Hydrodynamic models are forced through interaction with the atmosphere and by solar radiation. Processes occurring outside the model domain (entering through the boundaries of the model and in coastal domains) also drive models, as do the body forces associated with tidal flows and freshwater inputs such as precipitation and river discharge. Inaccurate specification of forcing conditions remains one of the greatest sources of error for simulations, due to large uncertainties in the observed and modeled atmospheric fluxes, and due to the difficulty of adequately representing wave dynamics at the boundaries of the model.
The interaction with the atmosphere can be prescribed with observed atmosphere fluxes that are interpolated over the domain. Unfortunately, most of these observations occur over land and are biased away from marine conditions. They also fail to resolve the spatial heterogeneity in the interaction of the atmosphere with surface waters under most circumstances due to data scarcity. Models are also run with 'bulk forcing' algorithms that compute momentum and evaporative and heat fluxes using, for example, sea-surface temperature of the ocean model as well as atmospheric characteristics. These bulk formulas tend to damp the ocean model back to observed surface temperature and salinity; so they may underestimate climate sensitivities. Alternatively, the fluxes of heat and freshwater as well as momentum can be individually prescribed based on direct measurements or atmospheric simulations that are often run in a fully coupled mode, or over prescribed sea-surface temperatures. However, there is no guarantee that these fluxes will result in a stable simulation, that is, without drift.
The simplest and most frequently employed lateral model boundary conditions are a damping back to observed or modeled climatology. This approach is generally effective; however, it isolates the model domain from remote influences, such as the influence of remotely forced coastally trapped Kelvin waves on local coastal upwelling. More complex boundary conditions include nesting as described above, and approaches that aim to allow waves to propagate freely out of the domain.
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