Validating Sargassum forecasts with Sentinel-2 data

11 June 2019
Category: Sargassum

Since April 1st, a group of 40+ users have been testing the Sargassum detection and monitoring service developed by CLS and Novablue Environment within the framework of an ESA Open Call Science for Society project. One of these users, Juerg Lichtenegger, Switzerland, uses ESA EO Open Data from Sentinel-2 and -3 (optical) satellites to evaluate and validate CLS Group’s sargassum drift forecasts and help CLS improve their service. Learn more about the drift model and the results in the article below.

How does the CLS Group sargassum detection and drift model work?

To detect sargassum, the CLS service relies on a unique combination of ocean color sensors (OLCI on Sentinel-3 and MODIS on Aqua) and high resolution optical sensors on Sentinel-2 and Landsat-8. These sensors make it possible to calculate an NFAI (Normalized Floating Algae Index), inherited from references works of Jim Gower (MCI) and Chuan Min Hu (AFAI) but using different wavelengths to obtain the highest possible pixel resolution for each sensor.

Our drift model uses surface current models such as HYCOM and CMEMS (depending on the region) and wind speed data from ECMWF/NCEP. Sargassum mats detected from satellite are advected with MOBIDRIFT model (Lagrangian advection model) using a probabilistic approach to take into account the uncertainties on wind and current forcing fields). This allows us to produce 2-3 day forecasts via the online user interface in 11 user-dedicated zones in the Caribbean and Gulf of Mexico. The resolution of the model currently being used is 10 km. CLS is working to improve the resolution of our model in order to provide fine-meshed forecasts in the near future.

Are the forecasts reliable?

The work of Juerg Lichtenegger helps CLS to validate the drift model with free high-resolution optical imagery (Sentinel-2 and Landsat-8). CLS and Novablue Environment are also working with a number of users in the Caribbean and Gulf of Mexico to obtain photos from local sources to confirm the forecasts.

Photo courtesy of Jean-Philippe Maréchal

barbados sargassum — **Barbados, June 3rd.** On the right: A Sentinel-2 image on June 3rd shows new sargassum landings on the beaches of central eastern coasts of Barbados (see red squares). On the left: A screenshot from the CLS SeeWater interface with small scale inlet. Black dots represent the original positions of sargassum mats detected by CLS with ESA/Sentinel-3/OLCI and the NASA/Aqua/MODIS sensors on May 28. The white lines are the projected drifts of sargassum mats based on ocean current and wind models. The red dots represent predicted sargassum landings. Yellow dots represent intermediate drifting positions on May 31. For this example, the landings mainly happened more to the south than CLS forecasted, and landings on the southeast coast are not confirmed or could not be verified because of cloud coverage.

domenica sargassum — **Dominica 22nd May.** On the left, the CLS drift model forecasts sargassum landings in many beaches on the east coast of Dominica (red dots represent sargassum landings, white lines are the projected drift trajectory). On the right, Sentinel-2 optical imagery confirms that sargassum have landed mostly on the southern part of the predicted beaches (red circles include sargassum landings, lines are drifting mats).

Free trial of the sargassum detection and forecasting service until June 15th, 2019

If you are interested in trying out this service for free (until June 15th, 2019), please submit a request for access via the following address: sargas-ops@groupcls.com

Learn more

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About CLS

CLS, a subsidiary of the French Space Agency (CNES), ARDIAN & IFREMER, is a worldwide company and pioneer provider of monitoring and surveillance solutions for the Earth since 1986. Our mission is to deploy innovative satellite-based monitoring solutions to understand and protect our planet, and to manage its resources sustainably.

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