The role of altimetry observations in constraining the Mercator Global Ocean analysis and forecasts

Yann Drillet (Mercator Ocean, France)

CoAuthors

Jean-Michel Lellouche (Mercator Ocean, France); Mounir Benkiran (Mercator Ocean, France); Antonio Bonaduce (Mercator Ocean, France); Mathieu Hamon (Mercator Ocean, France); Olivier Legalloudec (Mercator Ocean, France)

Event: 2017 Ocean Surface Topography Science Team Meeting

Session: Application development for Operations

Presentation type: Type Oral

Altimetry data is one of the key observing network to constrain ocean analysis and forecasts from meso scale to large scale. Since October 19, 2016, and in the framework of Copernicus Marine Environment Monitoring Service (CMEMS), Mercator Ocean delivers in real-time daily services (weekly analyses and daily 10-day forecasts) with a new global 1/12° high resolution system. The ocean modelling component is the NEMO platform driven at the surface by the IFS ECMWF atmospheric analyses and forecasts. Observations are assimilated by means of a reduced-order Kalman filter with a 3D multivariate modal decomposition of the forecast error. Along track altimeter data, satellite Sea Surface Temperature and in situ temperature and salinity vertical profiles are jointly assimilated to estimate the initial conditions for short term numerical ocean forecasting. A 3D-VAR scheme provides a correction for the slowly-evolving large-scale biases in temperature and salinity.
The current system benefit from significant improvements compared to its previous version thanks to updates such as: i) new freshwater runoff from ice sheets melting, ii) global steric effect added to the model sea level, iii) new Mean Dynamic Topography taking into account the last version of GOCE geoid, iv) adaptive tuning of altimetry and SST observational errors. The estimated observation errors for the different altimeters are greatly reduced compared to the previous system version. Those improvements lead to a better representation of the Global Mean Sea Level and regional sea level variability with reduced model forecast – observation differences and a better coherency between the in situ and satellite observation data sets assimilated in the system.

Dedicated studies are also conducted to better understand and improve the impact of altimetry data on the real time forecast and analysis. Not only an increasing number of altimeter data is improving the analysis and forecast quality but also their accuracy and their spatial coverage. The coherence between the Mean Dynamical Topography and the assimilated T and S in situ profiles is important: the new MDT (CNES-CLS 2013) allows reducing the T and S bias at depth.

Future challenges include the assimilation of large swath altimeter data. Preliminary assimilation of simulated observations show a clear benefit of 2D swath data compared to 1D track. It does not only improve the Sea Surface Height analysis and forecast but also allow a better velocity field estimation which should represent a significant benefit for Mercator Ocean and Copernicus users. The sensitivity to the complex error of those data will be assessed in the near future but the first results that will be presented are very promising.
 

Oral presentation show times:

RoomStart DateEnd Date
Symphony II Wed, Oct 25 2017,09:30 Wed, Oct 25 2017,09:45
Yann Drillet
Mercator Ocean
France
ydrillet@mercator-ocean.fr