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

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.

Different Patterns in Daytime and Nighttime Thermal Effects of Urbanization in Beijing-Tianjin-Hebei Urban Agglomeration

“…This phenomenon meant that when background LST increased, urban area LST experienced faster increase than the nearby rural areas, similar with the heat wave exaggeration results on SUHII [69,70]. In our study, the SUHII reached a peak at 13:30 in summer for the two MODIS daytime observation times.…”

“…With the increase of background LST from 10:30 to 13:30, urban areas received more incoming shortwave radiation and longwave radiation compared to near rural areas, resulting in more absorbed radiation. Moreover, sensible heat increased more significantly at urban areas due to more impervious materials and scarcity of water, while latent heat increased more significantly at the rural areas due to abundant vegetation and water [70].…”

Characterization of Urban Heat and Exacerbation: Development of a Heat Island Index for California

“…For example, Li and Bou-Zeid, 2013 [51] used observational data from Baltimore, MD, to show that heat waves increase the UHI intensity. Their data demonstrate that the 2008 heat wave increased the nighttime UHI from 0.5 to 2.5 • C and the daytime UHI from 0.25 to 1.5 • C. Li et al, 2015 [52] also use observational data from China to show that heat waves increase the UHI. Thus if warmer weather increases the UHI, then mitigation measures such as cool cities will become more important in the future [53], when heat waves are expected to occur more frequently.…”

The Analytical Objective Hysteresis Model (AnOHM v1.0): methodology to determine bulk storage heat flux coefficients

“…Residual: practical difficulties of direct measurement of Q S in urban areas, result in the SEB residual (i.e. Q * +Q F −(Q H + Q E )) frequently being the "preferred" observations (Ao et al, 2016;Ching et al, 1983;Doll et al, 1985;Li et al, 2015;Oke and Cleugh, 1987) (where Q F is the anthropogenic heat flux).…”