COST 732 Model Evaluation Case Studies: Approach and Results

Urban emissions occur mainly within or slightly above the canopy layer i.e. within a zone where the atmospheric flow is heavily disturbed by buildings and other obstacles. It is wellknown that buildings can deflect plumes and result in sometimes increased and sometimes decreased dilution, dependent on the density, height and shape of the obstacles and their orientation with the wind flow. In comparison to unobstructed terrain, local concentrations can change by more than an order of magnitude. As a consequence, it is inappropriate to consider buildings within a surface roughness parameterisation only, particularly if predictions on the scale of a few streets or city blocks are being made. These facts, in conjunction with increasing computer power, have promoted the development of obstacle-resolving or obstacleaccommodating prognostic and diagnostic models, subsequently called micro-scale meteorological models. Nowadays these models are commercially available and widely used in environmental impact studies.