Interactions between urban heat islands and heat waves

Abstract

Heat waves (HWs) are among the most damaging climate extremes to human society. Climate models consistently project that HW frequency, severity, and duration will increase markedly over this century. For urban residents, the urban heat island (UHI) effect further exacerbates the heat stress resulting from HWs. Here we use a climate model to investigate the interactions between the UHI and HWs in 50 cities in the United States under current climate and future warming scenarios. We examine UHI 2m (defined as urban-rural difference in 2m-height air temperature) and UHI s (defined as urban-rural difference in radiative surface temperature). Our results show significant sensitivity of the interaction between UHI and HWs to local background climate and warming scenarios. Sensitivity also differs between daytime and nighttime. During daytime, cities in the temperate climate region show significant synergistic effects between UHI and HWs in current climate, with an average of 0.4 K higher UHI 2m or 2.8 K higher UHI s during HWs than during normal days. These synergistic effects, however, diminish in future warmer climates. In contrast, the daytime synergistic effects for cities in dry regions are insignificant in the current climate, but emerge in future climates. At night, the synergistic effects are similar across climate regions in the current climate, and are stronger in future climate scenarios. We use a biophysical factorization method to disentangle the mechanisms behind the interactions between UHI and HWs that explain the spatial-temporal patterns of the interactions. Results show that the difference in the increase of urban versus rural evaporation and enhanced anthropogenic heat emissions (air conditioning energy use) during HWs are key contributors to the synergistic effects during daytime. The contrast in water availability between urban and rural land plays an important role in determining the contribution of evaporation. At night, the enhanced release of stored and anthropogenic heat during HWs are the primary contributors to the synergistic effects.

Localized Changes in Heat Wave Properties Across the United States

“…In addition, we think that the suggested method in this study helps communities to understand various aspects of heat waves by comparing different locations and heat wave strengths side by side. Similar to other studies, we believe that global warming and increasing urbanization will result in more harmful heat waves across the United States (Habeeb et al, ; Li & Bou‐Zeid, ; Zhao et al, ). Particularly, we think that further urbanization will increase some heat wave components more than others, such as nighttime heat wave intensity from urban heat island effect.…”

“…Earth's Future by side. Similar to other studies, we believe that global warming and increasing urbanization will result in more harmful heat waves across the United States (Habeeb et al, 2015;Li & Bou-Zeid, 2014;Zhao et al, 2017). Particularly, we think that further urbanization will increase some heat wave components more than others, such as nighttime heat wave intensity from urban heat island effect.…”

Increasing Heat Stress in Urban Areas of Eastern China: Acceleration by Urbanization

“…• Perceived heat stress increases faster than air temperature across eastern China; both of them increase faster in urban than in rural areas • Urbanization contributes ≈30% to the increase in mean heat stress and the frequencies of extreme heat stress days and consecutive events • The probability distributions of extreme heat stress and temperature in urban areas exhibit larger shifts toward higher values solar radiation (Oke, 1982). Reduced evapotranspiration (with a smaller amount of vegetation) and surface evaporative cooling over urban areas further intensifies the UHI effect (Taha, 1997;Zhao et al, 2018). Also, urban buildings increase surface roughness, intensifies frictional drag, and reduce surface wind speed, thus inhibiting convective cooling (Fujibe, 2003).…”

Urban Expansion and Drying Climate in an Urban Agglomeration of East China

“…Urban land expansion is associated with urban population growth, economic development, and other regional or local factors such as land use policies, capital flows, and transportation costs (Bren d'Amour et al, 2017;Seto et al, 2011). Urbanization enhances sensible heat flux and soil heat flux at the expense of latent heat flux, thereby leading to the well-known urban heat island (UHI) effect (Jones et al, 2008;Luo & Lau, 2018;Yao et al, 2019;Zhou et al, 2004), with higher temperature in the urban core area than the surrounding rural areas (Kalnay & Cai, 2003;Oke, 1982;Zhao et al, 2018). Urbanization enhances sensible heat flux and soil heat flux at the expense of latent heat flux, thereby leading to the well-known urban heat island (UHI) effect (Jones et al, 2008;Luo & Lau, 2018;Yao et al, 2019;Zhou et al, 2004), with higher temperature in the urban core area than the surrounding rural areas (Kalnay & Cai, 2003;Oke, 1982;Zhao et al, 2018).…”