The emergence of increasingly powerful computers enabled the development of tools that have the potential to predict flow and transport processes within the urban canopy layer. These new tools are micro-scale meteorological models of prognostic or diagnostic type. Prognostic models are based on the Reynolds-averaged Navier-Stokes (RANS) equations, whereas diagnostic models are less sophisticated and ensure only the conservation of mass. These two model types are presently supplemented by even simpler engineering tools. It is to be expected, however, that the latter will sooner or later be replaced by RANS codes or the even more complex Large Eddy Simulation (LES) models.

Models have begun to play an important and often dominant role in environmental assessment and urban climate studies that are undertaken to investigate and to quantify the effects of human activity on air quality and the local climate. Their increasing use, however, is paralleled by a growing awareness that the majority of these models have never been the subject of rigorous evaluation. Consequently there is a lack of confidence in the modelled results.

The main objective of the COST action is to improve and assure the quality of micro-scale meteorological models that are applied for predicting flow and transport processes in urban or industrial environments.