Influence Mechanism Of Two Types Of Low-frequency Oscillations On Pre-flood Season Precipitation In South China And Its Application In Extended-term Forecasting

Based on the datasets of the persistent heavy rainfall cases identified by China Meteorological Administration(CMA),the National Oceanic and Atmospheric Administration(NOAA)Climate Prediction Center daily precipitation and outgoing longwave radiation(OLR),and the ERA-Interim reanalysis,The characteristics of low-frequency circulation and convection associated with persistent precipitation anomalies in the pre-flood period of South China from 1979 to 2018 are analyzed.The mechanisms and main physical processes responsible for the occurrence of persistent precipitation in the pre-flood period of South China,particularly the influences of two different types of low-frequency oscillations(i.e.,10–20-day and 30–60-day intraseasonal oscillation),are discussed in details.A statistical extended-range forecast model is further established based on the relationship between precipitation anomalies and two types of low-frequency oscillations in the pre-flood period of South China.The power spectrum of precipitation in the pre-flood period of South China shows significant signals with 10–20-day and 30–60-day period.This suggests that the two types of intraseasonal oscillation exert evident modulations on local precipitation and provide favorable background conditions for persistent heavy rainfall occurrence.The statistical analysis of 70 persistent heavy precipitation events in the pre-flood period of South China from 1979 to 2018 and associated 10–20-day and 30–60-day low-frequency convective and circulation fields indicates that both types of low-frequency oscillations contribute to the background environment for the occurrence of heavy precipitation in the pre-flood period in South China.The probability of heavy precipitation event occurrences is increased when both types of lowfrequency oscillations appear a convective phase.In addition,the 10–20-day quasi-biweekly oscillation has a more significant effect on the intensity of persistent heavy rainfall events,while the 30–60-day intra-seasonal oscillation shows a higher correlation with the duration of heavy rainfall events.Based on the diagnoses of scale-decomposed moisture equation and vertical velocity equation,we found that the main source of water vapor for 10–20-day low-frequency precipitation is the horizontal advection process,in which the 10–20-day low-frequency southerly winds deliver tropical abundant water vapor to South China.The vertical motion anomaly related to low-frequency precipitation mainly comes from the vertical gradient of vorticity advection.For 30–60-day low frequency precipitation,both water vapor advection and convergence processes provide positive contributions to moisture accumulation,but the role of water vapor advection is more obvious.The main source of moisture advection is from the interaction between the mean wind field and low-frequency water vapor anomaly.The dynamical conditions are mainly caused by the interaction between the mean wind field and the vorticity perturbations.Finally,based on the relationship between the two types of low-frequency oscillations and precipitation in the pre-flood period of South China,and the associated physical processes,we establish an extended-range forecast model using the spatial-temporal projection method(STPM).The hindcast experiments show that the model is skillful to predict the precipitation anomalies at the lead time of 30 days.The temporal correlation coefficient between the forecasted precipitation and the observational precipitation are significant at the 95%confidence level for most of grids over South China.