Observed Synergies between Urban Heat Islands and Heat Waves and Their Controlling Factors in Shanghai, China

Abstract

There is an ongoing debate as to whether the UHI intensity (UHII) is enhanced or dampened under heat waves (HWs). Using a comprehensive dataset including continuous surface energy flux data for three summers (2016–18) and automated weather station data for six summers (2013–18) in Shanghai, China, we find synergies between UHIs and HWs when either a coastal or an inland suburban site is used as the reference site. We further find that during HWs, the increase of net radiation at the urban site is larger than that at the suburban site. More importantly, the latent heat flux is slightly reduced at the urban site but is slightly increased at the suburban site, while the increase of the sensible heat flux is larger at the urban site. This change of surface energy partitioning, together with the increased anthropogenic heat flux during HWs, exacerbates the UHII. The change of surface energy partitioning is consistent with the observed decrease of relative humidity ratio between urban and suburban areas. The UHII is stronger when the regional wind speed is reduced and under sea breeze, both of which are found to be associated with HWs in our study region. This study suggests that there are multiple factors controlling the interactions between UHIs and HWs, which may explain why synergies between UHIs and HWs are only found in certain metropolitan regions and/or under certain HW events.

Effectiveness of Urban Hydrological Processes in Mitigating Urban Heat Island and Human Thermal Stress During a Heat Wave Event in Nanjing, China

“…Heat waves, periods of abnormally hot weather, exacerbate the UHI effect and cause cities to be more vulnerable, resulting in higher heat stress for urban residents (Li & Bou‐Zeid, 2013). In the Yangtze River Delta, UHI‐enhanced heat waves have shown an increasing trend in recent years (Ao et al., 2019; Tan et al., 2010; Xia et al., 2016). The combination of UHI effects and heat waves is becoming an important issue in this area because of its hot summer season.…”

Adapting to Extreme Heat: Social, Atmospheric, and Infrastructure Impacts of Air-Conditioning in Megacities—The Case of New York City

“…The UHI effect is a result of how built-up areas modify land-air processes as a result of high heat capacity materials, high density of vertical structures with varying heights, limited green spaces, and anthropogenic heat release [7][8][9]. This UHI may intensify periods of extreme heat locally due to a lack of surface moisture in urban areas, low wind speed associated with heat waves, and increased heat storage and generation [6,[10][11][12][13][14][15]. As a result, these intensified heatwaves are projected to cause an increased demand in air-conditioning (AC) [16,17] with a rise in global AC ownership of about two-thirds of all households by 2050 [18].…”

High‐resolution global climate simulations: Representation of cities

“…The frequency of occurrence and magnitude of very warm (e.g., heat waves) or cold air temperatures in cities can be affected by the UHI (e.g., Lemonsu et al ., 2015; Ramamurthy and Bou‐Zeid, 2017; Ao et al ., 2019) and can be further exacerbated by feedback mechanisms with Q F (e.g., through air‐conditioning; Takane et al ., 2020). Representing such processes in climate projections for cities is crucial for various urban applications, such as related to thermal comfort/heat stress and associated mitigation strategies (e.g., urban greening; Zölch et al ., 2016).…”