A numerical study of the rapid intensification of Typhoon Bopha (2012)

Typhoon Bopha (2012) originated unusually close to the equator and underwent two periods of rapid intensification (RI) during its lifetime. It is the second southernmost super typhoon on the record. In this study, the first 48-h RI process was reproduced reasonably well using the Advanced Research and Weather Forecasting model (ARW-WRF) with large-scale spectral nudging to ensure good track simulation. Processes responsible for the RI were analyzed. The results show that the vertical wind shear (VWS) in the inner core region decreased at the onset of RI and oscillated during the RI. The VWS in both the inner core and the large-scale environment started to increase rapidly near the end of the RI. On the storm scale, the simulated Bopha experienced rapid eyewall contraction 12 hrs before the RI, in which the radius of maximum wind (RMW) shrank from 45km to 24km. During the subsequent RI phase, although the RMW stayed almost the same, the center for the maximum azimuthal mean variables, such as tangential wind, inertial stability, vertical velocity and rain rate all expanded radially outward. The onset of RI shows no direct link to CAPE or the number of convective bursts (CBs) in the simulated Bopha. Rather, it is correlated with the reduction in the storm relative flow (SRF) at the upper troposphere where the warm core subsequently formed. Peculiarly, just 6 hrs before the termination of RI, spikes of high CBs started to occur, seemingly triggered by the increasing VWS, which could lead to enhanced entrainments, and thus stimulating more active downdrafts and updrafts in the inner core region.