Flooding Forecasting & Mapping

Sentinel-1 data for York overlaid in red with Pixalytics flood mapping layer based on Giustarini approach for the December 2015 flooding event. Data courtesy of ESA.

Sentinel-1 data for York overlaid in red with Pixalytics flood mapping layer based on Giustarini approach for the December 2015 flooding event. Data courtesy of ESA.

Media headlines this week have shouted that the UK is in for a sizzling summer with temperature in the nineties, coupled with potential flooding in August due to the La Niña weather process.

The headlines were based on the UK Met Office’s three month outlook for contingency planners. Unfortunately, when we looked at the information ourselves it didn’t exactly say what the media headlines claimed! The hot temperatures were just one of a number of potential scenarios for the summer. As any meteorologist will tell you, forecasting a few days ahead is difficult, forecasting a three months ahead is highly complex!

Certainly, La Niña is likely to have an influence. As we’ve previously written, this year has been influenced by a significant El Niño where there are warmer ocean temperatures in the Equatorial Pacific. La Niña is the opposite phase, with colder ocean temperatures in that region. For the UK this means there is a greater chance of summer storms, which would mean more rain and potential flooding. However, there are a lot of if’s!

At the moment our ears prick up with any mention of flooding, as Pixalytics has just completed a proof of concept project, in association with the Environment Agency, looking to improve operational flood water extent mapping information during flooding incidents.

The core of the project was to implement recent scientific research published by Matgen et al. (2011), Giustarini et al. (2013) and Greifeneder et al. (2014). So it was quite exciting to find out that Laura Guistarini was giving a presentation on flooding during the final day of last week’s ESA Living Planets Symposium in Prague – I wrote about the start of the Symposium in our previous blog.

Laura’s presentation, An Automatic SAR-Based Flood Mapping Algorithm Combining Hierarchical Tiling and Change Detection, was interesting as when we started to implement the research on Sentinel-1 data, we also came to the conclusion that the data needed to be split into tiles. It was great to hear Laura present, and I managed to pick her brains a little at the end of the session. At the top of the blog is a Sentinel-1 image of York, overlaid with a Pixalytics derived flood map in red for the December 2015 flooding based on the research published by Laura

The whole session on flooding, which took place on the last morning of the Symposium, was interesting. The presentations also included:

  • the use of CosmoSkyMed data for mapping floods in forested areas within Finland.
  • extending flood mapping to consider Sentinel-1 InSAR coherence and polarimetric information.
  • an intercomparison of the processing systems developed at DLR.
  • development of operational flood mapping in Norway.

It was useful to understand where others were making progress with Sentinel-1 data, and how different processing systems were operating. It was also interesting that several presenters showed findings, or made comments, related to the double bounce experienced when a radar signal is reflected off not just the ground, but another structure such as a building or tree. Again it is something we needed to consider as we were particularly looking at urban areas.

The case study of our flood mapping project was published last week on the Space for Smarter Government Programme website as they, via UK Space Agency, using the Small Business Research Initiative supported by Innovate UK, funded the project.

We are continuing with our research, with the aim of having our own flood mapping product later this year – although the news that August may have flooding means we might have to quicken our development pace!

Is This The Worst Global Coral Bleaching Event Ever?

Great Barrier Reef off the east coast of Australia where currents swirl in the water around corals. Image acquired by Landsat-8 on 23 August 2013. Image Courtesy of USGS/ESA.

Great Barrier Reef off the east coast of Australia where currents swirl in the water around corals. Image acquired by Landsat-8 on 23 August 2013. Image Courtesy of USGS/ESA.

It was announced last week that 93% of the Great Barrier Reef has been hit by coral bleaching due to rising sea temperatures from El Niño and climate change. We first wrote about the third worldwide coral bleaching in October 2015, noting this year’s event could be bad. Those fears would appear to be coming true with the results of Australia’s National Coral Bleaching Task Force aerial survey of 911 coral reefs which found 93% had suffered from bleaching; of which 55% had suffered severe bleaching.

