Urban areas are usually hotter than their surrounding rural areas, which is the well-known urban heat island (UHI) effect. With continuing urbanization, more people are threatened by the high thermal risks in urban areas, especially during heat waves (HWs). While there has been progress in improving our understanding of the physical processes controlling the urban thermal conditions, key questions remain, especially regarding the relative importance of different physical processes and their spatial variability. To bridge the gap, this dissertation aims to determine the key physical mechanisms that cause surface and near-surface air UHIs, and to quantify the contributions of different physical processes to the daytime warming rate of the urban boundary layer (UBL). In doing so, surface and boundary layer heat budget analyses are conducted using both observations and simulations with the Weather Research and Forecasting (WRF) model.
In the first chapter of this dissertation, we quantify the magnitude of surface urban heat islands (SUHIs), or sometimes surface urban cool islands (SUCIs), and elucidate their biophysical and atmospheric drivers based on observational data collected from one urban site and two rural grassland sites in and near the city of Nanjing, China. The study highlights that the magnitude of SUHIs and SUCIs can vary strongly with the biophysical characteristics of the rural land. In the second chapter, we study the WRF-simulated surface and near-surface air UHIs during heat waves in two cities with contrasting climates (Boston and Phoenix). We demonstrate that the magnitude of UHIs or UCIs during HWs is strongly controlled by urban-rural differences in terms of aerodynamic features, moisture availability and heat storage, which show contrasting characteristics in different regions. In the last chapter, we further apply the UBL heat budget analysis to the WRF simulated results and show that in both Boston and Phoenix the surface sensible heat flux dominates the daytime UBL warming rate, whose behavior is the key to understanding how the daytime UBL warming rate changes throughout HWs. In summary, this dissertation improves our understanding of the physical processes modulating the thermal conditions of urban areas from the ground to the top of the boundary layer and provides scientific guidance on model development and mitigation of extreme heat conditions in cities.
| Identifer | oai:union.ndltd.org:bu.edu/oai:open.bu.edu:2144/43984 |
| Date | 07 March 2022 |
| Creators | Wang, Liang |
| Contributors | Li, Dan, Salvucci, Guido |
| Source Sets | Boston University |
| Language | en_US |
| Detected Language | English |
| Type | Thesis/Dissertation |
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