Insights into urban heat island and heat waves synergies revealed by a Land-Surface-Physics-Based Downscaling method

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 degrees 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.