Natural and Anthropogenic Contributions to Urban Heat Islands

Land use/land cover change has significant influences on climate system on both local and global scales by altering the biogeochemical and biogeophysical processes. Urbanization has been one of the most significant anthropogenic modifications to the Earth's surface in recent decades. The influence of urbanization on surface climate has a profound impact on the lives of urban residents, who comprise more than half of the world's population now and will comprise 70% of the world's population by the year of 2050. Conversion of natural land to urban land leads various changes in surface climate such as changes in temperature, humidity, precipitation, and air pollution. Among them, urban heat island (UHI), a ubiquitous phenomenon in which surface temperatures are higher in urban areas than in surrounding rural areas, represents one of the most significant human-induced changes to the surface climate. It is also a long-recognized and widely studied phenomenon in the research field of urban climate.;Although the candidate causes of UHI have been known for a long time, there is little knowledge on the relative contribution of each factor to UHIs. Previous modeling studies mostly focused on a short time scale of UHIs rather than on a climate scale. The observation-based studies cannot mechanistically quantify the contributions of each biophysical process to UHIs. Nor have these contributions been contrasted in different climate regimes. This dissertation aims to fill these gaps, for the first time quantifying the contributions of primary causes to UHIs and contrasting among difference climate regions.;A long-held perception is that reduction in evaporative cooling in urban land is the dominant driver of UHI. In this dissertation, however, we used MODIS observations and climate modeling to show that this perception is somewhat erroneous. We conducted two separate climate simulations: one in current climate (1972-2004) and the other in future climate (2005-2100). The current-climate simulation was driven by a carefully revised climatology dataset, and the future-climate simulation was driven by the community earth system model outputs.;Results show that, for cities across North America, daytime UHI is strongly correlated with precipitation, whereas nighttime UHI is correlated with the logarithm of population but invariant with climate. The geographic variations in daytime UHI are largely driven by variations in the efficiency with which urban and rural areas convect heat to the lower atmosphere. This convection effect depends on the local background climate, contributing 3.0 +/- 0.3 K (mean and standard error, s. e.) warming to daytime DeltaT in cities in humid climate but causing 1.5 +/- 0.2 K cooling in dry climate. Our results also show that in the humid eastern United States, there is evidence of higher daytime UHI in drier years. These relationships imply that UHIs will exacerbate heatwave stress on human health in wet climates. During nighttime, our results reaffirmed that the release of stored heat is the dominant driver of UHIs. Results also support the city albedo management as a viable means of mitigating UHIs on large scales.;Results of the future-climate simulation show that under the high emission climate change scenario (representative concentration pathway 8.5), daytime UHIs generally decrease in all three climate zones, indicating that urban and rural areas respond differently to climate change. Nighttime UHIs do not show any significant trends. Attribution of UHIs demonstrates how urban land-atmosphere interactions change with climate change.