Topographic effetcs on Typhoon Nari (2001): Verification and sensitivity experiments

Typhoon Nari (2001) brought more than 1000 mm in one day over northeastern Taiwan, which caused widespread flooding and severe economical/societal damage. The PSU-NCAR MM5 model is used in this study to investigate the key topographic and precipitation processes responsible for heavy rainfalls of Typhoon Nari. Twenty-four hours after initialization, the simulated Nari made landfall over Kee-Lung, only 20 km off the observed landfalling position of I-Lan. The MM5 model quadruply nested down to a 2-km grid size can simulate the maximum 24-h rainfall of 1183 mm near I-Lan on September 17th, in close agreement with observed maximum of 1188 mm. When Nari made landfall, Taiwan's terrain induced an asymmetric structure, and lowered the level of maximum condensational heating over the mountain slope. Most of outer spiral rainbands were produced by melting of snowflakes, and rainfalls within the inner core and over mountain areas were mainly associated with heavy raindrops and graupels. A horizontal pressure (temperature) gradient of 7-8 hPa (4-5 K) within 50 km was simulated near the eye, in good agreement with the derived pressure (temperature) gradient of 5-6 hPa (3-4 K) 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. In a half-terrain experiment, the simulated typhoon track followed closely the observed track for the first two days, and then started to have a noticeable deviation on the third day; the simulated rainfall total was about 60% of the observed during the landfall period. If Taiwan's topography was completely removed, the sensitivity experiment showed a substantial change on typhoon track, and the simulated total rainfall amount over Taiwan was only about 40% of the observed total during the landfall period. More sensitivity experiments on the topography and microphysics parameterizations are in progress in order to further understand the complicated interactions between the topographic and microphysical processes of Typhoon Nari.