Quantifying the City-Scale Benefits of Urban Vegetation and Green Infrastructure using Novel Land-Atmosphere Simulations 

Urbanization substantially modifies surface water and energy cycles. Compared to natural vegetation, paved urban surfaces produce more runoff, trap more heat, and lower evapotranspiration. At the same time, increased heatwaves and rainfall due to climate change are amplified in urban areas due to feedbacks between cities and meteorological processes. Land surface models, the part of atmospheric models tasked with modeling the earth’s surface and hydrology, lack the fine-scale, ecohydrologic process representation in cities to capture important feedbacks between urbanization, hydrology, and near surface energy partitioning. For example, tree cover that shades pavement and enhances evapotranspiration is ubiquitous amongst many cities worldwide, but contemporary land surface models cannot allow for tree canopy to extend over pavements. Further, lateral transfers of surface water from impervious to permeable surfaces are critical for runoff reduction, like routing of rainfall to natural vegetation, but are similarly not represented. Lack of ecohydrologic processes is problematic because we are unable to predict the impact of increasingly common greening initiatives that feature both nature-based solutions, like increased tree cover, and green infrastructure practices, like permeable pavements and green roofs. These practices are targeted to reduce runoff and urban heat, but will likely modify other urban atmospheric processes like rainfall in unknown ways. Unfortunately, potential connections between urban greening initiatives and resulting changes to the urban climate have not been explored rigorously at city scale. In this project, we use Noah-MP for Heterogenous Urban Environments (HUE), a new land surface model capable of resolving fine-scale ecohydrologic processes like urban tree cover shading pavements and routing of surface water to permeable surfaces with multiple landcover types per grid cell (e.g. a mosaicking scheme) in urban spaces. We use HUE to examine the impact of widespread climate adaptation policy in multi-year WRF regional climate simulations centered on the coastal city of Milwaukee, Wisconsin, USA at convective permitting scales. Different landcover configurations that represent cases of city-wide greening are interpreted from an ambitious real-world regional urban greening “master plan.” We show that more greening leads to a reduction of runoff throughout the warm season, although partitioning of runoff reduction between evapotranspiration and deep drainage varies year to year. We also examine how changes in sensible and latent heat fluxes affect near surface meteorology within the city, generally increasing humidity and decreasing air temperatures. These differences are especially apparent during days of strong lake-breeze coupling between Milwaukee and nearby Lake Michigan. We further show that urban greening leads changes in rainfall event totals, peak intensities, and seasonal averages. While only for a single city, our results highlight that widespread urban greening changes not only urban hydrology but also urban hydrometeorology. This highlights that the evaluation of urban greening initiatives worldwide is critical for climate change adaptation and mitigation.