Abstract
Abstract. The impact of assimilating simulated wide-swath altimetry observations from the upcoming Surface Water and Ocean Topography (SWOT) mission is assessed using observing system simulation experiments (OSSEs). These experiments use the Met Office 1.5 km resolution North West European Shelf analysis and forecasting system. In an effort to understand the importance of future work to account for correlated errors in the data assimilation scheme, we simulate SWOT observations with and without realistic correlated errors. These are assimilated in OSSEs along with simulated observations of the standard observing network, also with realistic errors added. It was found that while the assimilation of SWOT observations without correlated errors reduced the RMSE (root mean squared error) in sea surface height (SSH) and surface current speeds by up to 20 %, the inclusion of correlated errors in the observations degraded both the SSH and surface currents, introduced an erroneous increase in the mean surface currents and degraded the subsurface temperature and salinity. While restricting the SWOT data to the inner half of the swath and applying observation averaging with a 5 km radius negated most of the negative impacts, it also severely limited the positive impacts. To realise the full benefits in the prediction of the ocean mesoscale offered by wide-swath altimetry missions, it is crucial that methods to ameliorate the effects of correlated errors in the processing of the SWOT observations and account for the correlated errors in the assimilation are implemented.
Highlights
Satellite altimeter observations of the sea surface height (SSH) have been available for over 25 years and are routinely assimilated in operational global ocean analysis and forecasting systems (e.g. Waters et al, 2015; Oke et al, 2015; Lellouche et al, 2018)
We will detail the impact of assimilating the simulated Surface Water and Ocean Topography (SWOT) observations by comparing each experiment with the truth provided by the full 4-D fields from the nature run
A nature run using different initial conditions and atmospheric forcing to the observing system simulation experiments (OSSEs) provided the truth against which the OSSEs were assessed
Summary
Satellite altimeter observations of the sea surface height (SSH) have been available for over 25 years and are routinely assimilated in operational global ocean analysis and forecasting systems (e.g. Waters et al, 2015; Oke et al, 2015; Lellouche et al, 2018). Satellite altimeter observations of the sea surface height (SSH) have been available for over 25 years and are routinely assimilated in operational global ocean analysis and forecasting systems The global coverage of a constellation of altimeters provides constraints on the modelled SSH and geostrophic velocity field, including the position and strength of mesoscale eddies. The complementarity of satellite altimetry and in situ measurements of the temperature and salinity was demonstrated by Lea et al (2014), where it was shown that assimilating SSH observations improves the model forecast of other variables even where more direct measurements are available. Various studies have attempted to quantify the impact of different numbers of standard along-track (nadir) satellite altimeters on ocean analysis and forecasting systems.
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