Relationship between upward heat transport and building arrangement in urban districts of Osaka as revealed by large-eddy simulations

Urban impacts on local weather are the aggregated influences of buildings and man-made structures, therefore requiring analyses with an explicit representation of building arrangements and an upscaling from urban-district scale to mesoscale. Using a building-resolving large-eddy simulation model, this study examines how local climate zones (LCZs) and building arrangement parameters are related quantitatively to the upward heat transport in the surface boundary layer, which can develop and/or enhance rainfall in urban districts. Several urban districts in Osaka, Japan characterized by diverse LCZs and building arrangements were selected as the computational domains. Analyses of the characteristics of turbulent flows and upward heat transport showed that the vertical profiles of wind and upward heat flux in the districts were similar if the LCZ categories were the same but exhibited various characteristics if the LCZ categories were different. The vertically integrated upward heat flux in the surface boundary layer differed by a factor of 2.6 among the different LCZ categories. Building arrangement parameters such as maximum and average building height, building height variability, and building density exhibited linear relationships with the vertically integrated upward heat flux, with correlation coefficients of approximately 0.9 for maximum and average building height and building height variability. This study suggests that assigning heat flux values based on building arrangement in urban areas is essential for accurately quantifying the influence of urban environments when simulating mesoscale phenomena such as local heavy rainfall with meteorological models. • Features of wind speed and upward heat flux depends on the local climate zones. • Some building arrangement parameters had liner relationships with upward heat flux. • A variety of building arrangement is important when estimating urban impacts.