Physical Modeling for Active Phase of Multiple Tropical Cyclone Events by Using an Anomaly-Based Method

Based on the analysis of multiple tropical cyclone (MTC) events in the South China Sea and Northwest Pacific Ocean during 1979–2019, this study classfies periods of the tropical cyclone (TC) events into active, normal, and inactive phases. To analyze the spatial distribution of associated anomalous variables and indices, an anomaly-based variable model is employed. Anomalies of 850 hPa vorticity, 850 hPa water vapor flux divergence, and 400 hPa vertical velocity are selected as optimal predictors with physical significance. From these predictors, a physical model for the original MTC development is established. The results show that during the period of MTC development, a stable warm-core anomaly persists at 300 hPa, with a "warm tongue" extending downward as far as 700 hPa. The upper-level high-pressure anomaly center does not completely overlap with the low-pressure anomaly center at lower levels until TC genesis approaches. In addition, the 500–1000 hPa water vapor flux initially exhibits a negative anomaly, which facilitates water vapor to accumulate. A negative vertical velocity anomaly then develops, promoting the upward motion that gradually stabilizes and expands to a wider range. Ultimately, these processes lead to the formation of a positive vorticity anomaly, signifying a TC's formation. Among the three key factors, the water vapor flux divergence anomaly serves as the primary indicator for extended-range monitoring and forecasting of MTC events. It accounts for the highest proportion of TCs, with its anomaly values most frequently exceeding the critical thresholds at TC genesis locations. Meanwhile, the water vapor flux divergence anomaly also provides the earliest anomalous signal and demonstrates the most sustained and stable indicative effect.