Hydrodynamics

The core of any numerical water model is the "hydrodynamic-physical" part. This takes care of calculating the four basic parameters: currents, sea level, temperature and salinity. A few important aspects of this part can be summed up as follows:

• Currents: A distinction may be made between two types. Tidal currents occur periodically and are important in particular for short-term forecasts. "Residual" currents are usually slower and display a constant, seasonal pattern for weeks and even months. A typical residual circulation pattern, calculated using a computer model, is illustrated for a winter and a summer situation in the North Sea.
  
  
• Fronts: By analogy with meteorological situations, these form the dividing line between various water masses and can be observed using satellite pictures. Thermal fronts occur in the summer months and reflect the division between shallow, mixed water, where the temperature remains constant under the influence of turbulence, and deeper layered water consisting of a warm upper layer and a cold lower layer. The two layers are separated by a thin layer with sharp variations in temperature, known as the 'thermocline'. Fresh water fronts form the dividing line between fresh river water and salty seawater and are therefore found mainly along coastlines. An accurate simulation of the intensity and location of fronts is important for calculating residual currents and for the production of microplankton, which lies at the basis of the food chain.
  
  
• Turbulence arises primarily in reaction to the wind and tides and generates a complex pattern of micro-fluctuations, varying from a few millimetres to several meters. These fluctuations in turn influence the currents and the distribution of temperature, salinity and other substances on a macro-scale. It is virtually impossible to provide a precise description of turbulence for an area like the North Sea using the current generation of computers, which means that so-called 'turbulence models' in the form of complex mathematical comparisons have to be used.
  
  
• Interactions with the atmosphere: When calculating the temperature of the water, account has to be taken of the sun's rays that warm up the top 10 to 20 meters of the water column, and of other effects of heat exchange on the surface of the sea. The wind in the lowest layer of the atmosphere causes waves and turbulence. To include these effects in the model, meteorological data are required (wind speed and direction, atmospheric pressure, atmospheric humidity and temperature, cloud volume) and are usually provided by means of a weather model.

The development of faster supercomputers makes it possible to design and test more complex 'integrated' three-dimensional (length, breadth and depth) models with important components for biology, sediments and contaminants. One example of this is the recent COHERENS computer program developed by MUMM. The model is currently employed by almost 600 users worldwide and has been used for various applications.