Urban thermal climate and associated urban heat island circulation

Abstract

Increasing urbanization has altered the land surface-atmosphere heat and water fluxes and the dynamics of atmospheric boundary layer. This thesis aims to address the scientific question of the dynamics of the temporal variation of urban air temperature. Numerical and analytical methods are developed to better understand the thermal and dynamic responses of the urban climate, with a specific focus on the highly compact and highly dense city of Hong Kong. The mesoscale weather research and forecasting model is coupled with a multilayer urban canopy model (BEP/BEM) to better understand the physical mechanisms of the urban heat island (UHI) circulation. A dome-shaped feature is well captured with an improved urban heterogeneity parameterization under weak synoptic wind conditions. A new drag coefficient based on the plan area per unit ground area, λ_P, is used to capture the wind stagnation in the lower-level convergence zone. The local synergistic warming phenomenon induced by the strong interaction of the UHI circulation and foehn wind and the weakening sea-land breeze circulation is found to be the main contributor to the recently observed extreme high–air temperature events via numerical and simple theoretical models. The impact of the anthropogenic heat fluxes, especially from the central air conditioning systems in commercial areas, on the urban thermal and dynamic environment is then studied. The implementation of two alternative air-conditioning systems is shown to achieve a 30% reduction in energy consumption during extreme high-temperature events and can also modify the surface air temperature distribution, heat, and momentum of turbulence, which further inhibits the evolution of the planetary boundary layer height and the secondary local circulation, such as the sea breeze circulation. A simple city-scale zero-dimensional model is developed to understand the physical effects of urban environment on the variations in the annual and daily air temperatures. The urban thermal storage is found to have no effect on the mean urban air temperature but significant impact on the amplitude and phase terms of the daily temperature cycle. Two important parameters for engineers-the relative convective heat transfer number, λ, and time constant number, τ-are examined to better characterize the influence of the urban morphology on the urban air temperature. Finally, the impact of urban shape, Coriolis force, and twin-urban areas on the development of UHI circulation is investigated in an ideal setting. The asymmetrical structure of the inflow from the diagonal and divergence flow at the upper level from the four sides over an idealized square city is revealed. The effects of Earth’s rotation are significant during the night and increase the transverse wind velocity. A chain flow is found for the two-different sized urban shapes, and with the development of the UHI circulation, the outflow of the smaller city on the upper level is found to merge with the inflow from the large city. The results of this thesis are expected to have significant implications for the choice of a city site, the design of the city morphology, and the general urban planning for the development of climate-resilient cities.