Contrasting responses of urban and rural surface energy budgets to heat waves explain synergies between urban heat islands and heat waves

TLDR

Heat waves are projected to become more frequent and longer, posing serious public health concerns, especially for urban residents who experience amplified heat due to urban heat islands, yet the differential responses of urban and rural surface energy budgets to heat waves remain largely unknown. This study analyzes observations from two flux towers in Beijing, China to reveal significant differences between urban and rural (cropland) ecosystem responses to heat waves. The study finds that heat waves amplify urban heat islands—especially at night—by increasing shortwave and longwave radiation and sensible heat fluxes, while rural sites see greater latent heat fluxes, and that these contrasting surface energy budget responses drive the observed synergies, underscoring the need for mitigation measures such as green and white roofs.

Abstract

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 synergies between HWs and urban heat islands (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 Beijing, China and reveals significant differences between the responses of urban and rural (cropland) ecosystems to HWs. It is found that UHIs increase significantly during HWs, especially during the nighttime, implying synergies between HWs and UHIs. Results indicate that the urban site receives more incoming shortwave radiation and longwave radiation due to 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 diverge strongly for 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. As a result, urban mitigation and adaption strategies such as the use of green roofs and white roofs are needed in order to mitigate the impact of these synergies.