Altimeter Sea Surface Height

The along-track altimeter data undergo an extensive series of pre-processing steps to prepare the data for use in the assimilation. The measured sea surface height

(SSH) is corrected for geophysical effects (wet and dry troposphere, ionosphere, inverted barometer, and winds), and the tidal signal is removed. The corrected SSH from each satellite altimeter mission are then intercalibrated with a global crossover adjustment using Topex/Poseidon data as the reference. Next, the data are resa-mpled every 7 km (1 sec intervals) along the tracks. A mean SSH is removed from the individual SSH measurements producing sea surface height anomalies (SSHA). The mean SSH contains both the unknown geoid signal and the mean dynamic topography over the averaging period. For most satellite missions a mean SSH calculated over a 7-year period is used, although the averaging period continues to be extended in time as the altimeter satellite missions continue. These altimeter preprocessing steps are typically performed by the data provider.

Altimeter SSHA observations are of lower accuracy or are not interpretable near the coasts due to inaccurate tidal corrections and incorrect removal of atmospheric wind and pressure effects at the sea surface in shallow water. The coastal region for altimeter data assimilation is often defined as everywhere shallower than 400 m depth. Altimeter observations also have significant along-track correlated errors that must be taken into account in the assimilation. The along-track altimeter SSHA data are very noisy at the full 7 km resolution. Accordingly, altimeter SSHA data are smoothed along-track using a median or Lanczos filter to reduce the measurement noise. In addition, the altimeter data are often sub-sampled or bin-averaged to remove redundant observations. Finally, altimeter SSHA measurements are scaled by a hyperbolic tangent operator using local dynamic height variability limits that have been computed from the historical profile archive. This operation attempts to remove spurious altimeter SSHA outliers and maintain the data within the range of known baroclinic variability limits.

A final issue with altimeter SSHA measurements is the fact that different versions of the data are reported in both near real-time and in delayed mode. Real-time SSHA observations are computed using less precise, predicted orbits rather than the more precise, observed orbits, which are not available for several days after real-time. Although less precise, real-time SSHA observations still have significant value in the analysis. However, when the more precise delayed mode SSHA observations are available, the corresponding real-time SSHA data should be identified and replaced by the delayed mode data. This procedure ensures that the higher quality, delayed mode SSH observations are incorporated into the altimeter SSHA data archive for use in hindcast studies. Satellite altimeter SSHA observations are a critical data source in GODAE assimilation systems and timely access to the most complete, highest quality data is essential.

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