Contrasting responses of urban and rural surface energy budgets to heat waves

Heat waves (HWs) are projected to become more frequent and last longer over most land areas in the late 21st century, which raises serious public health concerns. Urban residents face higher health risks due to the urban heat island (UHI) effect and recently reported synergies between HWs and UHIs (i.e., UHIs are higher under HW conditions). However, the responses of urban and rural surface energy budgets to HWs are still largely unknown. This study analyzes observations from two flux towers in the Beijing metropolitan area in China and reveals significant differences between the responses of urban and rural (cropland) ecosystems to HWs. Results indicate that the urban site receives more incoming shortwave radiation and longwave radiation under HWs as compared to the rural site, resulting in a larger radiative energy input into the urban surface energy budget. Changes in turbulent heat fluxes also differ between the urban site and the rural site: latent heat fluxes increase more significantly at the rural site due to abundant available water, while sensible heat fluxes and possibly heat storage increase more at the urban site. These comparisons suggest that the contrasting responses of urban and rural surface energy budgets to HWs are responsible for the synergies between HWs and UHIs.