Effects of Building-Height Variability on Turbulence Generation in the Boundary Layer over an Urban Area

Geometrical features of surface roughness due to natural terrains and man-made buildings cause turbulent airflows in the planetary boundary layer. In this study, we investigated the effects of geometrical features of surface roughness on turbulent generation in the boundary layer over an actual urban area by conducting observations and large-eddy simulations (LESs). The City of Kyoto was chosen for our analysis, and 2-m resolution digital surface dataset was used to reproduce Kyoto City in the LES model. The morphological characteristics of Kyoto were compared with European, North American, and other Japanese cities, which indicated that the building morphology of Kyoto City is similar to that of European cities.

We conducted turbulence measurement by a sonic anemometer installed on a 55-m high meteorological tower at the Ujigawa Open Laboratory, DPRI and a Doppler lidar employed near the tower during the period from January to February 2016. The meteorological tower at the Ujigawa Open Laboratory, located in the southern part of Kyoto City, is a unique facility to observe atmospheric turbulence over an urban region. The Doppler lidar, temporally deployed during this specific period, can measure airflows and fluctuations in the vertical from the surface to the 200-m level. These observational data were used to validate the performance of the LES model. The LES results indicated that Reynolds stress increases with building density in Kyoto and that there is a threshold of the density to distinguish the effects of the building height variability. The quadrant analysis was employed to evaluate the physical processes related to turbulent generation, which revealed that sweeps contribute to the increase in the Reynolds stress with the building-height variability. Although the extreme momentum flux appears around buildings, it contributes little to the total Reynolds stress.

Further LES experiments were conducted for turbulent flows over idealized arrays of roughness obstacles with variable heights. It was found that the characteristic spatial scale in such turbulent flows significantly varies with the increase in building-height variability. At the levels of the roughness tops the characteristic spatial scales are significantly reduced, while they are increased at levels between the ground surface and the roughness tops. In this way, characteristic spatial scales in turbulent flows are significantly affected by the building-height variability.