Influence of Subfacet Heterogeneity and Material Properties on the Urban Surface Energy Budget

Abstract

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.

Exploring historical and future urban climate in the Earth System Modeling framework: 1. Model development and evaluation

“…At the Baltimore site, the model initial state is taken from Ramamurthy et al . [] with the initial LAI set to be about 1, which was also used by Ramamurthy et al . [] (Dr. P. Ramamurthy, personal communication, 2015; since Ramamurthy et al .…”

“…[] with the initial LAI set to be about 1, which was also used by Ramamurthy et al . [] (Dr. P. Ramamurthy, personal communication, 2015; since Ramamurthy et al . [] did not report LAI in their paper).…”

Effect of pavement thermal properties on mitigating urban heat islands: A multi-scale modeling case study in Phoenix

“…In addition, land-atmosphere interactions are largely neglected in offline models, i.e., meteorological conditions do not respond to changes in building physics. Due to theses limitations, upscaling the results of offline studies for guidance at city and regional scales becomes challenging [23]. Accurate quantification of the effect of pavements on the urban thermal environment necessarily requires studies in a fully-interacting environment, i.e., a coupled atmosphere-urban modeling system.…”

Inter-annual variability in urban heat island intensity over 10 major cities in the United States

“…The built surfaces that dominate the urban areas, particularly black roofs, asphalt roads and parking lots decrease the overall albedo and as a result more energy is stored within the urban environment. While storage heat over rural surfaces account for less than 10% of the total energy, in urban areas, it is the most dominant term during the convective period (Ramamurthy et al 2014). The storage heat is the primary factor that affects the UHI during the nighttime (Offerle et al 2005) and it can be seen in all the cities compared here.…”