Typhoon Nari struck Taiwan on 16 September 2001. Its record-breaking 24-48 hour accumulated rainfalls more than 2000 mm in some parts of Taiwan caused widespread flooding, resulted in severe economical and societal damage. The PSU-NCAR MM5 model is used in this study to investigate the key precipitation processes responsible for heavy rainfalls of Typhoon Nari. The simulated Nari made landfall over Kee-Lung (24 hours after initialization), only 15-20 km off the actual landfalling position of I-Lan. Numerical simulations with different horizontal resolutions show that the ability of the model to successfully predict the observed rainfall maximum is increased with the refinement of grid size. The MM5 model with a 2.2-km grid size can simulate the maximum 24-h rainfall of 860 mm near I-Lan on September 17th, in close agreement with observed maximum of 940 mm.

When Nari made landfall, Taiwan°¦s terrain induced an asymmetric structure, and lowered the level of maximum heating over the mountain area. Most of outer spiral rainbands were produced by melting of snowflakes, and rainfalls within the inner core and over mountain slopes were mainly associated with raindrops and graupels. A horizontal pressure gradient of 7-8 hPa within 50 km was simulated near the inner core, in comparison with derived pressure gradient of 5-6 hPa from radar data using a thermodynamic retrieval method. Simulated vertical divergence profile also compares fairly with that estimated by radar observations using the VAD technique. A series of numerical experiments are conducted to examine the sensitivity of simulated typhoon intensity, track, rainfall amount and precipitation structure to the choice and details of microphysics parameterizations used in the model. Analyses of air-parcel and hydrometeor trajectories over the open ocean and mountain area are performed to investigate the complex interaction between the microphysical and topographic processes.