The Third International Conference on Urban Air Quality Abstract Intercomparison of numerical urban dispersion models - Part II: street canyon in Hannover, Germany M. Ketzel, P. Louka1, P. Sahm2, J.-F. Sini1, N. Moussiopoulos2 Department of Atmospheric Environment, National Environmental Research Institute, Roskilde, Denmark 1Fluid Mechanics Laboratory, Ecole Centrale de Nantes, Nantes, France 2Laboratory of Heat Transfer and Environmental Engineering, Aristotle University Thessaloniki, Greece Microscale computational fluid dynamics (CFD) models have become a useful and popular tool for assessment and prediction of air quality in urban areas. The proper validation of such a model is a crucial prerequisite for its practical application. Within the framework of the European research network TRAPOS (see http://www.dmu.dk/AtmosphericEnvironment/trapos) a working group on computational fluid dynamics modelling was established and model intercomparison exercises were launched (http://www.dmu.dk/AtmosphericEnvironment/trapos/cfd-wg.htm). Different numerical models employing the widely used 'standard k-e -model' were applied to well defined test cases comprising a variety of 2 and 3 dimensional configurations for which measurements from wind tunnel or field studies were available. The main goals of the model intercomparison are: (1) to assess and allocate the source of differences that appear when different CFD codes using the same turbulence model are applied to well defined test cases, (2) to improve the knowledge base for model development and application and (3) to demonstrate the level of agreement that can be expected from CFD modelling in urban environment. This paper, which is the second in a sequence of two, presents the results of three models (CHENSI, MISKAM, TASCflow) for a street canyon in Hannover, Germany (Göttinger Strasse). Firstly the characteristics of the flow field predicted by the different codes are compared; secondly the calculated concentration fields are compared with field and wind tunnel data. Both agreements (e.g. for the general flow and concentration fields) and disagreements (e.g. for the level of concentration) are observed in the comparison. The discussion aims at explaining the differences along with giving some suggestions to CFD model users on how to calculate such complex flows, but also to experimentalists on where concentration measurements should be taken in order to be more representative for a whole street and to avoid distinct local effects.