Comparative Analysis of XGBoost-based Upper-Level Outflow Influencing Rapid Intensity Changes of Typhoons in the Western North Pacific

Utilizing multi-source observational data for typhoons over the Western North Pacific (June–October, 1979–2022), this study employs the XGBoost algorithm to objectively identify the four upper-level outflow patterns of typhoons proposed by Chen and Gray (1985) and compares the statistical characteristics, large-scale environmental conditions, and internal structural differences of typhoons during rapid intensification (RI) and rapid weakening (RW) events under different outflow patterns. The Single-channel Outflow equatorward pattern (SE-pattern) occurs most frequently, predominantly in August–September, while the Dual-channel Outflow pattern (D-pattern) and the Single-channel Outflow poleward pattern (SP-pattern) are less frequent but more common in October. Strong outflow in the D-pattern and SP-pattern is more conducive to RI and RW events, respectively. RI events are mostly concentrated within 12–60 hours after typhoon formation, with the No Outflow Channel pattern (N -pattern) having the weakest initial intensity. RW events are concentrated within 12–72 hours after the peak typhoon intensity, with the SE-pattern occurring the earliest and the SP-pattern the latest. RI events are zonally distributed in tropical regions, but the SP-pattern is located further north. The distribution of RW events exhibits a southwest–northeast orientation, but the N-pattern is situated further south. Composite comparative analysis indicates that high sea surface temperature, moist mid-level air, and weak vertical wind shear contribute significantly to RI events across all patterns. In contrast, RW events vary with outflow patterns: the D-pattern and SP-pattern are mainly influenced by declining sea surface temperature, dry air intrusion at mid-levels, and increased vertical wind shear; the N-pattern is affected by both dry air and wind shear; the SE-pattern exists in an environment relatively favorable for RI, and the mechanisms of RW-SP events require more attention. The evolution of the warm core and secondary circulation in typhoons shows that the RI-D typhoon and RI-SE typhoon exhibit significant warming of the warm core with deep vertical extent and enhanced low-level convergence. During RW, the warm core of the D-pattern typhoon weakens and descends rapidly, changes in the SP-pattern are delayed, and the structure of the SE-pattern remains relatively stable, with weakened upper-level divergence and outflow being important features. This study reveals the characteristics and differences of typhoon intensity rapid change events under the influence of different upper-level outflow patterns, providing observational evidence for a deeper understanding of the physical mechanisms of RI and RW events.