Localized synergies between heat waves and urban heat islands: Implications on human thermal comfort and urban heat management

Abstract

Heat waves (HWs) and urban heat islands (UHIs) can potentially interact. The mechanisms behind their synergy are not fully disclosed. Starting from the localized UHI phenomenon, this study aims i) to reveal their associated impacts on human thermal comfort through three different definitions of HW events, based on air temperature (airT), wet-bulb globe temperature (WBGT) and human-perceived temperature (AppT) respectively, and ii) to understand the role of air moisture and wind. The analysis was conducted in four districts (NH, JD, MH and XJH) with different urban development patterns and geographic conditions, in the megacity of Shanghai with a subtropical humid climate.

Results evidenced the localized interplay between HWs and UHIs. The results indicate that less urbanized districts were generally more sensitive to the synergies. JD district recorded the highest urban heat island intensity (UHII) amplification, regardless of the specific HW definition. Notably, during AppT-HWs, the increment was observed in terms of maximum (1.3 °C), daily average (0.8 °C), diurnal (0.4 °C) and nocturnal UHII (1.0 °C). Nevertheless, localized synergies between HWs and UHIs at different stations also exhibited some commonalities. Under airT-HW, the UHII was amplified throughout the day at all stations. Under WBGT-HW, diurnal UHII (especially at 11:00–17:00 LST) was consistently amplified at all stations. Under AppT-HW conditions, the nocturnal UHII was slightly amplified at all stations. Air moisture and wind alleviated the synergistic heat exacerbation to the benefit of thermal comfort. The extent depended on geographic condition, diurnal and nocturnal scenarios, temperature type and HW/normal conditions. Stronger HW-UHI synergies indicate the necessity to develop specific urban heat emergency response plans, able to capture and intervene on the underlying mechanisms. This study paves to way to their identification.

Introduction

Nowadays, many cities face the combined threat of heat waves (HWs) and urban heat islands (UHIs). Periods of extremely hot weather (HWs) are becoming increasingly frequent with global climate change (Perkins et al., 2012; Xu et al., 2016). In cities extra heat is generated/stored/absorbed/entrapped with respect to surrounding rural/suburban areas and this phenomenon (UHI) is also increasingly significant with the urbanization (Oke et al., 2017). The temperature increase caused by either HWs or UHIs has enhanced exposures of human bodies to dangerous overheating (Tong et al., 2010a; Habeeb et al., 2015). In particular, the HW has been recognized as the deadliest weather-related disaster in various countries across different continents (De Bono et al., 2004; City of Sydney, 2015; Shafiei Shiva et al., 2019; Anderson and Bell, 2011). UHIs are also a severe cause of worsened thermal comfort and increased morbidity and mortality (Johnson and Wilson, 2009; Tan et al., 2010).

On top of that, HWs and UHIs can potentially interact. The threats of each, taken individually, can be further magnified if they can foster each other (Li and Bou-Zeid, 2013; Li et al., 2016; Zhao et al., 2018; An et al., 2020). For instance, in temperate cities of the US, the near-surface UHI and surface UHI during HW periods can be 0.4 °C and 2.8 °C stronger than that during non-HW (NHW) periods, respectively (Zhao et al., 2018). In Athens (Mediterranean climate), Greece, the airborne UHI under HW conditions could be 3.5 °C stronger than that under summer background condition (Founda and Santamouris, 2017). Nevertheless, the synergies between HWs and UHIs are not consistent (McGregor et al., 2007; Liao et al., 2018; Kumar and Mishra, 2019; Jiang et al., 2019). For instance, during HW periods, diurnal UHIs in Beijing (humid continental climate) and Shanghai (humid subtropical climate) could be respectively magnified by 0.2 °C and 0.9 °C, while the diurnal UHI of Guangzhou (humid subtropical climate) could be alleviated by 0.2 °C. In comparison, the nocturnal UHIs of Beijing, Shanghai and Guangzhou were aggravated by 0.9 °C, 0.3 °C and 0.8 °C, respectively (Jiang et al., 2019). Data collected in India also evidenced such phenomenon, where both diurnal and nocturnal surface UHIs were mitigated by ~0.3 °C under HW conditions, compared with non-heat wave (NHW) conditions (Kumar and Mishra, 2019).

