Sentinel-2A (and Landsat-8) capture a giant ice avalanche in Tibet

After reading my previous post about the Rutog ice avalanche, my distinguished colleagues Antoine R. and Olivier H. challenged me to look for a pre-event image to better highlight the avalanche area. The closest clear-sky image that I could find is a Landsat-8 image that was acquired on June 24 (23 days before the slide).

 

Sequence of two Landsat-8 and Sentinel-2A images. Both images are level 1 product displayed as natural color composites. Click to enlarge.

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Sentinel-2A captures a giant ice avalanche in Tibet

The Nature News website reported yesterday on a massive ice avalanche that happened in Rutog, Tibet, on 17 July 2016. This ice avalanche killed 9 people and may be one the largest ever observed. The ice and rock mixture spread over 6 km from the collapse point up to the Aru Co lake shoreline.

Sentinel-2A image of the Rutog ice avalanche acquired on 21-Jul-2016 (4 days after the event). Click on the image to see at full resolution (1 pixel = 10m).

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Snow and Fire in the Dragon mountains

NASA's blog "Image of the Day" recently featured two beautiful MODIS images of the snow cover in Lesotho. In late July Lesotho experienced its heaviest snowfall in two decades. The snow is not uncommon in Lesotho given that over 80% of the country lies above 1800 m (wikipedia). However the frequency of such snow events has been reducing over the past decades due to the ongoing climate change. As a result the shepherds are less accustomed to the snow conditions so that "a severe storm like the one in July 2016 has greater potential to kill sheep and shepherds" [1]. Continue reading

Mapping the Glacier Bay landslide using Sentinel-1 and Sentinel-2

Dave Petley wrote a nice article in the AGU's Landslide blog about a massive landslide in Glacier Bay, Alaska. This huge avalanche of debris was spotted by a local pilot Paul Swanstrom on June 28. When Paul was above it, "dust was still flying". Later, based on earthquake records in Alaska, the experts figured out that it probably happened at 8:21 am on the same day.

Photo of the the Glacier Bay landslide in Alaska by Paul Swanstrom - MountainFlyingService (click on the image to see the Facebook photo album)

 

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Mapping flooded areas using Sentinel-1 in Google Earth Engine

Exceptional rainfall in May caused heavy flooding in the Paris region. Newspapers and TVs reported that the Seine flood forced the Louvre staff to move away from rising waters the art pieces that were stored in their cellar. But they did not tell you that about 50 km east of the Louvre museum, the flood of the Grand Morin river in Coulommiers also inundated the cellar of my parents-in-law. I'm really concerned about this cellar because I care about my parents-in-law of course, and also because I have let some of my bottles of wine in their cellar. Continue reading

Sex ratio at LPS16

I did not have the chance to attend the latest ESA's Living Planet Symposium so I was curious to read Olivier's report. The picture in his post shows a crowded room... but what stroke me most is that it looks like 90% of the people in the room are middle-age men. There is no public data on the age or the sex of the participants to the LPS16. But there is a page on the LPS16 website listing all the authors.

A room at LPS16

A room at LPS16

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Série temporelle de chats

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La télédétection satellite optique est un outil formidable pour suivre l'étendue du manteau neigeux en montagne... sauf quand il y a des nuages ! La télédétection radar du manteau neigeux (indifférente aux nuages) n'est pas encore opérationnelle en zone de montagne, notamment en raison du fait que le signal rétro-diffusé par le manteau neigeux varie très fortement avec son contenu en eau liquide. Sur le plancher des vaches, en revanche, de nombreuses personnes observent le manteau neigeux, même sous un ciel couvert. Certains sont même assez gentils pour prendre des photos, les télécharger sur un site web de partage, et les mettre à disposition sous licence publique. Une bonne partie des photos est géolocalisée, soit parce que l'appareil photo est équipé d'une puce GPS, soit parce que le photographe a lui-même ajouté les coordonnées de la prise de vue lors de la publication de son album.
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Kittens time series

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Optical remote sensing is great to map the snow cover extent in mountain regions as long as there is no cloud above the land surface. Radar remote sensing of the snow cover is not operational yet mainly because the backscatter from the snow surface is strongly dependent on the snowpack liquid water content. On the ground, however, thousands of people are observing the snow cover in the mountains, everyday. Some of them take photographs and kindly upload them to photo-sharing websites with a public license. Many of these photos are geotagged, either because the cameras have built-in GPS, or because the users added geographical coordinates when publishing their album.
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First Sentinel-2 snow map

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In the framework of the THEIA land data center, we have developed a simple but robust method to map the snow cover from Sentinel-2-like level 2A products. This code was tested with SPOT-4 Take-5 and Landsat-8 series, but it remained to adapt it so that it can run on real Sentinel-2 images! This is now done thanks to Manuel Grizonnet, which allowed us to process the Sentinel-2A image acquired on 06-July-2015 in the Pyrenees as a first example. This image was produced at level 2A by Olivier Hagolle using the MACCS processor. The snow mask from Sentinel-2 images is calculated at 20 m resolution after resampling the green and red bands that are originally at 10 m resolution while the NIR band is at 20 m.

How to make sure everything went well? We can control the snow mask by superposing the mask boundaries on a false color composite:

 

The Sentinel-2A image of 06-July-2015 (level 2A, tile 30TYN) and its snow mask. The snow mask is in magenta and the background image is a color composite RGB NIR/Red/Green. We also show a zoom in the Vignemale area.

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Premier masque de neige Sentinel-2

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Dans le cadre du Centre d'expertise scientifique THEIA "surface enneigée" nous avons développé une méthode simple et robuste pour détecter la neige à haute-resolution à partir des produits de niveau 2A de type Sentinel-2. Ce code a été testé sur des séries SPOT-4 Take-5 et Landsat-8, mais il restait à l'adapter pour qu'il puisse tourner sur de vraies images Sentinel-2 ! C'est chose faite grâce à Manuel Grizonnet, ce qui nous a permis de traiter l'image Sentinel-2A du 06-juillet-2015 sur les Pyrénées. Cette image avait été produite au niveau 2A par Olivier Hagolle avec la chaine MACCS. Le masque de neige est calculé à 20 m de résolution après ré-échantillonnage des bandes vertes et rouges qui sont d'origine à 10 m de résolution alors que la bande MIR est à 20 m. Continuer à lire