The Formation Process of Principal Rainbands of Typhoon Hagupit (2008) as Revealed by a Convective Allowing Cloud Model Simulation

The formation process of principal rainbands of Typhoon Hagupit (2008) was investigated through a 1-km cloud model simulation. One such rainband in the southeastern quadrant of the storm was observed by the NSF NCAR Electra Doppler Radar (ELDORA) airborne dual-Doppler radar, and its structure was documented in detail previously. Complementing this observational study, our model reproduced the rainband realistically, and multiple rainbands form in succession at nearly the same spot relative to the typhoon center, on the inner side, and upstream of an existing rainband. This study examines the favorable conditions in the formation area through diagnosing the momentum equation in both radial and tangential directions in a quasi-Lagrangian frame following the movement of Hagupit. The abovementioned rainbands exhibit characteristics resemble midlatitude squall lines and propagate outward, with tilted updraft/downdraft and a surface mesohigh at the inner side (i.e., the backside). The cold mesohigh and its outflow act to enhance the surface-based convergence inside the boundary layer in both radial and tangential directions, and the convergence is coupled with rising motion and divergence above near 1–3 km. Along the trajectory of the inflow, it warms diabatically by sensible heat flux from the ocean below, so the convergent region is also warmer and has lower pressure. This lower pressure reduces the inward-directed pressure gradient force in the radial direction and thus enhances the convergence. Therefore, the aforementioned formation region is more favorable to trigger a new rainband inside the parent rainband as illustrated in the conceptual model. After formation, the new rainband propagates out and provides similar conditions to trigger another rainband.