Coral bleaching occurs when water stresses cause coral to expel the photosynthetic algae, which give coral their colours, exposing the skeleton and turning them white. The stress is mostly due to higher seawater temperatures; although cold water stresses, run-off, pollution and high solar irradiance can also cause bleaching.

Bleaching does not kill coral immediately, but puts them at a greater risk of mortality. Recovery is also possible if the water stress reduces and normal conditions return, which is what is hoped for in the Northern Sector of the reef above Port Douglas, where around 81% of corals had suffered severe bleaching – the water quality in this area is good, which should also aid recovery. The reefs fared better further south. Within the Central Sector, between Port Douglas and Mackay, 75 of the 226 reefs suffered from severe bleaching. Whilst in the Southern Sector below MacKay only 2 reefs suffered severe bleaching and 25% had no bleaching.

The news is not all bad. A survey of the coral reefs of the Andaman and Nicobar Islands, a territory of India that marks the dividing line between the Bay of Bengal & Andaman Sea, also published this week shows no evidence of coral bleaching. This survey is interesting for remote sensors as it was undertaken by a remotely operated vehicle, PROVe, developed by India’s National Institute of Ocean Technology. As well as mapping the coral reefs, PROVe has a radiometer attached and is measuring the spectral signatures of the coral in the area, which could be used to support the monitoring of corals from satellites.

Monitoring coral bleaching from space has been done before. For example, Envisat’s MERIS sensor was determined to be able to detect coral bleaching down to a depth of ten metres, or the Coral Bleaching Index (Ziskin et al, 2011) which uses the red, green and blue bands to measure increases in spectral reflectance in bleached corals. Given the size, geographical area and oceanic nature of corals, satellite remote sensing should be able to offer valuable support to the monitoring of their health.

Following the second global bleaching event, in 1997/98, research confirmed that 16 percent of the world’s coral died. Who knows what the outcome of the current event will be?

Jason-3 Sets Sail for the Oceanographic Golden Fleece

Artist rendering of Jason-3 satellite over the Amazon. Image Courtesy NASA/JPL-Caltech.

Artist rendering of Jason-3 satellite over the Amazon.
Image Courtesy NASA/JPL-Caltech.

The Jason-3 oceanographic satellite is planned to launch on Sunday 17th January from Vandenberg Air Force Base in California, aboard the Space-X Falcon 9 rocket. Named after the Greek hero Jason, of the Argonauts fame, Jason-3 is actually the fourth in a series of joint US-European missions to measure ocean surface height. The series began with the TOPEX/Poseidon satellite launched in 1992, followed by Jason-1 and Jason-2 which were launched in 2001 and 2008 respectively.

Jason-3 should provide a global map of sea surface height every ten days, which will be invaluable to scientists investigating circulation patterns and climate change.

The primary instrument is the Poseidon-3B radar altimeter, which will measure the time it takes an emitted radar pulse to bounce off the ocean’s surface and return to the satellite’s sensor. Pulses will be emitted at two frequencies: 13.6 GHz in the Ku band and 5.3 GHz in the C band. These bands are used in combination due to atmospheric sensitivity, as the difference between the two frequencies helps to provide estimates of the ionospheric delay caused by the charged particles in the upper atmosphere that can time delay the return.

Once the satellite has received the signal reflected back, it will be able to use its other internal location focussed instruments to provide a highly accurate measurement of sea surface height. Initially the satellite will be able to determine heights to within 3.3cm, although the long-term goal is to reduce this accuracy down to 2.5cm. In addition, the strength and shape of the return signal also allows the determination of wave height and wind speed which are used in ocean models to calculate the speed and direction of ocean currents together the amount and location of heat stored in the ocean.

In addition, Jason-3 carries an Advanced Microwave radiometer (AMR) which measures altimeter signal path delay due to tropospheric water vapour.

