The particles are carried along by the model winds and diffused using random walk techniques. The accompanying Fig. 1 illustrates the complex evolution of a plume over a few days. The source in this case is areal, representing North American CFC emissions. The material over Europe was carried ahead in strong high-level flows. As well as producing forecasts of pollutant dispersion, NAME can analyse or hindcast the plume spread using an archive of the Unified Model's initial fields, which are derived via the operational assimilation procedures from the observational base.
Older data have to be restored from off-line storage.
Options can be made to run the Model using the global, regional or mesoscale grid, or any combination of nested grids. Provision has been made to allow the particles to stream from low-resolution grids to high, and vice versa. This automatically solves the problem of boundary inflows for the smaller domains in a very effective manner. The timestep is variable, reflecting the scale of the turbulent motion in which a particle is embedded, and the perturbation applied to a particle's motion giving effect to the turbulent diffusion includes a damped 'memory' of its velocity at the preceding timestep as well as a fresh turbulent impulse. This allows a simpler formulation, and a larger timestep to be used.