Influence of subfacet heterogeneity and material properties on the urban surface energy budget

Urban facets-the walls, roofs, and ground in built-up terrain-are often conceptualized as homogeneous surfaces, despite the obvious variability in the composition and material properties of the urban fabric at the subfacet scale. This study focuses on understanding the influence of this subfacet heterogeneity, and the associated influence of different material properties, on the urban surface energy budget. The Princeton Urban Canopy Model, which was developed with the ability to capture subfacet variability, is evaluated at sites of various building densities and then applied to simulate the energy exchanges of each subfacet with the atmosphere over a densely built site. The analyses show that, although all impervious built surfaces convert most of the incoming energy into sensible heat rather than latent heat, sensible heat fluxes from asphalt pavements and dark rooftops are 2 times as high as those from concrete surfaces and light-colored roofs. Another important characteristic of urban areas-the shift in the peak time of sensible heat flux in comparison with rural areas-is here shown to be mainly linked to concrete's high heat storage capacity as well as to radiative trapping in the urban canyon. The results also illustrate that the vegetated pervious soil surfaces that dot the urban landscape play a dual role: during wet periods they redistribute much of the available energy into evaporative fluxes but when moisture stressed they behave more like an impervious surface. This role reversal, along with the direct evaporation of water stored over impervious surfaces, significantly reduces the overall Bowen ratio of the urban site after rain events.