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 (\(T_a\)) to anthropogenic heat flux (\(Q_{AH}\)) 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 \(\Delta T_a/\Delta Q_{AH}\) (where ∆ 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 \(\Delta T_a/\Delta Q_{AH}\) simulated by the Community Land Model Urban over the contiguous United States (CONUS). In summer, the median \(\Delta T_a/\Delta Q_{AH}\) is around 0.01 \(\mathrm{K}\,(\mathrm{W\,m^{-2}})^{-1}\) over the CONUS. Besides the direct effect of \(Q_{AH}\) on \(T_a\), there are important feedbacks through changes in the surface temperature, the atmosphere-canopy air heat conductance (\(c_a\)), and the surface-canopy air heat conductance. The positive and negative feedbacks nearly cancel each other out and \(\Delta T_a/\Delta Q_{AH}\) is mostly controlled by the direct effect in summer. In winter, \(\Delta T_a/\Delta Q_{AH}\) becomes stronger, with the median value increased by about 20% due to weakened negative feedback associated with \(c_a\). The spatial and temporal (both seasonal and diurnal) variability of \(\Delta T_a/\Delta Q_{AH}\) as well as the nonlinear response of \(\Delta T_a\) to \(\Delta Q_{AH}\) are strongly related to the variability of \(c_a\), highlighting the importance of correctly parameterizing convective heat transfer in urban canopy models.