The Ozone Model REGOZON

REGOZON, developed by the former FIRST/GMD (GMD-Forschungszentrum Informationstechnik und Informationstechnologie GmbH), Berlin, was integrated with AirWare for the ECOSIM project.

The principle design of the model REGOZON is similar to the /MEMO/DYMOS system. A wind field model is coupled with a dispersion model and a chemical module: REGOZON includes a dynamic meteorological model.

The "time-variable meteorology" comes mainly from the internal energy budget computation. If there is any information about changing geostrophical winds or cloud cover they can be incorporated by an "objective method".

Initial meteorological input includes:

vertical soundings (potential temperature profile)
geostrophic wind
cloud cover
humidity
surface water temperature.

For the computation of meteorological values and the dispersion, an enhanced version of the Eulerian grid model REWIMET is used. It is based on the conservation laws for impulse, mass, energy and passive constituents. A hydrostatically stratified atmosphere is assumed, which is dry and incompressible. The model equations are expressed in three vertical layers. The first (surface layer) follows the ground level and has a fixed vertical thickness of 50 m above ground. It is turbulently mixed and its physical behaviour is strongly coupled with the surface characteristics.

Emissions from traffic, from households and from industrial sources with low emission heights are introduced into the surface layer.

The second layer (mixed layer) reaches from the upper level of the surface layer to the upper level of the atmospheric boundary layer, up to the mixing height. This layer is also turbulently mixed and shows the characteristic diurnal variation of the thickness of the atmospheric boundary layer. Emissions from higher emission sources, for example high stacks from power stations, enter the mixed layer.

The third layer (temporary layer) is located above the mixed layer. It is assumed to be free of turbulence. Since the atmospheric boundary layer can expand to the suprascale inversion, it is possible for the temporary layer to disappear. It will be recreated when the atmospheric boundary layer sinks. No substances are emitted in this layer but it transports the suprascale background concentrations of ozone and ozone precursor substances above the atmospheric boundary layer. For the computation of the temperature regime a surface energy budget routine has been added.

The computation of the dispersion is carried out immediately after the determination of the meteorological values. The transport model uses the same vertical structure. The dispersion equation is solved in two dimensions but with an allowed vertical exchange according to the determined stability. The chemical changes form a source or sink term in the dispersion equation. The photochemical scheme of CBM IV is applied; the chemical module is nearly identical with the DYMOS module. As characteristic time steps for the solution of the chemical system are much smaller, the chemical computations have been decoupled from the determination of transport. Usually, chemistry is not computed every transport time step.

Dry deposition velocities and biogenic emissions are computed as a function of land use. The forecast system REGOZON is applicable under the following restrictions:

relatively flat terrain without strong orographic features and land/sea breeze effects
fair weather conditions with moderate wind velocity
stable, barotropic synoptic situations.

Because of the strong constraints and the rough vertical model resolution, the computational time for the REGOZON model is comparatively small. A comparison between this relatively simple model and a nonhydrostatic 3D model with 35 vertical layers shows a similar ozone production for the selected episodes which conform with the above restrictions. The extent of computation time required by the 35 layer model system is approximately an order of magnitude higher. The REGOZON forecast model for a 100x100 horizontal grid needs approximately 8 hours for a 24-hour-period on a state-of-the-art workstation.