Code 613.1 Branch Seminar: Professor Ming-Jen Yang

Institute of Hydrological Sciences, National Central University, Taiwan, ROC
The extremely heavy rainfall of Typhoon Nari (2001) wth 3-day-total amount of more than 1400 mm over northern Taiwan caused widespread flooding and resulted in severe economical and societal damage. The fifth-generation Pennsylvania State University‚ÄĒNational Center for Atmospheric Research Mesoscale Model (MM5) quadruply nested down to 2-km grid size is used in this study to investigate the topographic processes responsible for Nari's heavy rainfall. A 108-h integration was made, which covers the whole landfall period. As verified against satellite, radar, and rain gauge observations, the model can simulate reasonably well the inner-core features and structure changes during the landfall processes. The features successfully captured include the storm track, the eye, the eyewall, the spiral rainbands, the shrinking of the typhoon eye, the rapid pressure rise (~ 1.67 hPa h-1) within the 12 hours after landfall, and the central pressure of 960 hPa and maximum surface wind of 40 m s-1 prior to landfall. Twenty-two hours after initialization, the simulated Nari makes landfall over Keelung, only 30 km northwest of the observed landfall location over Yilan and 3 hours earlier. The model captures the area-averaged rainfall? [horizontal rainfall distribution] fairly well; in particular, the 6-km (2-km) grid can reach 60% (80%) of the observed amount of the 24-h rainfall averaged over Taiwan. A radial pressure gradient of 10 hPa within 45 km across the eyewall is simulated, in good agreement with the derived gradient of 6 hPa obtained using a thermodynamic retrieval method from the radar data. As Nari makes landfall, Taiwan's terrain induces strong asymmetry on the kinematic and precipitation structures, enhances the radial wind, and results in a lower level of maximum latent heating along mountain slopes. Four sensitivity experiments of reduced Taiwan topography are performed to examine the topographic effects on Nari's track, intensity, rainfall distribution and amount. The impact of island terrain on Nari's intensity is quite linear, with stronger storm intensity in the lower-terrain run. On the other hand, the effects of Taiwan topography on Nari's track and accumulated rainfall amount are nonlinear. Nari's track around Taiwan thus results from the complex interactions between the environmental steering flow, Taiwan topography, and the terrain-induced mesoscale forcings. The orographic rainfall amount is increased with higher terrain, albeit at a nonlinear enhancement rate.