Study On Microclimate Effects Of Trees In Humid And Hot Regions Based On Evidence-Based Design

Urban residents are suffer from increased heat stress because of urban heat island intensification in combinationwith global climate change.To alleviate extremethermal stress and to improve residents’ quality of life,urban greeninfrastructures such as trees are frequently used for urban areas andhave received increasing attention by more and more urban plannersand landscape designers.How can we use the microclimate effects of trees in the planting design to improve the thermal comfort is worth exploring.This study used the ideas of evidence-based design and investigated how can we find the evidence in the study of microclimate effects of trees.First,this study identified the most suitable evaluation indicator in a humid subtropical climate region.Second,this study established a simulation method for microclimate effects of trees,which including the evaluation of ENVI-met and its post-process method named COMFA,and the simplified approach for building tree models in ENVI-met.In addition,this study used the established method to investigate what characteristics of trees are most important or how trees should be arranged to optimize thermal conditions in a high-rise residential area in Guangzhou.Moreover,the researched evidence has been used in an optimization study.The main work of this study was listed below:(1)The four most frequently used indicators in tree microclimate research wereanalyzed in detail,including their theoretical basis and calculation method,considerations,resolutions and legibility.In addition,the correct prediction rate was calculated by using the field measurement data,which is from four common tree species(Michelia alba,Mangifera indica,Ficus microcarpa,and Bauhinia blakeana)in Guangzhou,China.This part of work identified that COMFA is the most suitable evaluation indicator in humid subtropical climate regions.(2)The suitability of ENVI-met(V4.2 Science)and its post-process method COMFA in humid subtropical climate regions was evaluated.First,the mathematical model of ENVI-met was analyzed.After that,this study comprehensively compared the modeled and observed physiological parameters(leaf surface temperature and vapor flux)and microclimate parameters(solar radiation,air temperature and humidity)of four common tree species in Guangzhou,China,using the field measurement data collected previously.This study shows that the tree model of ENVI-met with its post-process method COMFA is accurate enough to be the predicting tool in tree microclimate research.(3)An ENVI-met tree model database for the common tree species in Guangzhou was established preliminarily.This study focused on the method by which landscape architects can build a tree model in ENVI-met quickly and accurately without any professional instruments.First,the orthogonal analysis has been done to ensure whether the default value can be used for some parameters which may hard to observe.Second,for those observed-data-needed parameters,a database with observation value reference and regression prediction model was established.(4)After established the evidence-finding method,the thermal environment of a high-rise residential area in Guangzhou was investigated.Fifty scenarios were evaluated in terms of how each tree and each configurationaffected the energy fluxes of a person on hot sunny days.Results showed that the largest effect in allscenarios was on radiation flux,with a much smaller effect on the convective and evaporative fluxes.An evendistributionof trees was found to be overall most effective as it maximized the shaded area by avoiding unnecessaryoverlap.Moreover,when the ratio of planting pitch to crown width is 1: 1,it has the highest cooling efficiency.While the solar radiation reduction was the largest effect,a detailed analysis provided additionalinsight into the sometimes-large effect of terrestrial radiation.Taller trees with low branching heightand dense crowns had the largest effect on solar radiation reduction.Terrestrial radiation was most affected byleaf albedo and sky view factor.Buildings surrounding a treed area increased both solar radiation(throughreflection)and terrestrial radiation(through emission from hot surfaces).The results are universally applicableas the tree characteristics are not species-specific,and the energy fluxes are based on first principles.The established evidence-finding method in this study can be used in simulation,model building,evaluation and application of tree microclimate.This is a research method suitable for humid subtropical climate regions.Additionally,this method can expand the application of evidence-based design in tree microclimate field.