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Case Study: Using Coastal Altimetry as Virtual Tide gauges

In recent years there has been much interest in coastal altimetry. This is the technique of retrieving altimetry measurements close to shore, which had previously been rejected as being corrupted by land interference, or because some of the corrections on which the measurements depend were not accurate enough in the coastal zone.

Things are rapidly improving with the development of specialized techniques and corrections to recover useful data in the coastal zone, and eSurge features its own coastal altimetry processor, built over the experience gathered for Envisat in the COASTALT Project, and improved with the novel ALES algorithm.

As coastal altimetry gives the total water level close to shore, storm surges are a natural application for these techniques, allowing measurements of sea heights where no in-situ data are available.  

To take a specific example, during the Xaver (also known as Bodil) storm of December 2013, ESA's Cryosat-2 spacecraft passed over the Kattegat on the night on 6th December, just as the storm surge was passing through the straits, measuring the water level with its SIRAL instrument. The resulting water level profile (right) clearly shows the water piling up north of Funen island.

This profile closely matches that expected from numerical modelling, as shown in the image on the left, which shows Cryosat's measurements against the DMI model of the surge. This in turn has been validated against tide gauge measurements.

In the case of the Kattegat there are many tide gauges which measure a surge. (Though even here altimetry can bring benefits when blended with in situ data; see here.) However in many parts of the world such in-situ measurements are lacking. If coastal altimetry can be used to measure a surge, this can be valuable confirmation of a model's accuracy. 

An example is cyclone Mahasen (right), which made landfall in India in May 2013. Cryosat was able to measure water levels around the coastline as Mahasen approached, showing the water profile at different stages.

Unfortunately however Cryosat did not get a direct overpass of the cyclone. This is a known problem with using coastal altimetry: there are too few altimeters to always guarantee a direct hit on an event of interest. However this will become less of an issue as more altimeters are added to eSurge's ALES processor, and also once the Sentinel-3 altimeters become available.

The coastal altimetry community has also looked at many other surge events, including Hurricanes Katrina, Isaac and Sandy. More examples are given in this presentation from Paolo Cipollini of NOC.

For more information, see: