Large scale circulation and low frequency signal characteristics for the persistent extreme precipitation in the first rainy season over South China in 2013

From April to June, widespread severe rainstorms events mainly concentrate over South China (SC). During the period, the persistent extreme precipitation events (PEPEs) are in high incidence and rainfall during this period accounts for nearly 40%-50% of the annual total. It is usually called the first rainy season (FiRS) over SC. From late March to mid-June in 2013, the precipitation frequently occurred in SC with multiple precipitation processes, severe intensity and long duration. Based on the statistical methods such as the wavelet analysis, the cross wavelet transform, the wavelet coherence, the ensemble empirical mode decomposition (EEMD) and the band pass filter, the PEPEs are analyzed. The results show that two persistent extreme precipitation processes occurred during the period of 26 March-11 April (which is called the first stage hereafter) and 23 April-30 May (which is called the second stage hereafter), respectively. During the first stage, rainfall amount increased from the north to the south region and accounted for 24.7% of the rainfall in FiRS with rainbelt extending from the southeast to northwest. However, during the second one, the precipitation intensity was stronger than that in the former and the amount reached 54.0% with rainbelt over the north and southeast coast region in SC. There are the predominant factors played an important role in the two persistent extreme precipitation processes. In the first stage, precipitation was continuously influenced by cold airflow from the north. In the latter, however, it was mainly affected by the tropical convection system from the south. Moreover, two PEPEs have been affected by the different large-scale atmosphere circulations. For the former one, the meridional circulation pattern with the high/low value system in the eastern/western region presented in the middle troposphere. A cold vortex continuously maintained over Northeast China, and the cold surge merged with the warm current from the northwest flank of the West Pacific Subtropical High (WPSH). It suggested that, the cold frontal precipitations hold a leading post in the first stage. Nevertheless, in the second one, the meridional circulation is rectified into the pattern with one trough between double ridges in the 500 hPa geopotential height. Before the monsoon breakout over the South China Sea (SCS), precipitation was still controlled by the cold front. There existed a duration transition between the first and the second stages. From perspective of precipitation low frequency feature, it transited from the intraseasonal oscillation into the quasi-biweekly one. After the monsoon breakout, the cold air decreased in severity. The WPSH has retreated from the SCS and the water vapor passage from the Bay of Bengal (BOB) established. In the meantime, the convection systems began active in the low latitudes. The troughs over the south branch were more active from the Indo-China Peninsula to SC. At the moment, an anomaly cyclonic circulation presented over the SC region. The warm convective precipitation occurred more often under the unsteady stratified atmosphere with remarkable convective activity developing in the second stage. Low frequency oscillation characteristics of precipitation and their predominant factors are further analyzed. Precipitation in the first stage showed a 20-50 d low frequency oscillation, which was affected by the active cold surge from the north region. In the second stage, it showed a 8-15 d low frequency oscillation, which was impacted by the strengthened WPSH and the SCS monsoon onset from the south. By using of the cross wavelet transform, the cold vortex over Northeast China and the zonal component of water vapor transportation over SCS are likely able to be used as possible early signals for the FiRS precipitation over the SC region for the extended range forecast.