end of the year as a demonstration. The products will be useful for weather centres to complement the accuracy of their sea forecast models.
Envisat is equipped with an advanced version of the SAR instrument, Advanced Synthetic Aperture Radar (ASAR), flown on the ERS-1 and ERS-2 missions. Its wave mode acquires 10 by 5 km small images, or 'imagettes', of the sea surface every 100 km along the satellite orbit. These small ‘imagettes’, which depict the individual wave heights, are then mathematically transformed into averaged-out breakdowns of wave energy and direction, called ocean-wave spectra, which ESA makes available to scientists and weather centres.
A typical SAR satellite images a swath of 400 km, enough to capture complete 'mesoscale' phenomena such as tropical storms. While optical satellite images show the swirling cloud-tops of a hurricane, a SAR image pierces through the clouds to show the sea surface roughness and its modulation through the combination of wind wave and currents.
Click image for larger version
Credits: ECMWF, NASA QuikscatWaves of all wavelengths and travelling in several directions are generated by storms. Upon leaving the storm, they disperse and those with the longest wavelengths travel the fastest. During the Envisat Symposium held in Montreux, Switzerland, from 23 to 27 April 2007, Chapron and Collard presented for the first time a demonstration of swell tracking across the Pacific Ocean over a period of 12 days using Envisat ASAR Wave Mode.
The same swell tracking was applied for the identification of the waves that hit Reunion Island last weekend. Independent satellite wind observations confirmed the position of the storm as the origin of the huge waves.
As part of the Global Monitoring for Environment and Security (GMES), a joint initiative of the European Commission and ESA, the space agency has undertaken the development of Sentinel-1, a European polar-orbiting satellite system for the continuation of SAR operational applications. The Sentinel-1 SAR instrument will have a dedicated wave mode allowing the Near Real Time tracking and forecasting of swell for European users.Source: ESA - News
Waspie_Dwarf
May 31 2007, 03:18 PM
Huge waves from one storm slam coasts some 6000 km apart
30 May 2007
Huge waves that struck Reunion Island and coastlines across Indonesia earlier this month all originated from the same storm that occurred south of Cape Town, South Africa, and were tracked across the entire Indian Ocean over a nine-day period by ESA’s Envisat satellite.
Credits: IFREMER - BOOST Technologies
Huge waves that struck Reunion Island and coastlines across Indonesia earlier this month all originated from the same storm that occurred south of Cape Town, South Africa, and were tracked across the entire Indian Ocean for some 10 000 kilometres over a nine-day period by ESA’s Envisat satellite.
Waves reaching up to 11 metres devastated France’s Reunion Island in the Indian Ocean when it slammed into the southern port of Saint Pierre on 12 May. Six days later waves created from the same storm measuring as high as seven metres began crashing into Indonesia coastlines from Sumatra to Bali, killing at least one person and causing some 1200 people to flee their homes.
Dr Bertrand Chapron of IFREMER, the French Research Institute for Exploitation of the Sea, and Dr Fabrice Collard of France's BOOST Technologies in Brest located and tracked the swells using standard processed Synthetic Aperture Radar (SAR) ESA Wave Mode products, as shown in the animation above.
"The extreme swell systems originated from the same storm, which moved rapidly and had two main strong wind periods," Chapron said. "As illustrated in the animation, the resulting waves were organised into two main swell systems that followed each other across the entire Indian Ocean, hitting Reunion Island, Mauritius, Australia and Indonesia."
Fishermen race against high waves to reach to safety in Denpasar, on Bali island, after a large swell combined with a higher than normal tide hit the coastlines of the island on 18 May 2007.
Credits: AFP
The waves that hit Reunion Island were forecasted, but their intensity was predicted to be 20 to 30% below measurements, Collard explained.
"Although swells are still surprise factors, these particular swells were created by natural events so they could be tracked," Chapron said. "By using the SAR Wave Mode product, we can locate and systematically track swells globally, making it possible to put a network of early warning systems in place in the near future."
"Because of its unique capacity to restitute wavelength and directional information of the propagation of swells, the SAR instrument is about to bring a remarkable contribution to the monitoring of energetic wave systems," Collard said.
Chapron and Collard are working on a global swell-tracking project, which was presented for the first time at the Envisat Symposium held in Montreux, Switzerland, from 23 to 27 April 2007, using the Advanced Synthetic Aperture Radar (ASAR) aboard Envisat to follow these waves in order to refine their propagation paths and determine their arrival times and intensities.
Picture taken on 14 May 2007 in Saint-Leu, on the French Indian Ocean island of Reunion, of a giant wave breaking on the coast.
Credits: AFP
Once in place, this system will be the equivalent of deploying a global network of virtual buoys that are able to detect and track large swell systems carrying large energy from all available remote sensing measurements of waves, such as SAR and radar altimetry.
Each virtual buoy will have the capacity to detect and measure the wavelength and the direction of propagation as well as the height of the swell systems crossing the oceans, complementing the sea forecast models used by weather centres and allowing alarms to be raised a few hours before these devastating swells hit coasts.
Storms are capable of generating waves of different wavelengths that travel in several directions upon leaving the storm system, with the longest wavelengths travelling the fastest. As these waves cross open seas, they can accumulate energy at precise locations and become very dangerous for marine safety.
In addition, wave systems slow down as they approach the coastline, and individual waves increase to reach at least two times the mean average of their initial wave height. For instance, a 5-metre significant wave height system can hit the coast with the height of 10 metres.
Sentinel-1 artist's impression
Credits: ESA - P. Carril
Today, the ASAR Wave Mode acquires 10 by 5 km small images, or 'imagettes', of the sea surface every 100 km along the satellite orbit. These small ‘imagettes’ are then mathematically transformed into averaged-out breakdowns of wave energy and direction, called ocean-wave spectra, which ESA makes available to scientists and weather centres.
As part of the Global Monitoring for Environment and Security (GMES), a joint initiative of the European Commission and ESA, the space agency has undertaken the development of Sentinel-1, a European polar-orbiting satellite system for the continuation of SAR operational applications.
The Sentinel-1 SAR instrument will also have a dedicated enhanced Wave Mode capability to double the present ASAR Envisat Wave Mode samplings, to improve the actual demonstrations for a Near Real Time tracking and forecasting of swell for European users.
ESA and IFREMER will sponsor an ocean wave data user workshop at IFREMER in Brest, France, on 20 and 21 September 2007 that will address and capture user requirements for a possible future wave data service.
In January 2008, ESA will host its second SAR oceanography workshop, SEASAR 2008, entitled "Advances in SAR Oceanography from Envisat and ERS missions," at ESRIN, ESA’s Earth Observation centre, in Frascati, Italy.Source: ESA - Observing The Earth
truethat
May 31 2007, 03:29 PM
wow those are amazing! Thanks for posting them.
contactismade
May 31 2007, 03:35 PM
nice pics thanks
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