Assessing the Cooling Effects of Urban Vegetation on Urban Heat Mitigation in Selected U.S. Cities

Urban Heat Island (UHI) is a growing problem worldwide. Mitigation of UHI is necessary for cities to adapt to climate change and enhance sustainable development at a city scale. Cooling cities with urban vegetation management is a sustainable solution for urban heat mitigation. Urban vegetation influences urban microclimate through the shading effect, surface roughness, and evapotranspiration. The differences in horizontal and vertical structures of urban vegetation determine the shading effect, surface roughness, and evapotranspiration. Enhancing the cooling effect of urban vegetation requires a comprehensive understanding of how vegetation structure affects UHI. The effects of horizontal structure on land surface temperature (LST) have been extensively documented, however, the cooling effect of vertical structure has rarely been studied. Therefore, this knowledge gap poses a serious challenge for UHI mitigation. Meanwhile, the solutions to alleviate UHI by urban vegetation management vary among cities due to the difference in their climate and urban settings. Examining the cooling effect of urban vegetation structure in different cities will help explore how vegetation interacts with local settings and climate and give rise to efficient urban vegetation management strategies. The objective of this research is to improve the understanding of the effects of vertical structure of urban vegetation on LST at a city scale. The research will be carried out in three specific phases as follows: (1) developing preliminary research to investigate whether urban vegetation height can cool down LST or not, (2) quantifying the spatial and temporal patterns of shade cast by trees and examining the cooling effect of shade in two different cities, and (3) investigating and comparing the effects of vertical structures related to surface roughness on LST at three cities with different climatic conditions and urban settings. The results of the first phase revealed that vegetation height has a significant effect on LST. Additionally, the optimal height and fractional cover at which vegetation can exert the greatest influence on LST were identified. Since it is extremely difficult to achieve very high vegetation cover in cities, managing vegetation height is important to exert efficient UHI management strategies. This study established a baseline for exploring the importance of vertical structure of urban vegetation. The second phase of study examined the cooling effect of tree shade in the two cities: Tampa, Florida and New York City (NYC), New York. A hillshade algorithm of geographic information system was employed to quantify the spatial and temporal pattern of shade cast by trees by integrating the sun location and tree height. The cooling effects of tree shade were investigated by statistical analyses. Hillshade tool combining the sun location and tree height can accurately capture the spatial and temporal variation of tree shade. Shade cast by trees has significant cooling effects in Tampa and NYC and shade at 07:30 am was the most significant cooling factor. Comparing the cooling effects of tree shade in the two cities, shade has a stronger cooling effect in Tampa than NYC. This difference is possible due to the distinct ratio of tree canopy to impervious surface cover, different spatial arrangements, and relative heights of trees and buildings. This initial comparison can provide significant insights to efficient management of urban climate moderation and allow further study of the mechanisms of UHI in different cities. The last phase of this dissertation research explored the cooling effects of urban tree vertical structural metrics related to surface roughness in the three different cities: NYC New York, Seattle Washington, and Tampa Florida. Maximum tree height and standard deviation of tree height were used to characterize the vertical surface roughness of trees. The study found that the two height metrics have important and but varied cooling effects in the three cities. This study suggests that there are different cooling effects of tree height metrics among cities with different biomes and urban characteristics. The study accomplishes three main objectives. First it provides significant knowledge about the effects of urban vegetation structures, especially vertical structure, on UHI. Second, it enhances understanding of the interaction between the cooling effect of urban vegetation vertical structure and urban settings and local climate. Finally, it provides important insights to urban planners and natural resource managers on how to mitigate the impacts of urbanization on UHI through vegetation management