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A European Commission DGXII MAST 3 Project

Wave Model Simulations

Steady improvements both in theory and computer power have led over the last 20 years to very sophisticated ocean wave models, now commonly used to produce daily wave forecasts on global, regional and sometimes site specific scales.

With respect to wave measurements, the biggest advantage of wave modelling is its capability to reconstruct the past (hindcast). Reliable meteorological information, in practice the surface wind fields, are available for at least the last two decades. These fields can then be used to derive, through wave modelling, similarly extended wave data. These past predictions can, of course, only be verified for the period and locations where measured wave data are available. At best, a few long term series exist which stretch back to the 1970s. However, if no substantial discontinuity is present in the quality of the wind fields, it is often assumed that the same is true for the wave fields, in practice validating the results for the whole hindcast period.

In practice, the limiting factor is the accuracy of the input wind fields, to which the wave results are extremely sensitive. As a result, while we have quite good results in the open ocean (see figure), the quality decreases in the Mediterranean Sea and more generally in the enclosed basins.

Figure. Comparison of significant wave height between WAM model data (y) against altimeter data from Topex for: left) the West of Ireland and right) the Mediterranean off Libya (after Mørk and Barstow, 1998a and 1998b).

This is where the synergy between the different sources steps in. As the accuracy of the measurements, both buoys and satellites, is independent of the dimensions of the basin, we can use them to validate and calibrate the model data. The result of this merging of data sets is then a reliable long-term data set, expanded in time and space relative to the measurements alone, and providing exactly what is required for the first component of Eurowaves. For our purposes, the main source of data is the European Centre for Medium-Range Weather Forecasts (ECMWF, Reading, UK), where the WAM wave model (Komen et al., 1994) has been operational since 1992. The data are available at 6 hourly intervals, with a resolution that has been changed with time, from 3 to 1.5 then 0.5 degrees in the open ocean, and from 0.5 to 0.25 degrees in the Mediterranean and the Baltic. In addition, hindcast data sets, at both large and local scale, are being considered as a source in Eurowaves, noteably the ERA hindcast at ECMWF and the WASA project hindcast (Gunther et al., 1997), the latter spanning a period of 40 years.

The model data, calibrated using the satellite and buoy data, provides the material for the next step, i.e. the statistics offshore.

National Technical University of Athens

Oceanographic Company of Norway

Istituto Studio Dinamica Grandi Masse

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