Deciphering the sensitivity of urban canopy air temperature to anthropogenic heat flux with a forcing-feedback framework

Abstract

The sensitivity of urban canopy air temperature ( Ta ) to anthropogenic heat flux ( QAH ) is known to vary with space and time, but the key factors controlling such spatiotemporal variabilities remain elusive. To quantify the contributions of different physical processes to the magnitude and variability of ΔTa/ΔQAH (where Δ represents a change), we develop a forcing-feedback framework based on the energy budget of air within the urban canopy layer and apply it to diagnosing ΔTa/ΔQAH simulated by the Community Land Model Urban over the contiguous United States (CONUS). In summer, the median ΔTa/ΔQAH is around 0.01 K W m-2-1 over the CONUS. Besides the direct effect of QAH on Ta, there are important feedbacks through changes in the surface temperature, the atmosphere-canopy air heat conductance ( ca ), and the surface-canopy air heat conductance. The positive and negative feedbacks nearly cancel each other out and ΔTa/ΔQAH is mostly controlled by the direct effect in summer. In winter, ΔTa/ΔQAH becomes stronger, with the median value increased by about 20% due to weakened negative feedback associated with ca . The spatial and temporal (both seasonal and diurnal) variability of ΔTa/ΔQAH as well as the nonlinear response of ΔTa to ΔQAH are strongly related to the variability of ca, highlighting the importance of correctly parameterizing convective heat transfer in urban canopy models.