A good knowledge of the tides is important in hydrography, especially in coastal areas. Near ports the tides are generally well measured but in between and further offshore good measurements are scarce. Satellite altimeters that measure the sea level worldwide are a valuable supplementary data source.
At HERMESS we have collected and analysed sea level time series obtained from satellite altimeters. This yielded a database of tidal constants, amplitudes and phases, of 21 major tidal constituents along the ground tracks of the satellites. The reliability of the results was assessed with the help of a bootstrap method.
The derived results are valuable for e.g. validation of tidal models but also provide input for such models. At HERMESS a dedicated tidal model was developed, based on the depth-averaged shallow water equations. The model is tailored to the assimilation of measurements, from satellites or in-situ, inside the model domain.
Below, a few examples are given of the capabilities of this method. The first figure shows the amplitudes of the M2 tidal constant in the waters of north-west Europe along the satellite ground tracks. Already here the main tidal patterns are visible but these become much clearer in the model results, computed on a 1/60 degree grid.
In the second example the same method is applied in the Arabian Gulf. The analysed altimeter data, together with available tidal data from coastal stations, are assimilated in the model on a 1/120 degree grid to provide a detailed image of the tidal amplitudes of M2.
The model also provides an assessment of the tidal constants for the depth-averaged currents. Surface currents are estimated by applying logarithmic vertical current profiles.
The computed tidal constants, for sea level and currents, are stored in separate model databases that enable a rapid response to customer requests for e.g. time series.