M. Rohinton Emmanuel

Summer microclimate variations at seven sites in Ann Arbor were evaluated using comfort indices based on wet, dry and globe temperatures. These indices were correlated to building density, ground cover and tree canopy cover. Further analysis of microclimate variations in the city was carried out using wet and dry bulb temperature data-sets obtained by an automobile-mounted instrument panel. Detailed evaluation of climate variations within the street canyons at each of the selected sites was also made. The aim was to determine the thermal comfort and climate effects of variations in building density, vegetation canopy cover and maturity and ground cover.

The results indicate that urban climate modifications at day and night were very different. A downtown-centered heat island was observed at night while there was a mix of cool and heat islands at day. The daytime variations were strongly correlated to the amount of canyon shading. During the night, urban climate at city-wide scale was highly correlated with the amount of tree cover. However, the sky-view factor and the thermal properties exerted greater influences upon nocturnal microclimate at the street canyon level. The daytime cooling associated with extensive tree canopy was not different from cooling by building shade. Furthermore, extensive tree-cover inhibited nighttime comfort due to the reduced the sky view.

Based on these findings, the study proposes four design strategies for the downtown core of Ann Arbor. They aim at optimizing shading on streets during the day while maintaining a large sky view to facilitate nocturnal cooling. This is achieved by proposing arcades around downtown city blocks with set-back building massing. The building massing is such that tall buildings are on the south side of the city blocks while green areas occupy the center. While shading at the street canyon level is desirable, a city-wide increase in tree cover is needed to reduce the overall magnitude of the urban heat island in summer. Towards this end, two more design strategies are proposed for the residential neighborhoods. These strategies propose an increase in tree cover and a courtyard built-form with grass-covered inner lawns to allow for building-shaded, soft ground-covered commons.

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Questions? Comments? Suggestions? Please contact me at rohemma@umich.edu

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