Environmental Effects Of Urban Geometry Changes On Air Temperature And Outdoor Thermal Comfort In Arid Climate Of Mashhad (Case Study Of Pachenar And Shahed)

Document Type : Research Paper


1 Department of Urban planning and urban designing,Mashhad Branch,Islamic Azad University, Mashhad, Iran

2 Faculty Member Of Arts & Architecture School, Tarbiat Modares University, Tehran

3 department of urban planning and urban designing, Mashhad Branch,Islamic Azad University, Mashhad, Iran

4 Faculty member of arts and architecture, Sanaati Shahrood, Shahrood, Iran


In recent years, natural disasters, such as the heat wave, mortality rates have increased due to rising air temperatures and environmental problems, such as air pollution due to the accumulation of heat in cities, and has led to increase attention to decling these environmental problems. It is a way to create an Outdoor thermal comfort, in addition to promoting human health as a factor in reducing the air temperature in urban environments. Therefore, man-made and natural spaces in cities can provide comfortable environments.We often think of people living their life inside buildings and we may not see that in fact for one reason or another we spend much of our time outside – between the buildings.The spaces and places of these, and many other everyday activities, have a special social, cultural and even economic significance.Thus we have to make great efforts by designers and planners to reduce air temperature and reduce air pollutants that enhance the presence of people in these spaces. Outdoor thermal comfort in urban open spaces is one of the most important indicator of quality of life in the city. Outdoor thermal comfort enhance presence of people, it can boost their health. Variables such as buildings height and orientations of buildings, space between the buildings, open spaces surface and building material have effects on outdoor thermal comfort. The main aim of this study was to examine the influence of urban geometry on outdoor thermal comfort in time- honored and state-of–the-art tissue in Mashhad. Moreover, urban geometry such as urban canyons and green spaces at pedestrian level are effective factors and there is limited attention in Iran. Evaluating thermal comfort of the locals may help architects and planners to deal with urban problems by identifying thermal comfort needs in indoor and outdoor environments, so that the intensity of temperatures is reduced in open spaces by using design elements. In Iran, thermal comfort issues in academic studies are more focused on indoor spaces, and in recent years, approaches to outdoor thermal comfort have increased.This study focuses on effects of urban geometry on outdoor thermal comfort in two urban areas PACHENAR and SHAHED tissue in Mashhad. The first part of the article examines researches carried out in the subject area of the paper, and then, in the next section, Theoretical foundations of the articles, thesis and books have been extracted. Air temperature, wind speed, mean radiant temperature (MRT), and the physiologically equivalent temperature (PET) are simulated with ENVI-met and Rayman to highlight the strengths and weaknesses of the existing urban geometry in summer season.
1. Matherials & Methods
The methodology is practical research based in two parts. Firstly the framework theory is extracted from researches such as papers, books and dissertation, The climatology data collected from weather station of Torogh which is located near Mashhad. After that Gis maps are used for neighborhood. In order to reach the PET values, the proposed method is applied to an urban canyon, which is simulated in terms of mean radiant temperature and PET, combining ENVI-met (http://www.ENVI-met.com) and Rayman. Firstly a simulation of the case study model are conducted in ENVI-met. Secondly, some of the key results and values from ENVI-met are imported into Rayman and other simulation runs.
2.1.ENVI-met Simulations
The three-dimensional micro climate model ENVI-met is designed to simulate the surface-plant-air interactions in urban environment. ENVI-met software is a useful tool for planners to analyze and predict the thermal comfort impacts of various factors. ENVI‐met is a 3D model, which seeks to replicate the major atmospheric processes that affect the micro climate. This model simulates wind flows, radiation fluxes, temperature, humidity and other parameters, based on the fundamental laws of fluid dynamics and thermodynamics. which is able to simulate the interactions between different urban surfaces, vegetation and the atmosphere. ENVI-met allows analyzing the effects of small scale changes in urban design on micro climate under different mesoscale conditions. A major shortcoming with ENVI-met is that buildings which are modeled as blocks where width and length are multiples of grid cells, have no thermal mass. Input data required to initiate ENVI-met simulations are:• Wind speed and direction at 10 m above ground level; • Roughness length (Zo); • Initial temperature of atmosphere; • Initial temperature and humidity of the soil; • Specific humidity at 2500 m; • Relative humidity at 2 m. The model calculation includes:• Short-wave and long-wave radiation fluxes with respect to shading, reflection and re-radiation from building systems and the vegetation;• Transpiration, evaporation and sensible heat flux from the vegetation into the air, including full simulation of all plant physical parameters. In this study, simulation was carried out in the summer (August) and based on the data of the meteorological station of Mashhad. This simulation was selected on August 28, 2017, during the sunrise and sunset times between 6 AM and 20 AM, , which represents the hottest day in summer.
2.2.Rayman Simulations and assessing PET
The Rayman is produced by Matzarakis. It model has been developed in the Meteorological Institute of the University of Freiburg in Germany in 1998. This model is developed to simulate the short and long wave radiation flux densities from the three dimensional surroundings in simple and complex environments. RayMan is in fact a freely available radiation and human bioclimate model. The aim Of the RayMan model is to calculate radiation flux densities , sunshine duration, shadow spaces And thermo-physiologically relevant assessment indices using only a limited number of meteorological and other input data. For such models to be applied in simple situations, the following atmospheric parameters are required:– direct solar radiation – diffuse solar radiation – reflected short-wave radiation – atmospheric radiation (long-wave) – long wave radiation from the solid surfaces.The following parameters describing the surroundings of the human body also have to be known:
– sky view factor – view factor of the different solid surfaces – albedo of the different solid surfaces – emissivity of the different solid surfaces
Assessment of microclimate and thermal comfort outdoors
Due to the wide use of the PET index in different climate studies and the ease of expression of oudoor thermal comfort of individuals, in this paper, the PET index is calculated. Since these numbers are not localized in Iran, and in particular to Mashhad. The range of 23 is based on the Matzarakis and Mayer studies as a thermal comfort range. On the other hand, trees play an important role in this study. The simulation carried out in Envi-met has also been based on field observations of the density of these trees, which is called the LAI index (LAI).Dense trees with 10 m height (LAI = 4.73) and very dense trees with 15 m height (LAI = 9.35) were designed for simulating the vegetation. In addition, grass with 0.1 m height (LAI = 0.03) was also used.
Model Calibration
In order to make the results of these models based simulations more credible, the model had To be calibrated. The input values have to be changed slightly, so that the output values turn Out to be a more accurate representation of reality. In order to do that The model was calibrated with on-site long-term measurements of air temperature (Ta), relative humidity (RH), wind speed (Ws) and direction (Wd) at two tissues as well as properties of ground surface materials


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