Diurnal evolution of synergistic interactions between urban heat islands and heat waves: An extreme heat wave case study in Seoul, South Korea

The synergistic interactions between urban heat islands (UHIs) and heat waves (HWs) refer to a phenomenon that UHIs are intensified during HWs, resulting in a greater temperature increase due to HWs in urban areas than in rural areas. Despite significant variations of the UHI–HW synergies on a diurnal timescale, their key physical processes are still not clearly understood. This study examines the diurnal evolution of the UHI–HW synergies and its underlying physical processes. For this, we simulate an extreme heat wave event with strong UHI–HW synergies in Seoul, South Korea using the Weather Research and Forecasting (WRF) model. The UHI–HW synergies rapidly strengthen in early evening and diminish in early morning, similar to the diurnal pattern found in many large cities. In early evening, the rural diabatic cooling is greatly enhanced under clearer skies during HW while the increases in both urban stored heat release and urban turbulent mixing result in only a slight change in urban diabatic heating. These contrasting responses are primarily responsible for the strengthening of the UHI–HW synergies. In early morning, the warm air advection from the rural residual layer by subsidence significantly increases under HW while subsidence rarely occurs in the urban boundary layer. These different responses to HW are mainly responsible for the weakening of the UHI–HW synergies. This study underlines the importance of differing changes in urban and rural near-surface thermodynamic processes during HWs on the temporal evolution of the UHI–HW synergies. • Synergies between urban heat islands and heat waves in Seoul are examined. • The synergies rapidly strengthen (weaken) in early evening (early morning). • Pronounced increase in urban stored heat release acts to strengthen the synergies. • Enhanced advection of warm residual-layer air in rural areas weakens the synergies. • Different urban and rural near-surface thermodynamic budget changes are important.