The Research Based On The Sealing Arch Border Tree's Shade Which Impacted On The Urban Street Canyon's Micro-climate In Xi'an City

The rapidly urbanized and population aggregated causing the climate degradation,and the contradiction between the need for outdoor comfortable climate becoming increasingly highlight. The importance of the border trees as an important strategy to slow down the urban heat island and improve the micro-climate has been widely emphasized. As long as the street buildings are not fully shielded from direct sunlight in summer the effects of tree on the micro-climate becoming particularly important and providing more options for users to choose the relative comfortable zone in the street valley. The research about the border trees in the valley street mostly focused on the beauty and greening form at present. Fewer studies had been done quantitatively which explained the impact of the border trees on the the urban street canyon micro-climate.To study the micro-climate effects and regulation mechanism of the border trees in the street canyon, which formed the" single arch" and enclosed tree-lined space. So this paper uses the border trees as the research object. Firstly, taken the spot investigation about the tree-lined in the valley in Xi' an city. In this paper,25 kinds of trees and 6 types of the border-trees shade space which were commonly used in Xi'an city. Stating the constituent elements in the border-trees shade space and the research on the elements of the trees were carried out furthermore to specify the function and effect of the border trees. Secondly, classified the 25 street trees which were commonly used in the Xi 'an city according to different influence indexes, and then to summarized the tree species which has better functions in cooling and humidifying. At the same time,established the typical form models of the typical tree species like Sophora japonica and Platanus acerifolia was to calculate the shade ratio of the crown to the branches and then concluded the tree form which provide the shade in summer and the sunlight in winter better. Lastly, chosen the basic "single arch" tree-lined and enclosed space units which were representative and typical to conduct the actual measurement and data analysis.And then validates the influence of the border trees on the micro-climate in the street from a quantitative perspective.According to the research on the border-trees in Xi' an city, which were adapted the Xi' an city climate characteristic should be given priority to deciduous trees which was should be chosen by some indicators like great Leaf Area Index and Transpiration Rate, like Platanus acerifolia?Sophora japonica?Prunus subhirtella?Acer truncatum?Koelreuteria paniculata?Magnolia denudata ; the longer column with the cup shape and heart-shape canopy form should be chosen preferentially. Through the measured analysis on the "single arch" and enclosed tree-lined space indicated that the period of temperature change under the border-tree can be divided into heating period in the morning?high temperature peak period in midday?cooling period in the afternoon and in the evening, and the lowest temperature in the early morning. The true cooling effect of the border trees is in the morning heating period and almost range from 1-7?which approaches the temperature of the weather station which in suburban. The border trees which height about 9-16m and the crown width about 5.5-13m that has a better cooling effect in" single arch" and enclosed tree-lined space than in" single arch" and semi-enclosed tree-lined space. There are cooling effects at daylight and thermal insulation at night. And also can alleviate the street valley thermal comfort in summer,improve the human body comfort level.The border tree' s canopy can shade the direct effects of direct solar radiation so that provided the best shading effect. On both sides there are buildings covered in the valley which orientation is east- west, the north side of the tree' s shading about 8 ?10h to play the main shading effect and meanwhile the south side of the tree' s shading time is limited. In the valley which orientation is south-north, the border trees to play the main shading effect at the front and back of the mid-day about 2 ?3h and the shading length about 4 ?5h. No matter what the direction of the street canyon,there is a "single arch" and tree-lined space on either side of the road or on both sides that the canopy radiation of the tree is the leading factor to regulate the micro-climate of the street valley in all-day.This paper summarized the 6 types of border trees space in the street canyon in Xi'an city. Based on the basic theoretical research and the constructed model analysis, the species and morphology of the trees which adapted to the climate characteristics of Xi'an city were obtained. And then provides a reference for the selection of the border-trees in Xi 'an city. The characteristics and mechanism of the period of temperature change were obtained through the measured analysis on the "single arch"and enclosed tree-lined space reveals the role of the street trees to the micro-climate in the urban canyon. Then the development of relevant theories will be promoted and has the practical significance.