Impacts of Landscape Modifications on Urban Boundary Layer Dynamics in Current and Projected Climate

The planetary boundary layer (PBL) is the lowest part of the troposphere and is directly influenced by surface forcing. Anthropogenic modification from natural to urban environments characterized by increased impervious surfaces, anthropogenic heat emission, and a three-dimensional building morphology, affects land-atmosphere interactions in the urban boundary layer (UBL). Ample research has demonstrated the effect of landscape modifications on development and modulation of the near-surface urban heat island (UHI). However, despite potential implications for air quality, precipitation patterns and aviation operations, considerably less attention has been given to impacts on regional scale wind flow. This dissertation, composed of three peer reviewed manuscripts, fills a fundamental gap in urban climate research, by investigating individual and combined impacts of urbanization, heat adaptation strategies and projected climate change on UBL dynamics. Paper 1 uses medium-resolution Weather Research and Forecast (WRF) climate simulations to assess contemporary and future impacts across the Conterminous US (CONUS). Results indicate that projected urbanization and climate change are expected to increase summer daytime UBL height in the eastern CONUS. Heat adaptation strategies are expected to reduce summer daytime UBL depth by several hundred meters, increase both daytime and nighttime static stability and induce stronger subsidence, especially in the southwestern US. Paper 2 investigates urban modifications to contemporary wind circulation in the complex terrain of the Phoenix Metropolitan Area (PMA) using high-resolution WRF simulations. The built environment of PMA decreases wind flow in the evening and nighttime inertial sublayer and produces a UHI-induced circulation of limited vertical extent that modulates the background flow. During daytime, greater urban sensible heat flux dampens the urban roughness-induced drag effect by promoting a deeper, more mixed UBL. Paper 3 extends the investigation to future scenarios showing that, overall, climate change is expected to reduce wind speed across the PMA. Projected increased soil moisture is expected to intensify katabatic winds and weaken anabatic winds along steeper slopes. Urban development is expected to obstruct nighttime wind flow across areas of urban expansion and increase turbulence in the westernmost UBL. This dissertation advances the understanding of regional-scale UBL dynamics and highlights challenges and opportunities for future research.