Variation in the urban vegetation, surface temperature, air temperature nexus

Our study examines the urban vegetation - air temperature (T_a) - land surface temperature (LST) nexus at micro- and regional-scales to better understand urban climate dynamics and the uncertainty in using satellite-based LST for characterizing T_a. While vegetated cooling has been repeatedly linked to reductions in urban LST, the effects of vegetation on T_a, the quantity often used to characterize urban heat islands and global warming, and on the interactions between LST and T_a are less well characterized. To address this need we quantified summer temporal and spatial variation in T_a through a network of 300 air temperature sensors in three sub-regions of greater Los Angeles, CA, which spans a coastal to desert climate gradient. Additional sensors were placed within the inland sub-region at two heights (0.1m and 2m) within three groundcover types: bare soil, irrigated grass, and underneath citrus canopy. For the entire study region, we acquired new imagery data, which allowed calculation of the normalized difference vegetation index (NDVI) and LST. At the microscale, daytime T_a measured along a vertical gradient, ranged from 6 to 3°C cooler at 0.1 and 2m, underneath tall canopy compared to bare ground respectively. At the regional scale NDVI and LST were negatively correlated (p<0.001). Relationships between diel variation in T_a and daytime LST at the regional scale were progressively weaker moving away from the coast and were generally limited to evening and nighttime hours. Relationships between NDVI and T_a were stronger during nighttime hours, yet effectiveness of mid-day vegetated cooling increased substantially at the most arid region. The effectiveness of vegetated T_a cooling increased during heat waves throughout the region. Our findings suggest an important but complex role of vegetation on LST and T_a and that vegetation may provide a negative feedback to urban climate warming.