AUTHORS: Peter Ibsen *, University of California Riverside; Mary Santelmann, Oregon State University; Michelle Talal, Oregon State University; Chris Swan, University of Maryland Baltimore Campus; Dorothy Borowy, University of Maryland Baltimore Campus; David Hondula, Arizona State University; Mary Wright, Arizona State University; Darrel Jenerette,University of California Riverside
ABSTRACT: The combination of increasing urbanization and global warming make cities a hot-spot for climate research. A major research focus is on mitigation of increasing urban warming through vegetation-derived microscale cooling. While vegetation has a documented cooling effect on land surface temperature (LST) and air temperature (Tair), uncertainty exists over the mechanism behind that effect and scale of the drivers. Here we ask how does shading or transpiration drive vegetation cooling, and how cooling effect strength varies from regional to continental scales? To resolve these issues, we placed ~400 Tair and ~100 relative humidity (RH) sensors in four large cities representing varying regional climates (Los Angeles, Portland, Phoenix, Baltimore) during the summer of 2017. To capture a broad range of urban vegetation density, we stratified sensor placement to binned categories of vegetation density. Vegetation density was quantified via satellite measurements of the normalized difference vegetation index (NDVI). Sensors recorded microclimate variables of Tair and RH hourly for ~84 days in each city. Microclimate variables in each city were regressed against local NDVI to assess how vegetation affects regional climate. We then regressed mean city-specific effect size (slopes of NDVI~Tair) with regional classifications of citywide vapor pressure deficit (VPD). Initial results show a pronounced difference of cooling effectiveness among cities, with strongest effects in cities with lower average VPD. All cities experienced changes in in cooling effect size during local heat waves, however, the slope direction differed depending on regional climate, with a negative nighttime effect in humid cities, and an positive effect in arid cities. These results show that vegetation-derived cooling cannot be explained through shading alone, as vegetation density does not explain the changes in effect size within different cities. Resolving mechanism and driver strength of cooling provides better resources for urban climate modeling and warming mitigation.
