Anthropogenic modification of natural landscapes to urban environments impacts land-atmosphere interactions in the boundary layer. Ample research has demonstrated the effect of such landscape transitions on development of the near-surface urban heat island (UHI), while considerably less attention has been given to impacts on regional wind flow. Here we use a set of high-resolution (1 km grid-spacing) regional climate modeling simulations with the Weather Research and Forecasting (WRF) model coupled to a multi-layer urban canopy scheme to investigate the dynamical interaction between the urban boundary layer (UBL) of the Phoenix Metro (U.S.) area and the thermal circulation of the complex terrain it resides within. We conduct paired simulations for the extremely hot and dry summer of 2020, using a contemporary urban representation and a pre-settlement landscape representation to examine the effect of the built environment on local to regional scale wind flow. Analysis of our simulation results shows that, for a majority of the diurnal cycle, 1) the thermo-topographical circulation dominates, 2) the built environment obstructs wind flow in the inertial sublayer during the nighttime, and (3) the built environment of Phoenix Metro produces an UHI circulation of limited vertical extent that interacts with the background flow to modulate its intensity. Such interaction is modulated by greater daytime urban sensible heat flux and dampens the urban roughness induced drag effect by promoting a deeper UBL through vigorous mixing. Our results highlight the need for future research – both observational and simulation based - into urbanizing regions where multi-scale flows are dominant.

The urban environment directly influences the evolution of the Urban Boundary Layer (UBL). Heat adaptation strategies proposed to help cities respond to global change and urban induced warming, are also expected to reduce the intensity of convective mixing and decrease UBL depth, thereby reducing the volume of air available to pollutant dilution and dispersion. We use 20 km resolution WRF-ARW decadal scale simulations that account for end of 21st century greenhouse gas emissions, urban expansion and intensive and uniform implementation of cool roofs, green roofs and street trees to investigate the individual and combined impacts of these drivers on the dynamics of the UBL over the Conterminous US (CONUS). Results indicate that combined impacts of climate change and urban expansion are expected to increase summer (JJA) daytime UBL depth in the eastern regions of CONUS (peak value: Δh ≅ 80 m over Atlanta metro area). When adaptation strategies are applied, summer daytime UBL depth is reduced by a few hundred meters (peak value: Δh ≅ -310 m over Dallas and Fort Worth metro areas) in all CONUS regions as a consequence of decreased surface sensible heat fluxes. Adaptation impacts are greater inland and smaller over coastal cities. In arid regions, the adaptation induced increase in latent heat fluxes can counterbalance the projected decrease in UBL depth. Furthermore, adaptation strategies are expected to increase the static stability of both daytime and nighttime UBLs and decrease the magnitude of vertical winds, inducing earlier and stronger subsidence (peak value: Δm/s ≅ -0.05 m over Phoenix and Tucson metro areas). In light of these findings, ongoing work addressing these aspects with convection resolving, high-resolution simulations is needed to determine whether the widespread implementation of urban adaptation measures could have deleterious effects for urban air quality in the cities of the future Contiguous US.