CANYON GEOMETRY, THE URBAN FABRIC AND NOCTURNAL COOLING: A SIMULATION APPROACH

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Abstract
A simulation model for surface cooling in urban street canyons under calm conditions is described, based upon a simplified energy budget for the canyon facets containing only the net longwave and substrate heat flux densities. The former term is evaluated from the canyon radiation budget model of Arnfield (1976), the latter by numerically approximating the Fourier heat conduction equation. Equilibrium temperatures evolve through the nocturnal period for specified canyon and incoming longwave irradiance characteristics. Numerical experiments conducted with the model show that canyon geometry alone exerts a significant effect on cooling rates and, hence, on heat island intensity. Construction materials and internal building climate control tend to enhance spatial variations in nocturnal temperatures. The effects of wall thickness, sky radiance distribution and cloud cover also are investigated. Results show qualitative and quantitative correspondences with previous field and scale-model studies. Sources of error and desirable model refinements are discussed. [Key words: urban climatology, heat island, heat flux, nocturnal cooling, simulation model.]