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Urban Environment Research Group

Arizona State University

 

 

Urban Canopy Model

Sensor Network

Subsurface Transport

Turbulent Dispersion

 

Urban Canopy Model

 

Study of physics of flow and transport of energy/water budgets in urban area is very challenging, considering the surface heterogeneity, the irregular structure morphology and the hitherto obscure anthropogenic effect. It¡¯s not surprising that researchers in urban climate community started by treating urban areas as ¡°flat slabs¡± with modified surface aerodynamic and thermal properties, so that the knowledge of flows over rough surfaces can be borrowed and readily applied to this relatively new area. This group of methodology, known as ¡°slab models¡±, start to lose its attraction of simplicity with the advance of ever-more complicated numerical modeling as well as the accumulated knowledge of physics of the land-atmospheric interaction. In the 21st century, the emergence of so-called ¡°urban canopy model¡± (UCM) has all parameterization schemes in the urban canopy layer physically resolved, including the canyon radiative trapping, surface turbulent heat transfer and subsurface heat conduction, rather than empirically related as in slab models. The first comprehensively developed UCM was documented by Masson in 2000, which was implemented in the Weather Research and Forecasting (WRF) model in 2001 by Kusaka and co-workers. The last decade has seen a rapidly increasing research effort devoted to the investigation and improvement of UCM, one of the latest improved versions being our coupled energy balance and hydrological models (sketched below):

 

The model features sub-facet heterogeneity of each urban facet (roof, wall or ground), a prominent example being the incorporation of vegetated surfaces into the UCM. Our UCM consists of two major components, viz. the urban energy balance model and the urban hydrological model. The first one concerns the re-distribution and transport of energy budgets in the urban canopy layer, focused on the solution of the energy balance (or distribution, depending how you view it) equation, i.e. Rn = H + LE + G.

 

The urban hydrological model, on the other hand, focused on the surface and sub-surface transport of water (or moisture, or vapor, depending on the existing form of H2O) in both the subsurface soil and the atmospheric layer. Physical processes in the hydrological model include precipitation, surface runoff, infiltration and evapotranspiration, the last one being the intriguing link of coupling energy and water interactions.

 

 

One interesting practical application of this coupled UCM is to evaluate different urban heat island (UHI) mitigation strategies, popular practices being installation of cool and green roofs to reduce the surface temperature of the building envelope during hot summer days. It is found that, in general, cool roofs are capable surface temperature through albedo (reflectivity of solar radiation) effect, while effectiveness of green roofs is largely regulated by plant stomata through evaporative cooling. It is premature for the community to reach any conclusive statement on questions such as ¡°which type of roof is more green?¡± The answer lies in complicated patterns of local, regional and global climatic, ecological and socio-economic interactions.