Decomposition of urban climate and city-scale building energy consumption during heatwave : modeling, mechanism and implication

Abstract

Extreme temperature events nowadays become increasingly frequent that pose significant challenges, including intensified heat stress, increased energy demand, and infrastructure vulnerability. This dissertation delves into the complex interplay of various urban factors, such as landscape, sea breeze, terrain, and urban morpho lo g y during heatwave (HW) events. Their individual and collective effects on urban-scale climate and building energy consumption were decomposed and contrasted. The Weather Research and Forecasting (WRF) model was modified to develop a new urban-scale climate and building energy model. First, a new land use/land cover scheme integrating local climate zones (LCZs) and building categories (BCs) enhances the WRF simulation accuracy of urban climate and air-conditioning (AC) load during the 2016 HW event in Hong Kong. The integration of LCZs and BCs reduced the bias in temperature by 0.4 °C and 2.0 °C, relative humidity by 1.98% and 0.68%, and AC load intensity (ACLI) by 6.78 W/m2 and 13.09 W/m2, respectively. Second, Hilbert-Huang transform was employed to analyze acute and high- frequency temperature fluctuations from WRF outcome during a HW event. High- frequency temperature components are decomposed into 4 intrinsic mode functions (IMFθ1 to 4). Their temporal scales ranged from 2.63 hours to 22.72 hours and a spatial scale from 2.31 km to 6.6 km. These components are attributed to turbulence, heat storage/release, terrain-induced disturbances, land/sea breezes, and anthropogenic heat. Their effective areas were identified. Compact or open high-rise urban areas are more sensitive to IMFθ1. Foothill and coastal urban areas are more susceptible to temperature fluctuations in IMFθ2 and IMFθ3, respectively. Moreover, the interactio n between coastal winds, complex terrain, and urba n warmth (including buoyancy flows) together with their impact on urban temperature and AC load are explored. The results show that mountain blockage in foothill areas could increase temperatures by 1 °C to 2 °C and ACLI by 5 W/m2. Besides, the urban heat island (UHI) effect downstream could accelerate upstream channel winds by 1.66 m/s (50.26%). On the other hand, channel winds caused heat advection that increased the downstream temperature by 0.7°C and ACLI by 2.62 W/m2. In addition, the interaction of UHI-induced flows and terrain stagnates sea breeze on mountain leeward sides, slowing down the winds by 0.81 m/s and increasing the downstream temperature and ACLI by 0.9°C and 6.41 W/m2, respectively. Finally, the urban morphology parameters in LCZ classification were clustered into three categories: urban structure, vegetation fraction, and impervious surface thermal properties. Their impact on UHI intensity and ACLI is decomposed, quantified and compared. Urban structure significantly contributes to higher UHI intensity at nighttime than that in daytime. In compact and open high-rise areas, particularly in commercial zones, the UHI intensity and ACLI increases by 40.33% and 34.52%, respectively. Vegetation effectively mitigates UHI. Its benefit is most permanent in

middle-rise and low-rise residential areas, reducing UHI intensity and ACLI by 106.26% and 53.13%, respectively. Compared with rural areas, urban impervious surfaces show a relative ly minor effect. Overall, this dissertatio n provides a comprehensive understanding of the mechanisms driving urban climate and energy consumption during HWs.