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Case study: Assimilating altimetry in the Baltic Sea

Sea level assimilation into North Sea operational storm surge models is limited. Only the Dutch model assimilates tide gauges, and along-track altimetry is not used operationally yet. This is because data assimilation in coastal regions is challenging, due to the short temporal and spatial scales, with (often unknown) time dependent error statistics.

Using the newly available coastal altimetry data, DMI (Danish Meteorological Institute) have developed a new method of blending altimetry and tide gauges. This combines the high temporal resolution and real-time availability of tide gauges with the spatial cover of satellite altimetry, providing a now-cast of sea level based on observations, but which are independent of the weather forecast and of hydrodynamic ocean models.

Some initial work has been done on assimilating such a product into assimilation schemes. DMI have looked at both a continuous 2 year period (2003-2004) as well as 5 specific surge cases in the period 2002-2005. Since tides are not yet well handled, the assimilation is done in the Baltic where tides are small.

The 2 year case shows significent improvement in the correlation and of room mean square error, using independent (i.e. not used in model) tide gauges as validation.

Further work is needed on the technique includes fully including tides. This technique can then be extended to new areas, such as the Adriatic sea.

Altimetry can also be used for near-real time validation of storm surge predictions.

This work has been published in the Journal of Geophysical Research (Madsen et al., doi/10.1002/2015JC011070)

For more information, see this presentation, given by Kristine Madsen of DMI at eSurge's Training event in Cork (February 2014).