Insights Into Urban Heat Island and Heat Waves Synergies Revealed by a Land‐Surface‐Physics‐Based Downscaling Method

• Published in: Journal of geophysical research. Atmospheres Vol. 129; no. 13
• Main Authors: Xue, Lingbo, Doan, Quang‐Van, Kusaka, Hiroyuki, He, Cenlin, Chen, Fei
• Format: Journal Article
• Language: English
• Published: Washington Blackwell Publishing Ltd 16-07-2024
• Subjects: Bowen ratio; Coastal zone; Coasts; Computer applications; Daytime; Extreme heat; Extreme high temperatures; Heat; Heat waves; heat waves (HWs); Heatwaves; High temperature; high‐resolution land data assimilation system (HRLDAS); Metropolitan areas; Night; Night-time; Nighttime; Noah‐Multiparameterization land surface model (Noah‐MP LSM); novel downscaling approach; Physics; Rural areas; Rural environments; Simulation; Specific heat; Temperature differences; Temperature effects; Temperature gradients; Urban areas; urban canopy model (UCM); Urban environments; urban heat island (UHI); Urban heat islands; Urban populations; Vegetation
• ISSN: 2169-897X; 2169-8996
• DOI: 10.1029/2023JD040531

Researchers have recently focused on the interplay of the urban heat island (UHI) effect and heat waves (HWs). However, the synergies of these two phenomena remains inconclusive at present. To address this gap, this study investigated UHIs and HWs synergies during the last 30 years in the Tokyo metropolitan area, through a unique and novel approach named Land‐Surface‐Physics‐Based Downscaling (LSP‐DS). LSP‐DS integrates the widely used Noah‐Multiparameterization (Noah‐MP) land‐surface model coupled with urban canopy‐process physics, aiming to conduct high‐resolution, long‐term urban‐specific simulations with much less computational resources. Our comprehensive analysis combining observation data and numerous LSP‐DS simulations confirms exacerbated UHIs during HWs. Specifically, HWs amplify the temperature differences between urban and rural environments, which is quantified by UHI intensity (UHII). During HWs, UHII increased more at night in inland areas and more during daytime in coastal areas. HWs present especially a heightened threat to coastal regions where daytime UHII increased by approximately 1°C during HWs. The Bowen ratio can explain the increase in the daytime UHII, and the daytime accumulated storage heat increase during HWs can explain the increase in nighttime UHII. Based on future projections of the increasing frequency of high temperatures, our findings highlight the impending heat‐related health challenges faced by urban residents.
Plain Language Summary
In the past 30 years in Tokyo, we studied how heat waves influence the urban heat island (UHI) effect using a unique approach called Land‐Surface‐Physics‐Based Downscaling. This method helps simulate urban‐specific conditions in high resolution over a long period using fewer computational resources. Our analysis, combining observations and simulations, confirms that heat waves worsen UHI effects. During heat waves, temperature differences between urban and rural areas (UHI intensity (UHII)) increase more at night in inland regions and more during the day in coastal areas. Coastal areas, especially, face increased daytime UHII by around 1°C during heat waves. Less vegetation in urban areas explains the increase of UHII in the daytime during heat waves, and the higher heat capacity of urban land surface explains the nighttime increase of UHII during heat waves. As heat waves become more frequent and urban populations grow, our findings emphasize the health challenges urban residents may face due to extreme heat.
Key Points
Development of Land‐Surface‐Physics‐Based Downscaling approach for urban climate prediction
Reveals the Added Heat Load (AHL), that is, the enhancement of the urban heat island effect during heat waves (HWs) in the Tokyo region
Positive daytime AHL due to higher increase of urban Bowen ratio during HWs; positive nighttime AHL due to urban's higher heat capacity