Numerical Study On The Impact Of Muti-Dimensional Expansion Of Urban Landscape On Urban Thermal Environment

Expansion of urban space manifests in two aspects: densification and outward expansion.Replacement of natural/semi-natural surfaces with impervious surfaces alters the albedo,thermal conductivity,roughness,and water content,while building infrastructures modify airflow,energy absorption,and atmospheric heat transport within cities.These changes in local and regional climate processes further influence surface energy balance,water circulation,local circulation,and planetary boundary layer structure,resulting in complex thermal environmental effects and serious impacts on the thermal comfort of urban dwellers.In order to optimize urban landscape patterns and improve human living environment,it is urgent to investigate the impact of mutidimensional urban expansion on urban thermal environment.Based on a review of domestic and international research,this thesis takes Wuhan,an international wetland city and mega-city in central China,as the study area.In response to the worsening thermal environment during the urbanization in Wuhan,this thesis conducts research on the coupling effect of urban outward expansion and lake shrinkage as well as the impact of urban densification on urban thermal environment.To be specific,this thesis uses a mesoscale numerical model coupled with an urban canopy model and a lake model to investigate the process and mechanism of the multi-dimensional urban landscape expansion on urban thermal environment.It further aims to reveal the direction,magnitude,and extent of its impact on urban thermal environment.Finally,this thesis proposes urban planning strategies to mitigate urban thermal environment.The main findings of this thesis are summarized as follows:(1)From 2000 to 2020,urban expansion alone(excluding the presence of lakes)led to summer warming of 0.9℃,drying of 0.9 g/kg in the built-up area of Wuhan,with local peak warming and drying up to 2℃ and 1.4 g/kg,respectively,and maximum increases in nighttime wet-bulb globe temperature and heat index of 1.0 and 2.7°C.In comparison,urban expansion with lake shrinkage led to warming of 0.8℃,drying of 1.6g/kg,with maximum changes up to 1.4℃ and 2.0 g/kg,an insignificant change in nighttime wetbulb globe temperature,and a maximum increase in the nighttime heat index of 1°C.In2000,the presence of water bodies reduced the maximum daytime temperature by 0.6°C,while increased the minimum nighttime temperature by 1.0°C,enhanced the average daily water vapor mixing ratio by 0.9 g/kg,and maximum increases in nighttime wet-bulb globe temperature and heat index of 1.8 and 2.5°C.In 2020,after lake shrinking,the presence of water bodies had weaker impacts on urban thermal environment.Humidity has a greater impact on heat stress of urban residents in Wuhan,especially on outdoor workers.Changes in the urban landscape slightly mitigate daytime thermal risk,but exacerbate nighttime thermal risk.(2)From 2008 to 2018,building heights and volumes have increased considerably in central Wuhan,especially between the second and third ring roads.The high-rise,middensity expansion,the most intensive urban expansion mode in central Wuhan,led to warming of 1.3℃,drying of 0.5 g/kg,decreasing in wind speed by 1.7 m/s,and increasing in heat stress by 1.7℃ to a maximum extent through the day.The mid-rise,mid-density and low-rise,low-density expansions were the dominant urban expansion modes in central Wuhan.The mid-rise,mid-density expansion led to warming of 0.5℃,drying of0.2 g/kg,decreasing in wind speed by 0.4 m/s,and increasing in heat stress by 0.5℃ to a maximum extent through the day.The effects of the low-rise,low-density expansion on the thermal environment were weaker than those of the mid-rise,mid-density expansion.The low-rise,high-density expansion led to warming of 1.0℃,drying of 0.3 g/kg,decreasing in wind speed by 0.3 m/s,and increasing in heat stress by 0.7℃ to a maximum extent through the day.It had stronger impacts on air temperature,relative humidity,and human thermal comfort,while a weaker impact on wind speed than the mid-rise,middensity expansion.Overall,the increase in nighttime heat stress due to the threedimensional urban expansion was much greater than daytime.Wind speed and temperature were the dominant factors affecting the daytime and nighttime heat stress,respectively.Based on the findings of this study,we suggest to promote urban land use efficiency by adopting the mid-rise,mid-density urban expansion.Meanwhile,building height range and otherness should be reduced.(3)Both two-dimensional and three-dimensional urban expansions have greater impacts on the nighttime thermal environment than that of daytime.Two-dimensional urban expansion led to an increase in temperature during both daytime and nighttime(0.9– 1.4°C),while urban three-dimensional expansion led to a weak change in daytime temperature and a significant nighttime local warming(1.3°C).Two-dimensional urban expansion reduced a larger magnitude of drying(1.6 – 2.0 g/kg)than three-dimensional urban expansion(0.2 – 0.5 g/kg).Compared with two-dimensional urban expansion,three-dimensional urban expansion has a smaller impact on thermal environment.Consideration of changes in building height can enhance the modelling of radiation trapping,shading effects,and convective enhancement caused by buildings,thus altering the magnitude of urban expansion impact on temperature and humidity.Therefore,failure to consider the impact of the three-dimensional urban landscape expansion on urban thermal environment will considerably limit the understanding of the impact of urban expansion on urban thermal environment.