Influence of Projected Climate Change, Urban Expansion and Adaptation Strategies on End of 21st Century Urban Boundary Layer Dynamics in the Conterminous US

The evolution of the Urban Boundary Layer (UBL), located within the planetary boundary layer, is determined by solar radiation and the synoptic scale circulation, but also responds to additional forcings characteristic of the urban environment. Built structures and paved surfaces alter the ventilation patterns and the surface energy balance of cities. Additionally, human activities, like industrial production, fossil fuel based transportation and building heating/cooling systems, lead to the production of anthropogenic heat and the emission of pollutants and chemical compounds to the atmosphere. Because cities must deal with the twin pressures of global change and urban induced warming, strategies to reduce heating within the built environment have been proposed. Such strategies include increasing material reflectivity through albedo modification (e.g. cool roofs and streets), deploying evaporative (or green) roofs, reducing incoming solar radiation through increased street tree planting, or decreasing building heat storage through the use of low thermal inertia construction materials. In addition to consequences for near-surface temperatures these interventions are also expected to reduce the intensity of convective mixing, thereby leading to a decrease of the UBL depth that could lead to a degradation of perceivable air quality as emitted pollutants are confined to a smaller volume of air. 20 km resolution WRF-ARW simulation results that account for end of 21^st century greenhouse gas emissions, urban expansion and adaptation strategies (Krayenhoff et al., 2018) are used to investigate the extent of such impacts on the dynamics of the UBL, in order to quantitatively and qualitatively estimate the tradeoff between achieving thermal comfort and reducing convective mixing in future Continental US cities.

Results indicate that when adaptation strategies are applied, end of 21^st century summer (JJA) UBL depth is reduced by a few hundred meters (50-600 m reduction for Cool Roofs implementation, 30-250 m for Green Roofs and 100-800 m for Full Adaptation) as a consequence of decreased surface sensible heat flux. Such a reduction displays a longitudinal gradient increasing westward and a generally greater areal extent of depth change for the Green Roofs and the Full Adaptation scenarios than for Cool Roofs. Our results highlight the importance of considering the various impacts of adaptation measures from a multiple co-benefits perspective.

Krayenhoff, E. S., Moustaoui, M., Broadbent, A. M., Gupta, V., & Georgescu, M. (2018). Diurnal interaction between urban expansion, climate change and adaptation in US cities. Nature Climate Change. https://doi.org/10.1038/s41558-018-0320-9