Clouds as the Primary Driver of Recent Prolonged Summer Heat Waves in South Korea

Recent prolonged heat waves in South Korea have raised questions about the key factors influencing their duration. This study investigates the potential physical drivers affecting the duration of Korean summer heat waves over 50 years (1973–2022), categorizing two types of events: short-term (5–7 days) and long-term (≥16 days) events. Using JRA-55 reanalysis data, we examine the distinct characteristics of both event types in relation to components contributing to the surface energy budget. Additional attention is given to the role of soil moisture and clouds in interacting with these components. The primary cause of both heat wave events is an increase in net shortwave radiation flux, attributed to anticyclonic circulation over South Korea, resulting in decreased clouds. Nonetheless, notable differences emerge: the short-term event exhibits a rapid recovery in all-altitude clouds, while the long-term event displays a slower recovery in low-level clouds. Continuously fewer low-level clouds allow much more incoming solar radiation, mainly contributing to the prolonged heat waves. This is linked to a dry atmosphere and weak atmospheric instability, which inhibits the development of lower-level clouds. Moreover, long-term events also exhibit a sudden increase in clouds at 100–200 hPa, intensifying the trapping effect on outgoing longwave radiation in the atmosphere, and subsequently leading to surface warming. This study enhances a comprehensive understanding of the mechanisms behind prolonged summer heat waves in South Korea, providing valuable insights into the complex interplay of atmospheric components.