On the Processes Leading to the Rapid Intensification of Typhoon Megi (2010)

The processes leading to the rapid intensification (RI) of Typhoon Megi (2010) are explored with a convection-permitting full-physics model and a sensitivity experiment using a different microphysical scheme. It is found that the temporary active convection, gradually strengthened primary circulation, and a warm core developing at midlevels tend to serve as precursors to RI. The potential vorticity (PV) budget and Sawyer-Eliassen model are utilized to examine the causes and effects of those precursors. Results show that the secondary circulation, triggered by the latent heat associated with active convection, acts to strengthen the mid-to upper-level primary circulation by transporting the larger momentum toward the upper layers. The increased inertial stability at mid-to upper levels not only increases the heating efficiency but also prevents the warm-core structure from being disrupted by the ventilation effect. The warming above 5 km effectively lowers the surface pressure. It is identified that the strong secondary circulation helps to accomplish the midlevel warming within the eye. The results based on potential temperature (theta) budget suggest that the mean subsidence associated with detrainment of active convection is the major process contributing to the formation of a midlevel warm core. On the possible causes triggering the inner-core active convection, it is suggested that the gradually increased vortex-scale surface enthalpy flux has a leading role in the development of vigorous convection. The results also highlight the potentially dominant role of weak to moderate convection in the onset of RI, while the convective bursts play a supporting role. Based on the aforementioned analyses, a schematic diagram is shown to describe the plausible path leading to RI.