# Recent Spatiotemporal Changes of Subhourly Extreme Rainfall Events in Seoul
## 2 Study Area and Data Description  
Seoul, the capital and largest metropolis of South Korea, is strategically located along the Han River in the northwestern part of the country. The city is surrounded by mountains, which enhance its scenic beauty. Seoul has a humid continental climate characterized by four distinct seasons. Summers are typically hot and humid, while winters are cold and dry, with the city experiencing a monsoon season during the summer months. As one of the most densely populated cities globally, Seoul has a population exceeding 9 million.  
For disaster prevention and monitoring in urban areas, the Korea Meteorological Administration (KMA) has installed approximately 700 automatic weather stations (AWSs) across South Korea. To analyze extreme rainfall events in Seoul, AWSs located within and near the city were investigated. AWSs with an operational period of less than 20 years were excluded from the analysis. The AWSs were chosen to ensure that the Thiessen polygons formed by the AWSs could cover the entire administrative area of Seoul. The final selection included 37 AWSs located within and near Seoul. Figure 1 depicts the locations of the selected AWSs and their Thiessen polygons.  
The average area of each Thiessen polygon was 15.71 km\({}^{2}\), indicating that each AWS represented an area of approximately 4 km by 4 km. This spatial resolution is considered adequate for analyzing the characteristics of rainfall events in urban areas. The names, altitudes (elevation: EL(m)), and observation start years of the selected AWSs are summarized in Table S1. The KMA AWSs record cumulative rainfall data every minute, which are then aggregated every 15 min. In this study, 15 min cumulative rainfall data were collected for each station from their observation start year until 2024, with recordings taken at 0, 15, 30, and 45 min past each hour.  
In this study, extreme precipitation events were defined as AMP events. In urban areas, the impact of rainfall events on drainage systems varies based on their duration and intensity, which are influenced by the size of the drainage basin. Urban inundation can result from short-duration rainfall events lasting less than an hour or from longer events lasting 4-5 h [16; 17; 18]. Thus, using 15 min cumulative rainfall data, the AMPs for durations of 15, 30, 45, 60, 120, 180, 240, 300, and 360 min were calculated. The current study used seven durations: three subhourly durations (15, 30, and 45 min) and four hourly durations (60, 120, 180, 240, 300, and 360 min).  
Figure 2 presents basic statistics of the AMP events for the selected AWS stations in Seoul. As the duration increased, the mean and standard deviation of the AMPs also increased, accompanied by larger dispersion among stations. The coefficients of skewness and kurtosis showed the smallest values at 30 min, increased between 30 and 240 min, and then exhibited a decreasing trend after 240 min. The calculated statistics indicate that the  
Figure 1: Locations of selected AWS stations and their Thiessen polygons in Seoul.  
characteristics of extreme rainfall events in Seoul vary significantly depending on their duration and location.