Park et al. (2026) Diurnal evolution of synergistic interactions between urban heat islands and heat waves: An extreme heat wave case study in Seoul, South Korea

Identification

• Journal: Weather and Climate Extremes
• Year: 2026
• Date: 2026-01-09
• Authors: Kyeongjoo Park, Jong-Jin Baik
• DOI: 10.1016/j.wace.2026.100854

Research Groups

• School of Earth and Environmental Sciences, Seoul National University, Seoul, South Korea

Short Summary

This study investigates the diurnal evolution and underlying physical processes of synergistic interactions between urban heat islands (UHIs) and heat waves (HWs) using a numerical simulation of an extreme HW event in Seoul, South Korea. It reveals that UHI-HW synergies strengthen in early evening due to contrasting urban and rural diabatic processes, and weaken in early morning primarily due to differing urban and rural planetary boundary layer (PBL) processes, particularly enhanced warm air advection from the rural residual layer by subsidence.

Objective

• To examine the diurnal evolution of the UHI–HW synergies and its underlying physical processes in detail.
• To conduct a numerical simulation of an extreme HW event in Seoul and its surrounding areas, and perform a detailed analysis of HW-induced changes in urban and rural near-surface thermodynamic energy budgets.

Study Configuration

• Spatial Scale: Three nested domains with horizontal resolutions of 25 km, 5 km, and 1 km. The innermost domain (145 × 135 grid points, 1 km resolution) covers Seoul and its surrounding areas (60 km radius from Seoul center). All domains have 70 vertical layers, with the lowest model level at approximately 43 m and a model-top pressure of 20 hPa.
• Temporal Scale: A 45-day simulation from 0000 LST July 4, 2021, to 0000 LST August 18, 2021, including a 24-hour spin-up period. The extreme HW event occurred from July 21 to 31, 2021. Non-HW periods were defined as July 5–11 (pre-HW) and August 11–17 (post-HW). Daytime was classified as 1200–1700 LST, and nighttime as 0000–0500 LST. Specific periods for rapid changes in synergies were 1810–1930 LST (strengthening) and 0550–0650 LST (diminishing).

Methodology and Data

• Models used:
  • Weather Research and Forecasting (WRF) model version 4.1.3.
  • Dudhia shortwave radiation scheme.
  • Rapid Radiative Transfer Model (RRTM) longwave radiation scheme.
  • WRF single-moment 6-class microphysics scheme (WSM6).
  • Kain–Fritsch convection scheme (turned off in the innermost domain).
  • Yonsei University planetary boundary layer (PBL) scheme.
  • Revised MM5 surface layer scheme.
  • Unified Noah land surface model.
  • Seoul National University urban canopy model (SNUUCM).
  • Thermodynamic energy equation analysis (residual approach for diabatic heating/cooling).
• Data sources:
  • Initial and boundary conditions: Fifth-generation European Centre for Medium-Range Weather Forecasts (ECMWF) reanalysis (ERA5) data (0.25° × 0.25° horizontal resolution, 1-hour temporal resolution).
  • Topography: Shuttle Radar Topography Mission (SRTM) data (~90 m horizontal resolution).
  • Land use/land cover: Korea Ministry of Environment data (4 m resolution).
  • Anthropogenic heat: Gridded dataset for South Korea by Lee and Kim (2015) (1 km horizontal resolution).
  • Model validation: 2-m temperature observations from 13 urban and 14 rural weather stations, and air temperature and wind profiles from Osan radiosonde station.
  • HW event characteristics: Korea Meteorological Administration (KMA) data.
  • Synoptic analysis: ERA5 data for 500-hPa geopotential height, horizontal wind vector, 850-hPa wave activity fluxes, 850-hPa vertical pressure velocity, and 1000-hPa temperature.

Main Results

• The UHI–HW synergies (ΔTsyn) exhibit a significant diurnal variation, rapidly strengthening in early evening (1810–1930 LST) and diminishing in early morning (0550–0650 LST).
• ΔTsyn is prominent during nighttime (mean 1.10 °C) but weak during daytime (mean 0.15 °C).
• During early evening strengthening (1810–1930 LST), the rural diabatic cooling is greatly enhanced under HW due to clearer skies, while urban stored heat release and turbulent mixing increase, resulting in only a slight change in urban diabatic heating. These contrasting responses in diabatic heating/cooling are primarily responsible for the strengthening of ΔTsyn.
• During early morning weakening (0550–0650 LST), warm air advection from the rural residual layer by subsidence significantly increases under HW, whereas subsidence rarely occurs in the urban PBL. These differing responses in vertical advection are mainly responsible for the weakening of ΔTsyn.
• The simulated extreme HW event in Seoul (July 21–31, 2021) reached a maximum 2-m temperature of 39.2 °C and a mean daily maximum of 36.7 °C.
• Model validation showed good agreement with observations, with correlation coefficients of 0.92 (urban) and 0.96 (rural) for 2-m temperature, and root mean square errors of 1.37 °C (urban) and 0.95 °C (rural).
• Adiabatic heating by subsidence was identified as the most crucial physical process for elevating regional/local temperature during the HW event.

Contributions

• This study provides a novel analysis of urban and rural near-surface thermodynamic energy budget changes during HWs to unravel the underlying physical processes for the diurnal evolution of UHI–HW synergies, a topic not thoroughly explored in previous studies.
• It highlights the critical role of differing changes in urban and rural near-surface thermodynamic processes in the temporal evolution of UHI–HW synergies.
• The research employs a high-resolution anthropogenic heat dataset specific to Seoul, enhancing the realism of urban climate simulations.

Funding

• National Research Foundation of Korea (NRF) under grant RS-2025-00562044.

Citation

@article{Park2026Diurnal,
  author = {Park, Kyeongjoo and Baik, Jong-Jin},
  title = {Diurnal evolution of synergistic interactions between urban heat islands and heat waves: An extreme heat wave case study in Seoul, South Korea},
  journal = {Weather and Climate Extremes},
  year = {2026},
  doi = {10.1016/j.wace.2026.100854},
  url = {https://doi.org/10.1016/j.wace.2026.100854}
}