Numerical Study of the Rainfall Event due to the Interaction of Typhoon Babs (1998) and the Northeasterly Monsoon

A heavy rainfall event in the Taiwan area associated with the interaction between Typhoon Babs (1998) and the East Asia winter monsoon is studied. Typhoon Babs is a case in point demonstrating the often-observed phenomenon that heavy rainfall can be induced in the eastern and/or northeastern region of Taiwan. Such heavy rainfall was caused by the joint convergent flow associated with the outer circulation of typhoons and the strengthening northeasterly monsoon in late typhoon season, even though Babs remained distant from Taiwan when it moved through the island of Luzon in the Philippines and stayed over the South China Sea. This heavy rainfall event is simulated in this study using the fifth-generation Pennsylvania State University-National Center for Atmospheric Research (PSU-NCAR) Mesoscale Model (MM5) with three nested domains and a highest horizontal resolution of 6.67 km. The control experiments with Kain-Fritsch cumulus parameterization perform well in terms of both simulated track and intensity. The 20-km resolution simulation reproduces the correct rainfall distribution during the three days studied, and the fine domain with 6.67-km resolution further improves the maximum simulated rainfall to very close to the observations. A series of sensitivity experiments that include model physics, terrain effect, typhoon vortex structure, and monsoon strength is performed, aiming at investigating the predictability of this typhoon-monsoon-terrain system when some of its components are perturbed. The rainfall event is analyzed based on two rainfall modes of different dominant mechanisms: monsoon mode during 0000 UTC 24-25 October and topographic mode during 0000 UTC 25-26 October. Removal of the Taiwan terrain in one of the sensitivity experiments results in completely different rainfall distribution due to the lack of the convection by orographic lifting, and the terrain is also found to play a key role in changing the low-level convergence pattern between the typhoon circulation and monsoonal northeasterlies. When the radius of the bogus vortex is reduced, the cold front to the north migrates southward in a faster pace than in the control simulation, and rain rate at the front also increases such that total accumulated rainfall at northern Taiwan is comparable with that in the control simulation but with shifted maximum position. In the extreme case in which no bogus vortex is implanted at all, rainfall is mainly associated with evolution of the cold front (pure frontal mode). In addition, a technique is developed to modify the monsoon strength over China. It is found that low-level (1000-700 hPa) reduction in monsoon strength weakens interaction with the typhoon, and rain distribution remains the same as in the control simulation. However, the simulated typhoon track is considerably sensitive to the deep-layer (1000-300 hPa) monsoon strength.