Predicting air temperatures in city streets on the basis of measured reference data.

Abstract

Knowledge of site-specific conditions is essential for the development of an architectural design that responds to the local environment. However, while meteorological data are recorded by the weather service in stations that are assumed to be representative of the surrounding region, generally no account is made of changes in local conditions caused by urban development - though these may be substantial. Micro-climate in city streets can be predicted by computational fluid dynamics (CFD) with fine spatial resolution. However, CFD requires extremely detailed input, involves long computation times and is thus limited to simulating short periods. The aim of this project was to create a model capable of simulating weather conditions for extended periods, with simplified inputs and less detailed, yet accurate, outputs. The CAT (Canyon Air Temperature) computer model predicts site- pecific air temperature in a city street based on data from a rural reference station. In addition to a rudimentary description of the two sites, it requires only inputs measured at standard weather stations, yet is capable of predicting accurately the evolution of air temperature in all weather conditions for extended periods. It simulates the effects of urban geometry on radiant exchange; the effect of moisture availability on latent heat flux; energy stored in the ground and in building surfaces; air flow in the street based on wind above roof height; and the sensible heat flux from individual surfaces and from the street canyon as a whole. A monitoring program was carried out in Adelaide, South Australia, in which weather conditions were recorded continuously at two streets and at a reference location outside the city centre for nearly a year. In addition to providing data required to calibrate and to validate the CAT model, the measurements provided evidence of a substantial nocturnal heat island in the city, of up to 8.6 °C. The weather records also demonstrate the existence of an urban cool island during the daytime of up to 3.8 °C, the intensity of which is related to the diurnal temperature range. The CAT model may be used to generate realistic, site- pecific temperature inputs for building thermal simulation software, required to produce more accurate modelling of energy use. It may also be used to evaluate the effect on micro-climatic conditions of proposed development at new urban locations.