Dynamic Atmospheric Correction dedicated to SWOT CalVal Phase

The main objective of the Dynamic Atmospheric Correction (DAC) is to remove the high-frequency ocean dynamic signals forced by the atmosphere (pressure and wind) and aliased by the altimetric sampling.

The current operational DAC is composed from the high frequencies of a barotropic ocean model, forced by ECMWF operational wind and pressure fields and the low frequencies of the IB (Inverse Barometer, Carrere and Lyard, 2003). The cut-off of the operational filtering is 20 days and corresponds to the Nyquist frequency of the reference TP/Jason altimeters which have a 10-days cycle.

The SWOT CalVal phase has a 1-day revisit cycle and thus only ocean variability below 2 days period is aliased in longer periods. In this case, the operational DAC will remove much more variability than only the aliased part. To fulfill the primary DAC objective and focus on the aliased ocean signals, a specific DAC has been computed for the SWOT CalVal phase.

Main characteristics of the DAC for SWOT CalVal phase:

  • Available on the period 01/01/2023 00h – 11/07/2023 18h
  • Use the same barotropic model as the operational DAC: MOG2D.
  • Use the same model parameters as the operational DAC.
  • Use the same atmospheric forcing as the operational DAC: 6-hours ECMWF operational analysis.
  • Use a 2 days filtering cut-off for IB and sea level (instead of 20 days for operational DAC).
  • Filtered sea level grids are extrapolated on 2 pixels on the coasts to ensure a better coastal coverage (only 1 pixel for the operational DAC).
Figure 1 : Example of a MOG2D sea level map extrapolated on the coasts.
Figure 2 : Standard deviation of the differences between the operational DAC (20-days cutoff) and the SWOT CalVal phase specific DAC (2-days cutoff) in cm.
Figure 3 : Variance reduction of the 1-day SWOT Nadir SLA when using the SWOT CalVal phase specific DAC (2 days cutoff) instead of the IB, in cm². Mean global variance reduction is -0.67 cm².

References

F. Lyard, L. Carrere, M. Dabat, M. Tchilibou, E. Fouchet, Y. Faugère, G. Dibarboure and N. Picot (2023), Barotropic corrections for SWOT : FES2022 and DAC, oral presentation at SWOT meeting 09/2023.

Carrère, L., and Lyard, F. (2003), Modeling the barotropic response of the global ocean to atmospheric wind and pressure forcing - comparisons with observations, Geophys. Res. Lett., 30, 1275, https://doi.org/10.1029/2002GL016473 , 6.

 

Back to product page of the DAC for the SWOT Calval phase

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