Assessment and Improvement of the Accuracy of Radiation Heat Transfer Estimation in Simplified Urban Canopy Models

Detailed modelling of the interaction between buildings and the urban environment is best implemented using Computational Fluid Dynamics (CFD). However, due to the often prohibitive computational cost of CFD models of a realistic urban domain, a number of simplified Urban Canopy Models (UCM) have been proposed since 2000: Town Energy Balance (TEB), Square Prism Urban Canopy (SPUC), Temperatures of Urban Facets in 3D (TUF3D), Urban Weather Generator (UWG), etc. The main goal is to estimate the intensity of the Urban Heat Island (UHI) intensity, in other words, the differential between the urban air temperature and the temperature measured at a reference rural station. These models are a lot faster than CFD models. However, the spatial organization of the surfaces (slopes, orientations, shape factors), and their physical characteristics (albedo, emissivity, thermal conductivity) need to be simplified as well thereby affecting the accuracy of the outcome. In particular, the models necessarily rely on overly simplified representations of short-wave (solar) and long-wave (infrared) radiation heat transfer. The contribution of the paper is twofold. First, it assesses the comparative validity of the radiation heat transfer approximations proposed by the different simplified modelling approaches in comparison to exact solutions calculated for an actual urban district. Second, it proposes to improve existing simplified models by replacing their approximate radiation transfer calculations with much more accurate ones determined by reference to the detailed geometry of the urban district under consideration. This approach is validated using the case of a district in downtown Abu Dhabi.