Relationships between urban land surface temperature, air temperature, and NDVI across a coastal to desert climate gradient
Formation of the urban heat island (UHI) is an attribute of interest across scientific disciplines due to potential impacts to human health, peak energy demands, ecosystem function, and local weather. Vegetation can mitigate the heat island effect by reducing urban land surface temperatures (Ts), but the effects of vegetation on air temperature (Ta) are less well understood. Our study examines the effects of vegetation on urban Ta and Ts at local and regional scales across a coastal to desert climate gradient. We quantified air temperature by placing a network of >300 thermochron iButton sensors on urban trees in three cities spanning the gradient (Los Angeles, Riverside, and Palm Springs, CA). Additional sensors were placed in Riverside, CA along vertical gradients above bare soil, irrigated grass, and underneath citrus canopy of two heights (2 m and 4 m). Hyperspectral Infrared Imager (HyspIRI) preparatory data were collected concurrently in June and August and included data products from the Airborne Visible/Infrared Imaging Spectrometer (AVIRIS) and the MODIS/ASTER Airborne Simulator (MASTER) to estimate the Normalized difference vegetation index (NDVI) and Ts.
Relationships were observed between Ts and Ta at 1300 hr (a time period corresponding to airborne data acquisition encompassing all target cities) across the climate gradient in both June (r2 = 0.62, p < 0.001) and August (r2 = 0.22, p < 0.001). Across cities, Ta at 1300 hr in June was ~19 °C cooler than Ts; Ta at 1300 hr in August ranged from 14.9 – 17.3 °C cooler than Ts in Riverside and Los Angeles, respectively. Relationships were also observed between NDVI and Ts within Los Angeles during June (r2 = 0.18, p < 0.001) and across cities in August (r2 = 0.12, p < 0.001). Ta data along vertical gradients revealed that Ta was reduced underneath vegetation canopy relative to grass and bare ground. Daytime (i.e., between 900 to 2000 hr) Ta at 0.1 and 1 m was 5.8 ± 0.1 and 3.0 ± 0.1 °C cooler, respectively, underneath taller canopy compared to bare ground. Vertical profile data demonstrate that vegetation canopy reduces Ta at a microclimate scale, but the effects of vegetation on Ta at neighborhood and regional scales are less clear; resolving these differing effects of vegetation-microclimate dichotomy is an important research direction.