A Numerical Study of the Rapid Intensification of Typhoon Bopha (2012)

Typhoon Bopha (2012) originated unusually close to the equator and underwent three periods of rapid intensification (RI) during its lifetime. It made landfall on the southern Philippine island of Mindanao as a Category 5 super typhoon and caused widespread destruction and over 600 deaths. In this study, the first 24-h RI process was reproduced reasonably well using the Advanced Research and Weather Forecasting model (ARW-WRF) with large-scale spectral nudging to ensure a good track simulation. Processes responsible for the RI were analyzed. On the large scale, environmental vertical wind shear (VWS) stays below 6ms-1 throughout RI. On the storm scale, the simulated Bopha eyewall starts to contract rapidly 18 h before RI; and the radius of maximum wind (RMW) has shrunk from 40km to 20km by the end of RI. During RI, near the RMW, azimuthal mean tangential wind, radial wind and inertial stability increase rapidly, indicating a positive feedback between primary and secondary circulation. On the convective scale, in the presence of low shear, the rapid growth of the convective bursts (CBs) at the onset and early stages of RI is most likely buoyantly driven. It is suggested that as inflows continue to penetrate into the core, the supergradient winds start to transport high θe air from the eye to the eyewall, increasing the buoyancy and supporting the CBs. The high θ air detrained from the lower stratosphere surrounding the tall CBs can possibly contribute to the upper tropospheric warming and the RI onset. Trajectory analysis is currently being performed to further demonstrate this eye-eyewall exchange that leads to the onset of RI.