Development and Formation Mechanism of the Southeast Asian Winter Heavy Rainfall Events around the South China Sea. Part II: Multiple Interactions

About 44% of the cold-season heavy rainfall/flood (HRF) events around the South China Sea require six days or longer to develop from the formation time of their parent cold surge vortices (CSVs). Formations for both the parent CSV and HRF event are involved with interactions of the concerned vortices with two different cold surge flows. The occurrence frequency of the East Asian cold surge flow varies from 4.5 to 6 days. The longevous CSVs enable their developments to interact with the second cold surge flows between formations of these CSVs and HRF events. Two requirements for the formation of HRF events are 1) synchronized occurrence of the HRF event and the northwestern Pacific explosive cyclone and 2) simultaneous occurrence of the maximum speeds among westerlies of the northwestern Pacific explosive cyclone and easterlies of the tropical trade winds and the HRF event. These requirements cannot be met by the CSV at its second maximum peak intensity, but the CSV at this stage plays an indispensible role for the formation of the HRF event to make its intensity and rainfall amount larger than those HRF events without this relay intensification. The development of an HRF event through multiple interactions of CSVs with sequential cold surge flows may pose difficulties to numerically simulate/predict the occurrence of these HRF events over the cold-season rainfall centers around the South China Sea.