The Cool Island Effect Of Urban Blue-green Spaces And Impact Factors In Mega City

Due to the global warming and urbanization,extremely hot weather commonly appears arround the world,which brings negative effects to residents.Shanghai is one of the most urbanized and developed regions in China.In recent years,the intensity and area of heat island has been expanding,the average temperature has been rising and the intensity and frequency of summer high temperature calamity is significantly increased.The heat island relieving becomes urgent social and science problem to be solved.Urban green-blue spaces have“cool island effects”.It is beneficial to increase human settlement and level of ecological construction by heat island relieving through the method of making full use of urban green-blue spaces and the surrounding landscape pattern.This research focuses on the spatial and temporal pattern of green-blue spaces in Shanghai,discusses the spatial features and impact factors of their cool island effects and explores their distribution method in urban planning to take the maximum advantage of their cool island effects.The major research contents and results are:?1?Spatial and temporal pattern of urban surface temperature and impact factorsThe temporal variation feature is:in the recent 15 years,the heat environment of Shanghai becomes stronger and the area of heat island is expanding.The spatial variation feature is:The general trend of heat island distribution is expanding from the center to surrounding areas.The land surface temperature varies significantly with different types of land use.The ranking of land surface temperature within each year are the same,which is construction land>bare land>green land>agricultural land>water body.The construction land contributes the most to the heat island effect of Shanghai and is easier to form heat island centers.While Water body,green land and agricultural land have significant heat island relieving effects and are easier to form cool island centers.For green land,water body and agricultural land,larger area of patch,lower level of landscape fragmentation,more concentrated distribution and complicated landscape outline results in lower land surface temperature,which indicates a better heat island effect relieving.On the contrary,for construction land,larger area of patch,lower level of landscape fragmentation,more concentrated distribution and complicated landscape index leads to a higher land surface temperature,which indicates more intensive heat island effects.?2?Cool island effects of urban water body and impact factorsThe research on the temperature feature of water body and its relation with impact factors indicates that the temperature of water body is negatively correlated to its area?S_w?,shape index?LSI_w?and the percentage of surrounding vegetation area(PG_wo)while is positively correlated to the percentage of surrounding impervious surface area(PC_wo).The effective cooling distance and amplitude are negatively correlated to PC_wo while are positively correlated to PG_wo.The correlation between temperature gradient and other factors are not significant.When LSI_w is higher than 4 or PC_wo lower than 60%,the cooling distance increases significantly.So to enhance the cooling distance of water body with a specific area,it is the best to increase the sinuousness of its edge?LSI_w>4?and decrease the percentage of surrounding impervious surface area?<60%?as much as possible.Prediction model for the effective cooling distance and amplitude of water body and its impact factors is built through regressive analysis.The model is verified by predicting the rest 6 samples which are not used in regressive analysis.The relative factors between predicted and practice values are 0.879 and 0.762 respectively,which indicates the model is reliable.?3?Cool island effects of urban green land and impact factorsThe research on the temperature feature of green land and its relation with impact factors indicates that the temperature of green land is negatively correlated to its area?S_g?,shape index?LSI_g?,the percentage of inner vegetation area(PG_gi)and the percentage of inner water body area(PW_gi)while is positively correlated to the percentage of inner impervious surface area(PC_gi).S_g and PC_gi are the major impact factors of the temperature inside green lands.The impact effect of S_g on the internal temperature of green land has an obvious threshold.If S_g is lower than 40ha,the internal temperature of green land decreases significantly with the increasing S_g.However,when S_g exceeds 40ha,the temperature trend becomes flat.Thus,with the same area of green land,the effective measure to reduce the temperature of it is increasing the complexity of its edge and the percentage of its inner vegetation or water area or decrease the percentage of its inner impervious surfaces.The research on the cool island of green land indicates that S_g and external factors are the major impact factors of it.The effective cooling distance is significantly positively correlated to S_g and the percentage of surrounding vegetation area(PG_go)while significantly negatively correlated to the percentage of surrounding impervious surface area(PC_go).The cooling temperature amplitude is not significantly correlated to any researched factors except S_g positively.The temperature gradient is significantly positively to LSI_g while not significantly correlated to other factors.Prediction model for the effective cooling distance and amplitude of green land and its impact factors is built through regressive analysis.The model is verified by predicting the rest 28 samples which are not used in regressive analysis.The relative factors between predicted and practice values are 0.743,0.605 and 0.692 respectively,which indicates the model is reliable.?4?Simulation research of the cool island of urban green-blue spaceThrough compare between the cool island effects of green land and water body,it is concluded that water body has a higher temperature drop and effective cooling distance,while a lower cooling efficiency.By Computational Fluid Dynamic?CFD?simulation,it is concluded that facet lakes have stronger cool island effects than linear rivers.The lakes with higher LSIs have more intensive cool island effects.Thus,the effective measure to increase the cool island effect of water body is setting more facet water bodies.The cool island effect intensity ranking of different types of green land is:sphenoid>radial>ribbon>puntiform.Sphenoid green land has the most intensive cool island effects while ribbon and puntiform green lands have the least.However,the ribbon green lands have significant cool island effects in the formed corridors and puntiform green lands have significant cooling effect within the micro area around them.So sphenoid green lands shall be the first choice in urban construction.Puntiform ones are more flexible and very important to improve micro climate,thus they can be applied in residence communities.Ribbon green lands can form ventilation corridors,which enhances the airflow exchange between inside and outside of urbans,thus shall be used for road greening along prevailing wind.Sufficient buffer beside rivers shall be designed to make full use the effect of“cool ribbon corridor”.?5?The proposals and strategies for the plan of urban green-blue spaceFor the overall planning of urban green-blue spaces,the principles of adding“cool belts”,“cool points”and protecting“cool surface”shall be applied to relieve urban heat island effects.Add“cool belts”beside main and ring roads inside the city to improve ventilation,air quality and environment comforts.Add“cool points”in the city center to break up heat island areas,thus relieving heat island effects and improve micro climates around the“cool points”.Protect and add“cool surface”areas outside the city center to maximize its cool island effects and adjust the heat environment of the whole city.This research has important science value and wide applications.The results can be easily applied in the practice to direct the plan,construction and development of the green-blue spaces in future cities.It complements the theory system of urban planning,pushes forward the scientization of urban planning and provides a new concept for the research on urban heat island effects.