On July 18th, 2017, the Copernicus Marine Environment Monitoring Service (CMEMS) released the first real-time global wave product based on satellite data. It came after the launch last April (2017) of the wave product based on models, and is part of a series of releases listed in the WAVE product roadmap (see figure below). This new product from satellite altimeter data contains the Significant Wave Height (SWH) from Jason-3 and Sentinel-3A satellite altimeter data, provided within 3 hours after data acquisition. This new satellite wave product is available in the CMEMS catalogue (see here). It is also assimilated into the CMEMS numerical real-time Global Wave Models (see here) and allows CMEMS to provide wave forecasts with better accuracy.
This new global wave product based on satellite altimetry data is produced for CMEMS by their Sea Level Thematic Assembly Center (SL TAC), contract with Mercator Ocean (MO). This SL TAC is led by the private company CLS in collaboration with IFREMER (both located in France).
Dr. Romain Husson, responsible for wave products at CLS, tells us more about this new product built for CMEMS.
MO: What new insight is brought about by this new global satellite wave product?
Husson: It is the first time the Copernicus Marine Service (CMEMS) offers a wave product based on satellite data. Before, the CMEMS portfolio had only gathered data for wave products from models. The wave portfolio has now been enlarged (since July 2017) with the addition of this wave product based on satellite data.
In physical oceanography, the Significant Wave Height (SWH) is defined traditionally as the mean wave height (trough to crest) of the highest third of the waves. This mathematical definition of ocean wave height is intended to express the height that would be estimated by a trained observer, capturing the most significant waves over the water surface.
Satellite wave measurements come from two main techniques, altimetry and Synthetic Aperture Radar (SAR). The SWH can be found with altimetry and directional and spectral information with SAR. This new product (released in the summer of 2017) is based only on satellite altimetry data and includes wave products deduced from Jason-3 and Sentinel-3A altimeters, launched in January and February of 2016, respectively. This new product provides quality-filtered and inter-calibrated along-track high-resolution SWH (one measurement every 7 km, or every second). These measurements provide global ocean coverage, following satellite ground tracks with a 7km resolution.
These satellite wave products provide the actual measurements of the waves, over the whole Earth, regularly and homogeneously over several years. They often offer a better description of extreme events, which numerical models tend to under-estimate. In situ wave data, typically provided by buoys, are similarly very helpful but in many areas of open water, there are no such buoys available.
This new satellite wave product (as seen in Figure 1) is available in the CMEMS Catalogue (see here).
This new satellite wave product is also assimilated into the CMEMS numerical real-time Global Wave Models (see here). The added value of model products is that they can provide global coverage of SWH with a forecast capacity of a few days. Measurements either from satellites or in situ technology, contribute to the improvement of forecast accuracy thanks to real-time assimilation. Hence this new satellite wave product contributes to the improvement of forecast accuracy in the Global Numerical Wave Products in the CMEMS Catalogue.
My colleague Lotfi Aouf from Météo France operates the Global Ocean Wave Model for CMEMS. He is using our new satellite wave product for data assimilation in his global wave model. For example, the analysis increment of SWH after 1-day of assimilation of Sentinel-3A wave data in the Global Ocean Wave Model is provided in Figure 2. The maximum impact can reach more than 1.5 meters (see Figure 2). A strong impact is indicated in the North-West of the Pacific Ocean related to the typhoon season and in the Gulf of Mexico after the passage of the Hurricane Harvey.
MO: What are some interesting applications of this product?
Husson: High waves are dangerous, and not only extreme wave conditions can threaten delicate operations at sea. Wave information is crucial for all people at sea who need to know about past, present and future sea conditions. For oil & gas offshore platform operations, historic data and the forecasts of wave heights are vital for the safety of personnel, equipment and the environment. Similarly, marine renewable energy operations and site studies need wave information. Also, ship routing can be improved by such forecasts. The various sources, satellite, insitu and models, provide complementary wave products for such applications.
MO: What will come next?
Husson: Next December (2017), the satellite altimeter wave products will be extended to the remaining altimeter missions (i.e. Jason-2, Saral), which will improve the global coverage of the altimeter wave products. A new global and gridded product (level 4) will also be provided, containing the daily mean of the SWH. In addition, we will also deliver wave products derived from the Sentinel 1-A and 1-B SAR (Synthetic Aperture Radar) instrument. With respect to the altimetry, the SAR has the unique ability to measure the wave period and direction on top of the SWH and is particularly well suited for long waves (also sometimes referred to as swells). Such a product will enable CMEMS to make more detailed descriptions of the wave conditions, such as cross sea conditions (waves coming from different, oblique directions), which are particularly dangerous in navigating situations. This new product will provide ocean swell information on a daily basis both on irregular (level 3) and regular (level 4) grids.