Using ERA5 meteorological reanalysis to improve the Dynamic Atmospheric Correction for altimetry

Loren Carrere (CLS, France)

CoAuthors

Marine Lievin (CLS, France); Quentin Dagneaux (CLS, France); Cécile Kocha (CLS, France); Florent Lyard (LEGOS, France); Damien Allain (LEGOS, France); Yannice Faugère (CLS, France); Gérald Dibarboure (CNES, France); nicolas Picot (CNES, France)

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

Session: Tides, internal tides and high-frequency processes

Presentation type: Type Forum

Given its current accuracy and maturity, altimetry is considered a fully operational observing system dedicated to various applications such as operational oceanography and climate studies. Altimeter measurements are corrected for several geophysical effects in order to isolate the oceanic variability, and the Dynamic Atmospheric Correction (DAC) is the second most important one after the tide correction. The DAC allows for the removal of high frequency ocean variability induced by the atmospheric forcing and aliased by the altimetric measurements. The accuracy of the DAC has been much improved over the last 25 years leading to centimetric accuracy in open ocean. However significant errors still remain mostly in shallow waters and in polar regions, due to bathymetric errors, to atmospheric forcing errors, to local lack of resolution of the grid ...
In this context, several ways of improvements have been tested in the recent years (cf results presented at Chicago OSTST, 2019), and a new DAC reprocessing has been performed in 2020 to improve the performances of the correction on the older altimeter missions and to homogenize the time series on the entire altimetric period for mean sea level, climate and mesoscale applications. Several improvements have been taken into account including a new bathymetry, a new model version (TUGO instead of MOG2D) and a new atmospheric forcing (ERA5 ECMWF reanalysis recently available).
Several validation analyses have been performed using multi-mission (Topex-Poséidon, Jason-1, Jason-2, Jason-3, ENVISAT, ALTIKA, CRYOSAT-2) analysis of crossovers differences (SSH) and sea level anomalies (SLA) and also some global and regional mean sea level estimations. The results show that the new DAC-ERA5 allows a significant variance reduction of the residual signal on the first years of altimetry (for mesoscale studies) and has a strong impact on regional MSL estimations. Moreover, on most recent years, DAC-ERA5 still allows reducing the sea level variance compared to the operational DAC, likely thanks to the use of the new bathymetry and the 1-hour time sampling of ERA5 dataset compared to the 6-hours sampling of the operational ECMWF analysis. The new DAC-ERA5 product is used for the computation of the new DUACS L2P reprocessing currently in production.
 
Loren Carrere
CLS
France
lcarrere@groupcls.com