Street Canyon Modes And Surface Material Influence The Mechanism Of The Urban Heat Island Effect

With the speeding up of urbanization process,the urban heat island(UHI)has become increasingly serious problem of the decline of the urban livability greatly result from the increase of the city population,city buildings,square and roads.Urban heat island effect not only reduces city residents living comfort,but also harm to health and economic development.This paper in view of the high reflectivity coating as a potential technology to reduce urban island effect by means of experiments and numerical simulation.It is expected that a engineering measures can be found to reduce urban island effect.The main conclusions are as follows:(1)This paper proposes a new method of measuring the albedo of urban canyon prototype and prepares a flat model and four UC prototypes with south-north orientation,west-east orientation and cross-street orientation,respectively.The albedo measured from the proposed method is compared with that measured by the ASTM E1918-06 and by the adapted ASTM E1918-06.We found that when the variation of the incident solar intensity is less than 20W/m2(a tolerant error stated by ASTM E1918A),the ASTM E1918-06 can either underestimate or overestimate the albedo of the urban canyon prototype up to 0.08,verified the method of reliability.(2)In order to reveal the mechanism of variation and controlled by urban canyon albedo,we prepare twenty UC prototypes with different pavement(wall)reflectivity and with south-north orientation,west-east orientation and cross-street orientation,respectively.The albedo was measured from the proposed method outdoor in a sunny day.We found that for an UC with an aspect ratio of 1.0,an change from 0.15 to 0.65 of pavement(wall)albedo would cause an increase of the albedo of the UC from only 0.15 to 0.35 when the albedo of the roof and wall is about 0.40(represent the reflectivity of common roof and wall).Raising the albedo of the pavement in urban canyon is thus inefficient to increase the albedo of the UC,especially for UC with great aspect ratio.(3)To find out the relationship between reflectivity and street model fractal dimension.We calculate the UC prototype box dimension by the MATLAB graphics processing and observe the albedo of UC prototype with different shapes.The results show that:fractal dimension is a good way to describe the evolution of urban structure and the characteristics of time and space,but also can be used as a key parameter to improve the effective evaluation of the influence of street canyon reflectivity.(4)In order to scientifically evaluate the reflection road facing the city heat island effect and engineering significance,a one-dimensional urban canyon reflectance model was established to estimate whether increasing the pavement albedo can arise the urban canyon albedo(UCA)effectively and to evaluate the reflective diffuse radiation from the pavement albedo to adjacent building walls.The simulation results show that when the aspect ratio is less than 1.0,increasing the pavement reflectivity can effectively improve the urban canyon reflectivity,it is recommended that reflective pavements can be a potential technology to reduce urban heat island effect.Raising the albedo of the pavement in low UC or in parking lot introduces a sizable additional radiative flux to the pedestrians and causes glare problem.Therefore,it should be caution to develop reflective pavements in UC as an urban cooling strategy.