| With the intensification of urbanization,the current ecological environment has been affected by the deterioration of urban heat island(UHI)seriously.Lake is a key element to alleviate UHI which have been studied by many scholars.The current research focuses on the linear relationship between the lake landscape characteristics and its cooling effect,and rarely reveals the dynamic interaction between lakes and the surrounding built environment.The "source-sink" landscape theory lays a foundation for the coupling study of static pattern and dynamic process of lakes’ thermal mitigation,so as to establish a comprehensive identification mechanism of lake landscape pattern and cooling process,which can help us deeply understand the local climate effect of the lake.Taking Wuhan as an example,this study adopts WRF/ UCM to simulate the structure of surface layer on typical meteorological day in summer 2018.By constructing "Lake cold source-Thermal circulation-Urban cold sink" system which based on Lakes’ thermal mitigation process,this study explores the development mechanism and dynamic changes of interaction process between lakes’ cold island and UHI.(1)Cold sinks are concentrated in commercial areas and residential areas in the central city,which shows the distribution of "high-intensity with single core" in the morning to "low-intensity with wide distribution" at noon,and "high-intensity with single core" in the afternoon and night.Cold sources are mainly distributed in lakes,the area and intensity of which are basically stable after a short rise in the morning,and turn into cold sink in a large area at sunset.At night,the area of lake cold sources is less,which is mainly distributed in the upwind suburban lakes.Cold sources’ cooling effect is stronger in the daytime,while the warming effect of cold sinks is stronger at night.Specifically,the thermal mitigation capacity of the lake cold sources increases in the morning,and reaches the pinnacle at noon,then keeps decreasing until sunset.The thermal mitigation capacity increases briefly at night and keeps decreasing until the early morning of the next day.And the thermal mitigation capacity differs in lake cold sources,small lakes in the urban center,large urban lakes and suburban lakes have stronger thermal mitigation capacity,especially the upwind suburban lakes have stronger thermal mitigation effect at night.(2)The UHI circulation in cold sinks appears in the morning,and disappears at14:00 pm,which appears in the early morning again.It can form circulation loop in the morning and night,and the structure is the strongest in the morning.Lake breezes occur at noon,whose formation and disappearance characteristics are affected by lakes’ area,location and background wind.The lake cold source will promote the formation of UHI circulation,and the lake breeze circulation will weaken the vertical flow of the UHI circulation.The cold sink will delay the generation and development period of the vertical air flow of the lake wind circulation,and the UHI circulation will strengthen the lake breeze.When there is no cold sink around the lake,the lake breeze is easier to be observed in the downwind area of the lake,and the lake breeze circulation loop is easier to be generated in the upwind area.In terms of the thermal mitigation process of lake cold source,the uneven distribution of lake cold sources,the reduction of lake cold sources at night and the obstruction of high-density construction areas make the cooling range of lakes limited.The thermal mitigation process differs in lakes.The suburban lakes with large scale can produce cold air as cold sources.The presence of small lakes in large lakes surroundings promotes the form of cold chain between lake breeze circulations to expand lakes’ cooling distance.The cooling intensity of lake breeze circulations can be up to2-3 ℃ within 1-2.5 km and the cooling intensity and range are affected by the construction intensity and lake distribution.(3)According to the pattern and process of "Lake cold source-Thermal circulation-Urban cold sink",the lake spatial management and control system combining coverage,connectivity and permeability is established.Firstly,the coverage of the "source" is realized by constructing the lake water network,the cold air supply loop of suburban lakes and the five lake wind circulation chains.Secondly,according to the thermal regulation mechanism of lakes,large suburban Lakes(compensation area),small lakes(diversion point)and large urban lakes(action reservoir)are divided to realize chain ventilation and cooling(cold chain).Under high construction intensity,the cold chain gives priority to control the distance between lakes;Under low construction intensity,the cold chain gives priority to control the low temperature and orientation of the compensation area.Under non-compliant dominant wind direction,the cold chain gives priority to control the relative position of the lake to ensure the connectivity of "flow".Finally,the classified and hierarchical control strategy of lakes is established to ensure the permeability of "sink" and maximize the local climate effect of lakes. |
