Research On The Impact Of Urban Spatial Arrangement On The Temporospatial Distribution Urban Micro-climate

The rapid development of urbanization is drastically changing the properties of urban underlays and urban morphology.Coupled with a large amount of artificial heat and atmospheric pollutant emissions,the trend of urban warming is becoming more and more obvious,and urban heat island effects and air pollution problems are becoming increasingly serious.Although there are a lot of achievements in the current research on urban microclimate and air quality,there are also some shortcomings,which are mainly manifested in the following two aspects:（1）Firstly,the research scale is mainly concentrated on the macro-scale and micro-scale,and there are relatively few studies on the meso-scale（urban scale）.For example,research on the meteorology and environmental engineering is mainly to study climate change and air quality issues within a regional scale.However,due to the lack of awareness of urban spatial morphology,they often ignore the climate change of a single city caused by the differences in urban internal spatial morphology.Research in the field of urban planning and architecture focuses more on the impact of block-scale and building-scale microclimate.These researches can more accurately describe the microclimate changes caused by building forms,but lack of control over the climate change of the city,and the block-scale microclimate is affected by the urban-scale microclimate.On the other hand,a lot of urban microclimate researches were a summary of the status quo,and lacks the foresight and predictability of the impact on urban microclimate after the development of urbanization.The urban master plan needs to determine the city scale,urban spatial structure,land use layout,etc.These factors will have a decisive impact on the underlying surface and urban form,and even the rules of artificial heat emission and air pollutant emission.It will be difficult to change when it’s determined.Therefore,in the urban planning process,it is necessary to predict and evaluate the urban microclimate after the implementation of the plan,and this is the missing aspect of previous studies.In view of this,this study take Wuhan urban development area as an example,based on land use data and building vector data in 2015,firstly analyzed the spatial layout and urban morphological characteristics of Wuhan,and according to the"Urban Master Planning of Wuhan"（2010-2020）and the requirements of the“Regulations on the Management of Land Use Construction Intensity in the Main Urban Area of Wuhan”,the land use data of Wuhan was reclassified and sorted into three intensity levels（Industrial/Commercial,High-intensity Residential,and Low-intensity Residential）.Secondly,set the urban morphological parameters according to the intensity of various types of land,and apply them to the WRF/UCM model to simulate the temporal and spatial distribution of the urban microclimate in Wuhan.Furthermore,the Multi-resolution Emission Inventory of China（MEIC）and the simulation results of WRF/UCM were used to coupled with CMAQ to simulate the air quality in Wuhan.By setting up multiple cases,we analyzed and compared the effects of urban spatial arrangement on the temporospatial distribution of the thermal environment and PM_2.5 concentration in Wuhan.The results showed that the spatial structure of Wuhan is not a single model,but a composite model that combines“circle-layer”and“group-type”.In terms of land layout,with the relocation of industries and the renewal of old communities,the size of the city has continued to expand and the area of water bodies has continued to decrease.However,with the adjustment of the industrial structure,the industrial land in the main urban area of Wuhan has been continuously reduced and moved out,replaced by a large number of high-rise residential and commercial areas,making the construction intensity of the main urban area continue to increase.These measures have had a relatively obvious impact on the urban microclimate of Wuhan,making Wuhan’s urban heat island and air quality problems increasingly serious.The simulation results showed that from the inside to the outside,the intensity of urban heat island（UHII）and the PM_2.5 concentration gradually decrease.The UHII and the PM_2.5 concentration in the second ring are the highest,and the peak concentration of PM_2.5 is 4.3 times than the outer ring.The maximum UHII is 1.8°C higher than the outer ring.From the perspective of time changes,both the UHII and the average PM_2.5concentration show a trend of"low during the day and high at night".At around 07:00in the morning,PM_2.5 reached its peak concentration,then began to gradually decrease,and then rose slightly at around 20:00 at night.This pattern of change coincides with the appearance of local traffic peaks in the morning and evening.In response to the above problems,this study proposed optimization strategies for the spatial layout of Wuhan from a macro scale:（1）The"multi-center+cluster"spatial layout disperses high-intensity commercial and high-rise residential into different circles of the city,avoids excessive concentration of high-intensity areas,and is more conducive to long-wave radiation dissipation and artificial heat to dissipate,which can effectively reduce the strength of the heat island.The relatively dispersed layout can guide the infiltration of clean air from the suburbs into the downtown,which is conducive to the diffusion of PM_2.5 in the main urban area.Compared with the"single center+circle layer"spatial structure model,the heat island intensity of the"multicenter+cluster type"central urban area can be reduced by a maximum of1.4℃,and the peak PM_2.5 concentration can be reduced by about 48%.（2）Changes in the area of industrial and water bodies can significantly affect the urban microclimate.After the relocation of industrial land,the average air temperature in the main urban area of Wuhan dropped by about 1.2℃,and the average PM_2.5 concentration dropped by about 11%.After turning the lake water body into urban construction land,it changed the attributes of the underlying surface of the city and increased the surface roughness length,which caused the average air temperature to rise by about 1.6℃,and a large number of concentrated and contiguous high temperature areas appeared.The duration of high temperature is increased from 1 hour to 3 hours.In addition,the reduction in the area of lakes has greatly reduced the urban ventilation capacity,resulting in an increase of the average PM_2.5 concentration in the urban area by about17.5%compared to the current situation.（3）Among the four construction modes,the high-capacity and low-density mode is conducive to long-wave radiation heat dissipation and ventilation due to its larger building spacing,while the staggered buildings helped the development of atmospheric turbulence and can increase urban ventilation capacity.The simulation results show that,compared with the current situation,the high-volume and low-density construction mode can reduce the UHII of the main urban area at night by approximately 2.3°C,and reduce the average PM_2.5concentration by approximately 60%.This shows that the contradiction between the need for building capacity for urban development and the improvement of urban microclimate can be better balanced.（4）Using the lakes and low-intensity construction areas as ventilation corridors to separate high-intensity construction areas,and guide clean and cold air from the suburbs into the main urban area,which can reduce the intensity of UHI,avoid the formation of large high-temperature accumulation areas,and promote the diffusion of air pollutants.It’s an effective means to improve the urban microclimate,and the wider the corridor,the more obvious the effect.