A Scheme for Scalar Exchange in the Urban Boundary Layer

Abstract

We present a scheme for parameterising scalar transfer in the urban boundary layer, which is divided into an inertial layer and a roughness layer. The latter is further divided into a shear layer and a canyon layer. In the inertial layer, scalar transfer is determined by turbulence related to canyon macroscopic features, while in the roughness layer, it is determined by shear-generated turbulence, canyon vortex and vortex-generated turbulence. We first describe a conceptual model for the canyon flow and the aerodynamic resistance network, and then estimate the resistances from the point of view of drag partition and vortex advection. The results are compared with the measurements from wind-tunnel experiments. It is found that for small canyon aspect ratio, σ c , the transfer velocity increases with σ c , reaching a maximum at around σ c = 0.5 and then decreases with σ c . We also show that the scheme is not sensitive to adjustable parameters.

Experimental Determination of Heat Transfer Resistance in Urban Canyon Environments

“…More often than not, the average temperature appeared at 0.2z b . This is possibly the reason that the reference height of 0.2z b supports the agreement of our results with those of Yang and Shao (2006). As shown in previous studies, the flow pattern in an urban canyon depends heavily on the canyon aspect ratio.…”

“…Our results also agreed with the theoretical evaluation of the resistance values measured by Yang and Shao (2006) given a lower reference height of 0.2z b where z b is the canyon height. A reference height of 0.2z b is reasonable, considering the airflow pattern of the canyon vortex and the air temperature distribution.…”

“…Therefore, the key point in the H04 parameterization is how to acquire an adequate u * and ∆U for an urban canyon. The second study we discuss is that of Yang and Shao (2006). They showed the transfer resistance between a canyon recirculation region and the inertial layer above it.…”

“…z ref was set as the measurement height for the temperature and wind above the roof (1.2 z b ). Although Yang and Shao (2006) suggested that z c would be 0.2z b based on their trial calculation, we also tried 0.5z b because we used the averaged temperature value for the entire canyon layer. u * a is the friction velocity of the canyon air, which can be evaluated using the drag partitioning methodology (Yang and Shao 2006).…”

“…Although Yang and Shao (2006) suggested that z c would be 0.2z b based on their trial calculation, we also tried 0.5z b because we used the averaged temperature value for the entire canyon layer. u * a is the friction velocity of the canyon air, which can be evaluated using the drag partitioning methodology (Yang and Shao 2006). In our u * a evaluation, 0.7z b was adopted for the zero plain displacement height and wind speed at the 50-m tower was used.…”