The Process of High-Frequency Intraseasonal Oscillation Associated with a Persistent Rainfall Event over South China

In the present study, the moist static energy (MSE) budget associated with a persistent rainfall event that occurred over South China in late August 2018 (PR1808) was analyzed using ERA-Interim data. The results revealed that the PR1808 event was closely related to a 12-30-day intraseasonal oscillation (ISO), with a significant westward-propagating mode. The recharge of MSE occurred before ISO deep convection, and the MSE was discharged during and after the peak precipitation. This result indicates that the recharge-discharge cycle of MSE played an important role in regulating the high-frequency ISO (HF-ISO) process during the PR1808 event. However, the midlevel MSE tendency, rather than the low-level tendency, controlled the column-integrated MSE change in this case, and there was no low-level MSE tendency ahead of HF-ISO convection, which is different from previous studies, implying that shallow convection was not a key factor in this case. The recharge of MSE related to the midlevel MSE change mode was mainly attributed to zonal advection, while the forcing related to radiative heating (longwave and shortwave radiation) and heat fluxes (latent and sensible heat fluxes) contributed little to the change in MSE. Furthermore, for the zonal advection of MSE, the main contribution originated from the advection by the low-frequency zonal flow across the low-frequency MSE gradient, and interactions between high- (low-) frequency zonal flow and low- (high-) frequency MSE. In addition, the disturbances from higher latitudes enhanced the persistent rainfall in this case over South China through the southward shift of baroclinic vorticity.