An urban canyon energy budget model and its application to urban storage heat flux modeling

To obtain a local-scale urban energy balance by either measurement or modeling it is necessary to determine storage heat flux (ΔQs). This flux cannot be measured directly due to the complexity of the urban surface. The Grimmond et al. Objective Hysteresis Model (OHM) [C.S.B. Grimmond, H.A. Cleugh, T.R. Oke, An objective urban heat storage model and its comparison with other schemes, Atmos. Environ., 25B (1991) 311–326] of local-scale ΔQs combines empirical equations for individual surface types in the proportion that they are present within the urban area. One surface type for which there is very limited field data is the urban canyon, which consists of the walls of adjacent buildings, the horizontal street-level area separating them (roadways, gardens, parking lots, etc.) and the enclosed air volume. Here the storage heat flux of an urban canyon and the resulting OHM parameters are investigated with a numerical model of a dry urban canyon energy budget. Substrate heat fluxes are derived from simulated surface and substrate temperatures; the latter evolve through time according to the finite difference form of the Fourier heat conduction equation. When compared against measured fluxes, the model performed satisfactorily. Numerical experiments show significant effects on the OHM parameters due to changes in the ratio of building height to separation distance and building wall thermal properties. Effects of intermediate significance were attributable to canyon orientation, wind speed and the timing of the between-building air temperature regime. Air temperature and the timing of the wind speed curve showed only minor significance.