Impact of urban heat island on daily and sub-daily monsoon rainfall variabilities in East Asian megacities

This study evaluates the impact of the urban heat island (UHI) on the daily and sub-daily monsoon rainfall variabilities in East Asian megacities using the high-resolution ground- and satellite-based observations for the period of 1998–2015. The three representative megacity regions, i.e., Guangdong in China, Seoul/Gyeonggi in Korea, and Tokyo in Japan, are particularly considered. A strong UHI day, defined as a summer day with UHI index greater than one standard deviation, is typically drier than normal especially in the urban area. However, when rainy, a distinct rainfall peak appears in the early afternoon, contrasting to the climatological rainfall distribution with a maximum rainfall in the early morning and a secondary maximum in the late afternoon. A stronger early-afternoon rainfall in the urban area than in the rural area becomes more pronounced as UHI intensity increases beyond a certain threshold value. The UHI-induced extreme rainfall in the afternoon, which is larger than climatology, is also robustly found in all three megacity regions. The impact of the UHI on extreme rainfall is the largest in Tokyo (55–75%), followed by Seoul/Gyeonggi (35–65%) and Guangdong (25–50%). Such regional difference can be partly explained by the difference in the geographical location and urbanization progress. This result suggests that East Asian megacities are likely prone to more extreme UHI-induced rainfall with accelerated urbanization.