Real cities are comprised of a diverse, random arrangement of building positions, shapes, and sizes. Yet most of the urban parameterisations thus far developed share the assumption that a city is made up of either a regular array of parallelepipeds or infinitely long canopies. The inputs to these models, which include street width, building width, building density and a statistical representation of building heights, are generally obtained through quantitative field surveys (which are very slow and time consuming to perform) or qualitative estimates from Digital Elevation Model. But in performing this geometric abstraction there is no way to ensure that the total built surfaces and volumes of the simplified geometry match those of the actual city, or more importantly, that the energy and momentum exchanges are equivalent. In this paper, we aim to test the central hypothesis that cities can be accurately represented by a regular array of parallelepipeds or canopies. For this, we investigate, for a particular scenario, the effects of complexity in urban geometry on the spatially averaged drag forces and shortwave radiation exchange. For drag computation, we used the Immersed Surface Technique, while for computing the incident radiation we used the Simplified Radiosity Algorithm. After testing the above hypothesis, we propose a new approach for fitting an array of cubes to any complex (realistic) geometry, so that new or existing urban parameterisation schemes can be used with confidence. Copyright. (C) 2010 Royal Meteorological Society