Section title: A matter of scale
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Title" West Coast Models, by Karen Green

ROMS requires masses of computing power partly because it examines phenomena that need to be measured on different time scales. Air temperature and wind speed can change very quickly. Carbon dioxide levels in the water change more slowly and salinity levels slower still. In addition, other coastal phenomena happen on greatly different spatial scales. Local circulation patterns are influenced by localized winds, storms, coastline irregularities, and turbulence in larger-scale currents. These local patterns must be simulated with relatively high resolution, with each measurement covering perhaps five kilometers. Larger-scale circulation patterns, however, must be simulated with coarser resolution, say 15 or 20 kilometers per measurement.
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Looping lines follow float trajectories in an area off the coast of California during early summer. The trajectories are superimposed over mean sea surface temperature, as simulated in ROMS with a U.S. West Coast parent domain and a central California embedded child domain with horizontal grid resolutions of 15 and 5 kilometers, respectively. The grid boundaries are marked by the black rectangle. Note the general offshore and southward flow as well as mesoscale activity illustrated by the trajectories, particularly off Cape Mendocino.

For these reasons, ROMS was developed with a time-stepping algorithm that allows for a great variety in the size of steps and with numerical methods that allow high resolution locally and coarser resolution for larger-scale, or mesoscale, phenomena such as storm systems or circulations associated with a particular coastal area.

"The ability to have different spatial resolutions is very important in this kind of modeling because many of these events, such as cycling of biological materials, happen both locally and on a larger scale," explains McWilliams. "Localized events have an impact on mesoscale events and vice versa."

If the ROMS research team is able to account for these interactions among local and mesoscale events in their simulations, those simulations will be more meaningful and closer to what actually happens in regional coastal systems, he adds.