Such inconsistent interconnections between HWs and UHIs have motivated many studies to examine if there could be a synergistic interaction depending on the context and to reveal the mechanisms behind it. For instance, Li and Bou-Zeid (2013) pointed out that lack of surface moisture and weak wind were critical to UHI aggravation during HW periods. Zhao et al. (2018) examined 50 US cities, concluding that climatic condition is a critical factor: while there could be positive synergistic interactions in temperate climates, there was none in dry climates. Rogers et al. (2019) examined whether UHIs could be aggravated by HWs in Australia, pointing out that both Melbourne and Adelaide underwent stronger nocturnal UHIs under HW conditions, while the nocturnal UHI in Perth was weaker compared with NHW conditions, where wind speed, wind direction and station location affected the quantification of such synergies. Kumar and Mishra (2019) concluded that it was anthropogenic activities (e.g. irrigation) that led to the UHI intensity (UHII) reduction in India. Jiang et al. (2019) pointed out that solar radiation and wind direction were critical to UHI amplification during HW periods in Beijing, Shanghai and Guangzhou, China. Hong et al. (2019) highlighted that urban development pattern could reinforce HWs of Seoul, Korea and the interaction of HWs and UHIs depended on weather conditions and localized urban typologies. Pyrgou et al. (2020) concluded that moisture content could be a factor influencing HW and UHI synergies in Nicosia, Cyprus.

Nevertheless, the mechanisms behind synergies between HWs and UHIs are still not fully unveiled. UHIs are generally regarded as localized phenomena, which are sensitive to local variations of thermal, moisture, aerodynamic, radiant and anthropogenic properties (Oke et al., 2017) and explicit factors of land use/land cover (e.g. pervious, impervious surface) (He et al., 2019; Guo et al., 2020), urban development and typology (Stewart and Oke, 2012; Lemonsu et al., 2015), wind and urban ventilation (Yun et al., 2020; He, 2018; Yang et al., 2019a), construction materials (Santamouris, 2014; Qi et al., 2019), green and blue infrastructures (Zhang et al., 2020), etc. Therefore, localized UHIs may contribute to the variability of the interplay between HWs and UHIs. However, previous studies mainly examined the city as a whole with no consideration for its multiple microclimates (Pyrgou et al., 2020; Jiang et al., 2019; Kumar and Mishra, 2019; Founda and Santamouris, 2017). Here we suggest a local scale investigation to relate the specific interactive mechanism to homogeneous boundary conditions and reduce the climate-dependent uncertainties. Meanwhile, since existing studies have concluded that wind velocity and moisture are critical factors (Li and Bou-Zeid, 2013; Rogers et al., 2019; Hong et al., 2019), it becomes essential to further understand the local synergies between HWs and UHIs associated with air moisture and wind.

Moreover, both HWs and UHIs can generate adverse impacts on human thermal comfort and thereby trigger heat-related mortality and morbidity, especially among the vulnerable groups (e.g. elderly people, young children, patients) (Tong et al., 2010b; Basagaña et al., 2011). However, previous studies have mostly looked at HWs as meteorological events and neglected public health concerns (Robinson, 2001; Xu et al., 2016). As a result, the variability of human thermal comfort associated with the synergies between HWs and UHIs is still underexplored. It becomes particularly imperative as wind velocity and air humidity are two important factors among several that can influence heat stress and human thermal comfort (ASHRAE, 2010).

Motivated by above-mentioned knowledge gaps, this study aims to investigate localized synergies between HWs and UHIs correlated with air humidity and wind velocity and to reveal the associated influence on heat stress and human thermal comfort. Four districts in the megacity of Shanghai were considered, characterized by subtropical humid monsoon climate. The four districts are different in urbanization ratio (e.g. population, urban density) and proximity to the coast shore, thus featuring different patterns of urban development and typology, air humidity distribution and air circulation. HWs were defined in terms of air temperature (AirT), wet-bulb globe temperature (WBGT) and human-perceived temperature (apparent temperature, AppT), all metrics that can reflect heat stress and human thermal comfort (Jiang et al., 2019; Kang et al., 2020). The WBGT lumps the influence of AirT and relative humidity (RH), whereas the AppT considers the influence of AirT, RH and wind velocity.

The objectives of this study are (1) to characterize AirT-HWs, WBGT-HWs and AppT-HWs within four districts in a consistent climate context, (2) to reveal the localized synergies between HWs and UHIs and understand their interplay, (3) to understand the variations of air moisture and wind along with localized synergies between HWs and UHIs and (4) to quantify the influence of air moisture and wind velocity. Overall, this study is expected to add the knowledge of understanding the mechanisms through which HWs and UHIs can couple, mutually interact, feedback and self-sustain. Moreover, the findings of this study could practically support policymakers and urban managers to appropriately respond to the potential exacerbation of public health concerns.