Transitory Behaviors of June Rainfall Types over the Korean Peninsula

It was reported that the East Asian Summer Monsoon (EASM) has experienced a significant variability change in the mid-1990s. Then, as a part of EASM, the Changma, a summer rainy season over the Korean peninsula, is expected to undergo significant change as well. In fact, there has been a prevailing June drought over the Korean peninsula over last 20 years. Noting that diurnal variation of rainfall is one important characteristic of the monsoon, it is interesting to examine how the diurnal variation of rainfall has responded to such abrupt change of EASM in the mid-1990s.

We examined how the diurnal variation has evolved, using hourly rain gauge measurements at 60 stations over the peninsula during June from 1974 to 2017. Normally Changma starts in the middle of June. The rainfall data were averaged over 60 stations and associated normalized anomalies were calculated, in order to remove interannual variability. The comparison between the first and second half of the anomaly over the 44-year period (1974 - 2017), reveals that the evening peak becomes stronger while the morning peak becomes weaker, in the latter period. Also found is less precipitation around the noon, in which a precipitation minimum is located. It is noted that the diurnal variation of heavy rainfall greater than 10 mm/hr shows a similar result. Since the morning peak is associated with warm-type rainfall while the evening peak is related to cold type, results suggest that the cold-type rainfall becomes more prevalent over the Korean peninsula during June, likely after the EASM shift in the mid-1990s.