The refinement of a meteorological pre-processor for the urban environment

The meteorological pre-processor used routinely at the Finnish Meteorologic al Institute (FMI) has been modified in order better to represent urban con ditions. We have reevaluated the roughness length, introduced the zero disp lacement height and divided the surface layer into a roughness sub-layer an d an inertial sub-layer. The friction velocity and Monin-Obukhov length are evaluated using an empirically developed, exponential, Reynolds stress pro file in the roughness sub-layer. The effect of these modifications has been studied by computing the dispersion parameters used in the urban dispersio n modelling system UDM-FMI and comparing the revised parameters with the pr evious model computations.