A simple model of urban canyon energy budget and its validation

An urban canyon energy budget simulation model is described. The model is simple and computationally inexpensive enough to permit characterization of the spatial and temporal variability of canyon energy exchanges by running multiple cases. It is two-dimensional and dry and avoids the necessity for computational fluid dynamical calculations by using parameterizations for the down-canyon wind profile and cross-canyon vortex flow and by employing an exchange coefficient formulation for sensible heat loss from the walls. These are combined with radiation calculations using the procedure of Arnfield (1976. 1982) and substrate heat fluxes computed using the Fourier heat conduction equation and wall and floor thermal property information. The model is validated against the measured canyon energy budget data of Nunez and Oke (1977)and reproduces these data quite well, with Willmott indices of agreement in the range 0.91 to 1.00 and root-mean-square errors of 14 to 46 W m(-2) for individual facets and for the total canyon system. The poorest correspondence was for the substrate heat flux, the maxima for which were overestimated, suggesting use of excessively large facet thermal properties. These inputs were adjusted based on the measurement-simulation discrepancies, leading to further improvement in model performance. The adjusted thermal properties are consistent both with published data and with the available information on the morphological characteristics of the canyon.