Characteristics and possible mechanisms of diurnal variation of summertime precipitation in South Korea

Abstract

This study analyzes the characteristics of diurnal variation of precipitation in South Korea in June and July using 10-yr nationwide rain gauge data and investigates possible mechanisms for this. The diurnal variation of precipitation amount has two peaks, a primary peak at 05–08 LST and a secondary peak at 16–18 LST. Both the precipitation intensity and frequency contribute to each of the two peaks, but to the morning (late afternoon) peak, the contribution of precipitation intensity (frequency) is slightly larger. Spatially, afternoon-to-evening peaks (14–22 LST) appear in the mountainous regions and the south coastal region, and late night-to-morning peaks (02–10 LST) appear in the west and east coastal regions and the western inland regions. Rainy days are divided into days with late night-to-morning peaks (LNMP days) and days with afternoon-to-evening peaks (AEP days), which are associated with different mechanisms. The LNMP days are characterized by enhanced large-scale low-level southwesterlies toward South Korea and resultant large-scale and heavy precipitation, while the AEP days are characterized by relatively weak synoptic forcing for precipitation. On the LNMP days, the low pressure anomaly over the Yellow Sea in the late night and the nocturnal acceleration of low-level monsoonal southerlies driven by the boundary layer inertial oscillation cause the late night-to-morning precipitation peak. The radiative cooling at the cloud top and increased upper-level snow in the late night also contribute to the peak. On the AEP days, the destabilization of the low atmosphere due to daytime surface heating yields a higher chance of convective precipitation in the afternoon. Strong near-surface convergence in the afternoon over the mountainous regions due to sea breezes and upslope winds enhances afternoon precipitation. The downslope winds from the mountainous regions cause near-surface convergence over the inland region in the evening, contributing to evening precipitation.