Generation of 2020 Hydrology Targets databases for Sentinel-3A, 3B and Jason-3/Sentinel-6 OLTC

Denis Blumstein (CNES / LEGOS, France)

CoAuthors

Léa Lasson (Ocean Next / LEGOS, France); Sophie Le Gac (CNES, France)

Event: 2020 Ocean Surface Topography Science Team Meeting (virtual)

Session: Science IV: Altimetry for Cryosphere and Hydrology

Presentation type: Type Forum

The monitoring of water resources at the global scale is already a major challenge whose importance will increase in the next years. Continental waters are essential components of the water, energy and carbon cycles. The rivers and lakes water levels are identified as essential climate variables by the GCOS. Traditionally, this information was provided by gage data. However, the availability of these in situ measurements is very heterogeneous and is declining (GRDC).
For the last twenty years, numerous research teams have demonstrated that measurements from space altimeters could be used to mitigate this lack of in situ measurements. However, it is well known that the altimeters, being primarily designed and optimized to observe the oceans, can have some difficulties to track the surface of the inland water bodies especially in cases where the surrounding topography is rugged. In the past, this issue has limited the number of available measurements with respect to what could have been potentially acquired.
However, lots of progresses have been made on this subject in the last four years with the now operational use of a new tracking mode on recent radar altimeters (on the Jason3 and Sentinel3 satellites). These altimeters can be operated in two tracking modes: the classic Close Loop (CL) mode and the Open Loop (OL) mode. In OL mode the position of the tracking window is no more defined autonomously by the altimeter but is imposed by internal tables that are built from mean sea surface over ocean and a database of (mainly) hydrology targets over land. This database provides position, approximate elevation and width of the water bodies. The OL tracking mode improves the number of available measurements for hydrology and their accuracy.
Recently, the methods have been refined and extended at LEGOS, so that the number of Hydrology Targets has increased a lot. In 2020, 74 000 for Sentinel-3A (S3A), 72 000 for Sentinel-3B (S3B) and 31 000 for Jason-3 and Sentinel-6 have been defined as well. Targets at high latitudes (above 60°N) have now been defined. In addition, a value of elevation is now available (almost) everywhere above continental surfaces even when no water was identified in the databases of lakes, reservoirs and rivers used as input for the generation of our hydrology targets database.
These databases were delivered to CNES in March and April 2020 for conversion into OLTC tables used by the onboard software of the altimeter. In this presentation we will highlight the improvements brought by these new versions with respect to the old ones (2017 for Jason-3, 2018 for Sentinel-3) and provide a short description of the methods used and the quality control. And finally we conclude with the current plans for improving the current database for hydrology in the next few years in preparation for the future mission SWOT.
 
Denis Blumstein
CNES / LEGOS
France
denis.blumstein@free.fr