2. Data & Method
2.1. Data
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Observational data: ASOS (Automated Surface Observing System) — 71 stations with no missing records during the analysis period
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Reanalysis data: ERA5 (ECMWF), 0.25° × 0.25° horizontal resolution
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Analysis period: June–August 2024 (JJA)
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Spatial domain: Korean Peninsula
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Variable: Daily maximum 2 m temperature (Tmax)
2.2. Definition of Heatwave
A heatwave day is defined as a day on which the daily maximum temperature (Tmax) reaches or exceeds 33°C, following the criteria of the Korea Meteorological Administration (KMA).
A heatwave event is defined as two or more consecutive heatwave days at the same station. For each event, the start/end date, duration, and mean/maximum Tmax are recorded.
2.3. Spatial Matching via IDW Interpolation
Since ERA5 provides gridded data while ASOS observations are point-based, ERA5 values were interpolated to each ASOS station location using Inverse Distance Weighting (IDW). For each station, the four surrounding ERA5 grid points were identified, and a distance-weighted average was computed as:
T_IDW = Σ(wᵢ × Tᵢ), where wᵢ = (1/dᵢ) / Σ(1/dᵢ)
where Tᵢ is the ERA5 Tmax at each surrounding grid point and dᵢ is the great-circle distance between the station and the grid point.
2.4. Comparison of Heatwave Characteristics
Heatwave characteristics are compared between ASOS and ERA5 across three aspects:
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Frequency: Total number of heatwave days per station over JJA 2024
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Duration: Mean and maximum length of continuous heatwave events per station
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Intensity: Tmax magnitude on days identified as heatwave days in both datasets, evaluated via bias (ASOS − ERA5), RMSE, and correlation
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Spatial distributions of each metric are mapped across the 71 stations to examine regional differences, with particular attention to coastal vs. inland contrast.