417 / 2019-02-27 14:52:18

A Parametric Computer Experiment on Urban Form, Microclimate and Human Comfort in Downtown Atlanta

urban form,microclimate,urban heat island,human comfort,computer experiment

全体主题 > Urban Planning & Modeling

Urban form is known as a key factor that contributes to microclimate variation. The resulting urban heat island (UHI) effect can influence different aspects of thermal performance of the cities such as the building energy use and public health. A better understanding of how urban form influ-ences the two aspects is of great importance for sustainable urban development. However, the mechanisms and scales for the two types of ther-mal performance are different. While the building energy use is affected by microclimate conditions especially the air temperature and solar insola-tion at the building scale, the outdoor human comfort that causes public health issues is influenced by the same set of factors at the human scale. A review on the related literature shows there are three limitations in cur-rent studies: first, urban form in the literature is often too specific and the number of cases is often limited; second, most studies focused on thermal performances in typical summer days without considering other seasonal variations; and third, there’s a lack of comparisons between urban form’s influences on the two performance types. To close these gaps, this study aims to answer the following questions: how urban form influences the air temperature and the outdoor human comfort in summer and in winter? What are the trade-offs between the two performances and between sea-sons? To explore those questions, this study uses the parametric design and simulation tool to run computational experiments. The part of Atlanta downtown surrounding Forsyth St NW is selected as the case site due to its notorious UHI effect. Following Martin and March’s approach, a 3x3 urban grid structure is used as the design framework with the block size and street size the same as in the case site. Based on such a grid system, 400 design scenarios are generated parametrically with the cover ratio and the number of floors as urban form factors, based on which FAR (Floor Area Ratio) is then calculate. With reference to the site conditions, vege-tation cover is ignored. The study adopts the UWG (Urban Weather Gen-erator), a microclimate simulation tool developed and validated by MIT Sustainable Design Lab, to simulate the hourly microclimate conditions for each design scenario throughout a meteorological year. The UTCI (Universal Thermal Climate Index) is then calculated based on the simula-tion results to estimate the comfort level. The final results suggest that in the summer, with higher FAR, the air temperature generally drops until a certain FAR value and then increases, while in the winter the relationship is more complex. But the relationship between cover ratio and air temper-ature seems to be positive in both seasons. The relationship between ur-ban form and human comfort tells a different story. With increasing FAR, during the summer, the average human comfort level increases while dur-ing the winter, it increases first and then decreases. With increasing cover ratio, during summer and throughout the year, the average human comfort level increases. But such a relationship in winter shows a very complex pattern. For both relationships, in general the relationships for yearly average seem to agree with those in the summer. The results show that urban form’s influences on air temperature and outdoor human comfort fol-low different patterns with general trade-offs. Findings in this study can inform urban planners, designers and policymakers to better understand the influences of urban form on the UHI effect and the outdoor human comfort for sustainable urban development. Their trade-offs can further support urban planning and design processes for multi-objective design tasks.