The three location focused instruments aboard Jason-3 are:

  • DORIS (Doppler Orbitography and Radiopositioning Integrated by Satellite) – Uses a ground network of 60 orbitography beacons around the globe to derive the satellite’s speed and therefore allowing it’s precise position in orbit to be determined to within three centimetres.
  • Laser Retroreflector Array (LRA) – An array of mirrors that provide a target for laser tracking measurements from the ground. By analysing the round-trip time of the laser beam, the satellite’s location can be determined.
  • Global Positioning System – Using triangulation from three GPS satellites the satellites exact position can be determined.

The importance of extending the twenty-year time series of sea surface measurements cannot be underestimated, given the huge influence the ocean has on our atmosphere, weather and climate change. For example, increasing our knowledge of the variations in ocean temperature in the Pacific Ocean that result in the El Niño effect – which have caused coral bleaching, droughts, wet weather and movements in the jet stream in 2015, and are expected to continue into this year – will be hugely beneficial.

This type of understanding is what Jason-3 is setting sail to discover.

El Niño causing Coral Bleaching

Variations in Pacific Ocean sea surface height compared to a long term average for the 2015 and 1997/98 El Niño events. Courtesy NASA/JPL-Caltech

Variations in Pacific Ocean sea surface height compared to a long term average for the 2015 and 1997/98 El Niño events. Courtesy NASA/JPL-Caltech

Coral reefs are currently undergoing their third worldwide bleaching event linked to the El Niño effect in the last 20 years, and scientists fear this one could be the worst.

El Niño is the warm phase of the El Niño Southern Oscillation and is associated with a band of warm water that develops in the central and east-central equatorial Pacific. This means waters in the Pacific Ocean are nutrient-poor, and are accompanied by high air pressure in the western Pacific and low air pressure in the eastern Pacific. This causes disruption to weather patterns worldwide including: droughts in Indonesia and Australia, and altering the path of the atmospheric jet stream over America.

Coral reefs cover less than 1% of the earth’s surface, but are some of the most valuable, diverse and vulnerable ecosystems on the planet. Reef building corals thrive in water temperatures between 73° and 84° Fahrenheit, but struggle outside of this range. Climate change is providing a challenge, and when you add on a warming effect like El Niño, the danger for coral reef ecosystems is clear. Worldwide coral bleaching events have occurred in 1997/98 and 2009/10, both of which were El Nino years; and the fear is that this year’s event, which will extend into 2016, could be significant.

Warmer water stresses the coral causing them to expel the photosynthetic algae, called zooxanthellae, living in their tissues. This turns them completely white, hence the term bleaching. Although this does not kill the coral immediately, it does put them at greater risk of dying. For example, half the coral reefs in the Caribbean were lost following a local bleaching event in 2005 – a weak El Niño year.

Satellite data has provided a valuable source of data to monitor the changes in coral reefs. For example, the French Centre National d’etudes Spatiales (CNES) and the USA’s National Aeronautics and Space Administration (NASA), began a mission in 1987 to monitor global ocean changes including measuring sea height by radar altimetry. It began with the TOPEX/Poseidon mission launched in 1992, which provided major data on the way the El Niño effect operated. This was followed Jason-1, launched in 2001, and Jason-2 in 2008. The value of this type of data to monitoring effects like El Niño can be seen at the top of the blog that shows side by side the variations in the Pacific Ocean sea surface height compared to a long term average for the 2015 El Niño and the strong event of 1997/98, with data collected by TOPEX/Poseidon for 1997 and the OSTM/Jason-2 for 2015. Further images and animations from NASA/JPL-Caltech can be found here.

Coral reef ecosystems are a source of food, protection against coastal erosion and provide spawning and nursing grounds for fish. They also provide jobs through fishing and tourism, and are estimated to contribute $29.8 billion to the global economy every year. However, scientists estimate that between 40 and 50% of corals worldwide have been destroyed or lost in the last 50 years. They expect this decline to continue, which could have significant consequences to the human and marine populations that are dependent on them.