Spatial Variability and Long-Term Trend in the Occurrence Frequency of Heatwave and Tropical Night in Korea

Abstract

Extreme hot weather in summer is represented by heatwave and tropical night. Based on long-term (1973-2017) surface observations from 73 selected stations, including 60 automated surface observing systems (ASOSs) operated by Korea Meteorological Administration (KMA) and 13 by North Korea, this study examines the spatial variation of the occurrence frequency of heatwave and tropical night in Korea and their long-term trend. The spatial pattern of the heatwave occurrence frequency has different unique features from that of the tropical night. Heatwave develops more frequently in the southern inland regions, while tropical night occurs more in the southern coastal regions and in the urban areas. These hot spots for heatwave and tropical night exhibit a significant positive trend as well. It is also noticeable that the heatwave becomes more frequent in May and September in recent decades. Development mechanisms for heatwave and tropical night are further examined, particularly based on the relationship with cloud fraction. The occurrence frequency of heatwave is correlated negatively with the cloud fraction throughout the warm season, which indicates the daytime temperature is increasing by low cloud fraction. On the other hand, the occurrence frequency of tropical night shows a positive correlation with cloud fraction in May and September, suggesting that the nighttime temperature is effectively increased by enhanced greenhouse effect by cloud for early and late summer. However, the correlation becomes negative in mid-summer (June to August), in which the increased daytime temperature with low cloud fraction tends to affect the nighttime temperature as well. Because of this, the tropical night is more likely accompanied by the heatwave in recent decades.