A Modeling Study of Typhoon Nari (2001): Model Verification and Topographic Effects

Ming-Jen Yang
National Central University


Although there have been many observational and modeling studies of tropical cyclones, our understanding of their intensity and structural changes after landfall is rather limited. In this study, several 84-h cloud-resolving simulations of Typhoon Nari (2001), that produced extremely heavy rainfall over Taiwan, are carried out using a quadruply nested-grid mesoscale model with the finest grid size of 2 km. After evaluating the model performance in simulating Nari’s track, intensity and structures, the topographic effects on the torrential rainfall production are investigated.

It is shown that the model reproduces reasonably well the kinematic and precipitation features as well as the structural changes of Nari, as verified against satellite, radar, and rain gauge observations. They include the storm track, contraction of the eye and eyewall, and the spiral rainbands; and the minimum central pressure and maximum surface wind, the rapid pressure rise (~ 1.67 hPa h-1), and the nearly constant intensity prior to, during and after landfall, respectively. In addition, the model captures the horizontal rainfall distribution and some local maxima associated with Taiwan’s orography.

A series of sensitivity experiments of reducing Taiwan topography are performed to examine the topographic effects on Nari’s track, intensity, rainfall distribution and amount. Results show that the impact of island terrain on Nari’s intensity is nearly linear, with stronger storm intensity in lower-terrain runs. In contrast, the effects of Taiwan topography on Nari’s track and accumulated rainfall amount are nonlinear. Thus, we conclude that Nari’s track is determined by the complex interactions between the environmental steering flow, Taiwan’s topography, and the terrain-induced circulations, and that the local rainfall amount increases with terrain elevation.