UFEAST-3D: Urban Forest Effects on Anisotropy and Surface Temperature in 3D

Urban trees are important modifiers of urban climate through their shading, evapotranspiration and wind drag effects. They have particularly marked impacts on the radiation and surface temperature distributions within urban canyons, and can be important contributors to urban climate adaptation at multiple scales. The latest generation of urban canopy models have begun to explicitly represent the impacts of trees within the urban canyon, but they lack rigorous data for evaluation. Thermal anisotropy over urban surfaces is known to be large and impacts our ability to use remotely sensed temperatures of cities. Most previous observational and modeling studies of urban thermal anisotropy have focused on densely built downtown regions with little vegetation. Residential and suburban areas with greater tree cover fractions that make up a large part of the overall urban area in many cities have received much less attention. Modeling studies of thermal anisotropy for these neighbourhoods suggest that trees have the potential to increase or reduce thermal anisotropy, but again lack evaluation data.

In this presentation we report on a unique project that employed a suite of instrumentation to characterize the 3D urban surface temperature at scales ranging from individual facets to that of a neighbourhood. Two urban residential neighbourhoods in the Salt Lake City metropolitan area, Liberty Wells and White Sands, were chosen for study in early July 2018. These neighbourhoods are characterized by different levels of urban tree coverage, but similar street layouts and topographic settings. In each neighbourhood a set of temporary fixed microclimate observing stations (air and surface temperature, humidity, solar radiation, wind speed and direction) were deployed to sample representative components of the urban canyon (road, sunlit front lawn, tree shade), in conjunction with a ground-based thermal and LiDAR scanner that provided centimetre scale resolution of the urban surface structure and temperature. Vehicle traverses of road and wall temperature, air temperature, humidity and incident solar and longwave radiation were made within the neighbourhood to extend the spatial assessment of temperature to ~500 m x 500 m. These traverses were coincident with airborne sampling of the surface temperature at select times using a thermal scanner and non-scanning infrared radiometer from different viewing angles and azimuths over the same spatial domain. Above-canyon measurements of radiative and climate parameters were made from a temporary tower located in the study neighbourhood. This combination of observational systems provided a unique dataset that will be used to: characterize the full 3D surface temperature distribution in urban neighbourhoods with trees, assess the thermal anisotropy of such neighbourhoods, and to evaluate urban canopy-layer models with trees. In the context of both urbanization and climate change, better understanding of the impact of trees on city temperature and climate will be useful to city managers and planners as they seek to exploit the benefits of urban vegetation in adapting to our changing climate.