Sub-daily extreme rainfall downscaling and IDF curve estimation under AR6 scenarios

Reliable planning for hydraulic and urban drainage under climate change requires station-scaled IDF(Intensity-Duration-Frequency) curves that represent future sub-daily extremes. We develop IDF projections for 60 stations across Korea using long-term observations and an ensemble of regional climate model outputs under SSP2-4.5. Sub-daily annual maxima(1 24 h) are generated from daily maxima via a conditional copula-based temporal downscaling scheme that treats the 24-h annual maximum as the conditioning variable. Multi-day(48, 72h) extremes are taken from bias-corrected series using quantile delta mapping, and frequency analysis applies the Gumbel distribution with probability-weighted moments. Changes are summarized by return period and three future time slices(2015 - 2040, 2041 - 2070, 2071 - 2100). The results indicate widespread amplification of design rainfall, with larger percentage increases for longer durations and rarer events, and with clear spatial heterogeneity stronger signals along the eastern and southern coasts and more moderate responses over inland basins. The downscaling framework preserves the climate-change signal of the driving models while providing station-level, sub-daily estimates suitable for engineering application. These projections offer a consistent basis for updating rainfall design standards and reassessing flood risk in a warming climate. More broadly, the copula-based approach provides a practical pathway to translate daily climate scenarios into sub-daily extremes for adaptation planning, while highlighting the need to consider duration and return-period dependence, as well as regional variability, when revising design criteria.