Highly inhomogeneous interactions between background climate and urban warming across typical local climate zones in heatwave and non-heatwave days

TL;DR

This study investigates how radiative and dynamical processes contribute to urban heat island effects during heatwaves across different local climate zones in Sydney, revealing inhomogeneous interactions and the importance of tailored mitigation strategies.

Contribution

It provides a detailed analysis of the inhomogeneous radiative and dynamical interactions between urban heat islands and heatwaves across local climate zones using coupled climate models.

Findings

Maximum temperature difference up to 10 K between HW and non-HW days.

Cloud reduction is the most dominant factor in warming during HWs.

Latent heat flux mitigates warming, causing up to 10 K temperature differences.

Abstract

Urban heat island (UHI) in conjunction with heatwave (HW) leads to exacerbation of thermal stress in urban areas. Prior research on UHI and HW has predominantly concentrated on examining the thermal conditions at the surface and near-surface, with few investigations extending to the radiative and dynamical interactions of UHI and HW, particularly with a focus on the inhomogeneities across local climate zones (LCZs). Here, we analyse the temperature disparity between HW and non-HW conditions across LCZs in the Sydney area by quantifying the contributions of individual radiative and dynamical processes using the coupled surface-atmosphere climate feedback-response analysis method (CFRAM). Three HW events in 2017, 2019, and 2020 are simulated using the Weather Research and Forecasting (WRF) model coupled with the Single-Layer Urban Canopy Model (SLUCM). The maximum temperature difference between HW and non-HW days may reach up to 10 K, with the increased net solar radiation during HWs being comparable to the typical level of anthropogenic heat flux in urban areas. It is also found that the reduction of clouds, the presence of vapor, and the increase of sensible heat contribute to the warming effect at different levels, with the contribution of clouds being the most dominant. Conversely, the generation of dry convection and the increase of latent heat flux lead to mitigating effects, with the latter being more dominant and capable of causing up to 10 K surface temperature difference between LCZ1 (compact high-rise) and LCZ9 (sparsely built). The differences in the contributions of climate feedback processes across different LCZs become more evident during more severe and humid HWs. These findings underscore the necessity of implementing local climate zone-tailored heat mitigation strategies.