CERA-SAT: A coupled satellite-era reanalysis

Schepers, Dinand ; Boisséson, Eric de ; Eresmaa, Reima ; Lupu, Cristina ; Rosnay, Patricia de

Année de publication
2018
Résumé
ECMWF has completed the production of a new research reanalysis, CERA-SAT, which reconstructs the state of the atmosphere, the ocean, sea ice, ocean waves and the land surface between 2008 and 2016. Reanalyses are produced by combining models with observations in a process called data assimilation. The same technique is also used in numerical weather prediction (NWP) to establish the initial conditions on which forecasts are based. CERA-SAT demonstrates the application of coupled data assimilation in the satellite era. In this context, coupling means that atmospheric, ocean, sea-ice, ocean-wave and land-surface observations are assimilated in a consistent manner. First assessments show that coupling as implemented in CERA-SAT improves the quality of the reanalysis in the tropics but degrades it in the extratropics. The deterioration in the extratropics is believed to be caused by shortcomings in the representation of boundary currents in the coupled model. Insights gained from assessing the performance of CERA-SAT will be used to develop coupled data assimilation in operational weather forecasting and the next generations of ECMWF's reanalyses. Accounting for coupled processes between Earth system components is beneficial for weather forecasting, a clear example of which is tropical cyclogenesis. For climate assessment purposes, introducing coupling makes it possible to diagnose the key role that the oceans play in storing and transporting heat energy. Heat that is absorbed by the upper ocean eventually affects other components of the Earth system by melting ice shelves, increasing evaporation at the ocean surface, or directly reheating the atmosphere.

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