This study is part of the efforts undertaken to explain the rapid intensification (RI) onset of Typhoon Megi by the insight from subseasonal and seasonal SSTA effect. Control and sensitivity experiments are carried out at high resolution domain of WRF model forced by different SSTA contribution. The warm seasonal SSTA signal mainly presents a large-scale average distribution in most of the Northwest Pacific, and the warm subseasonal SSTA is concentrated in the area where the TC passes. Simulation result indicates that TC RI onset in the experiment with subseasonal SSTA (SSTA1) and real SST (CTRL, including both subseasonal SSTA and seasonal SSTA) occur simultaneously, 6-h earlier than the one with seasonal SSTA (SSTA2), although SSTA1 has the coolest inner-core (radius of 0-100 km) and outer-core SST (radius of 100-200 km). Axisymmetric thermodynamic process is demonstrated by the result of that there is a TC warming trend belt of azimuthally potential temperature at 4 km and 8 km earlier and nearly before RI onset, respectively. This heating band extends from the inner-core to outer-core region to offset the ventilation cooling and break the hindering effect of VWS. Comparing TC thermodynamic structures of SSTA1 and SSTA2, the lower-level warm core in SSTA1 is as strong as the other experiments, owing to its coolest outer-core SST and external SST environment. The stronger upper-level radial inflow in SSTA2 flushes to TC upper-level inner-core region, and strongly tilts TC structure. Then the alignment time of the TC structure is prolonged, prompting the ventilation structure around to be slightly strengthened again, delaying the time of RI onset by 6 h.

 

Rapid intensification onset, Typhoon Megi, subseasonal SSTA, seasonal SSTA.