How Many Facets are Needed to Represent the Surface Energy Balance of an Urban Area?
We investigate the question of how many facets are needed to represent the energy balance of an urban area by developing simplified 3-, 2- and 1-facet versions of a 4-facet energy balance model of two-dimensional streets and buildings. The 3-facet model simplifies the 4-facet model by averaging over the canyon orientation, which results in similar net shortwave and longwave balances for both wall facets, but maintains the asymmetry in the heat fluxes within the street canyon. For the 2-facet model, on the assumption that the wall and road temperatures are equal, the road and wall facets can be combined mathematically into a single street-canyon facet with effective values of the heat transfer coefficient, albedo, emissivity and thermodynamic properties, without further approximation. The 1-facet model requires the additional assumption that the roof temperature is also equal to the road and wall temperatures. Idealised simulations show that the geometry and material properties of the walls and road lead to a large heat capacity of the combined street canyon, whereas the roof behaves like a flat surface with low heat capacity. This means that the magnitude of the diurnal temperature variation of the street-canyon facets are broadly similar and much smaller than the diurnal temperature variation of the roof facets. Consequently, the approximation that the street-canyon facets have similar temperatures is sound, and the road and walls can be combined into a single facet. The roof behaves very differently and a separate roof facet is required. Consequently, the 2-facet model performs similarly to the 4-facet model, while the 1-facet model does not. The models are compared with previously published observations collected in Mexico City. Although the 3- and 2-facet models perform better than the 1-facet model, the present models are unable to represent the phase of the sensible heat flux. This result is consistent with previous model comparisons, and we argue that this feature of the data cannot be produced by a single column model. We conclude that a 2-facet model is necessary, and for numerical weather prediction sufficient, to model an urban surface, and that this conclusion is robust and therefore applicable to more general geometries.
KeywordsRoof and street canyon Simplified urban parameterization Surface fluxes Urban energy balance
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- Goward SN (1981) Thermal behaviour of urban landscapes and the urban heat island. Phys Geogr 2: 19–33Google Scholar
- Grimmond CSB, Best MJ, Barlow J, Baik J-J, Baklanov A, Belcher S, Bruse M, Calmet I, Chen F, Clark P, Dandou A, Erell E, Fortuniak K, Hamdi R, Kanda M, Kawai T, Kondo H, Krayenhoff S, Lee S-H, Limor S-B, Martilli A, Masson V, Miao S, Mills G, Moriwaki R, Oleson K, Porson A, Sievers U, Tombrou M, Voogt J, Williamson T (2007) Urban surface energy balance models: model characteristics and methodology for a comparison study. COST 728 Exeter workshop, 131 ppGoogle Scholar
- Masson V, Grimmond CSB, Oke TR (2002) Evaluation of the Town Energy Balance (TEB) scheme with direct measurements from dry districts in two cities. J Appl Meteorol 11: 1011–1026Google Scholar
- Oke TR (1987) Boundary layer climates, 2nd edn. Methuen, London, p 435Google Scholar
- Sparrow EM, Cess RD (1970) Radiation heat transfer, Chap. 3–4, appendices A & B. Thermal Science Series, Brooks/Cole, pp 75–136, 300–313Google Scholar