A Modeling Study of Typhoon Nari (2001) at Landfall. Part I: Topographic Effects

Abstract

Although there have been many observational and modeling studies of tropical cyclones, understanding of their intensity and structural changes after landfall is rather limited. In this study, several 84-h cloudresolving simulations of Typhoon Nari (2001), a typhoon that produced torrential rainfall of more than 1400 mm over Taiwan, are carried out using a quadruply nested-grid mesoscale model whose finest grid size was 2 km. It is shown that the model reproduces reasonably well Nari's kinematic and precipitation features as well as structural changes, as verified against radar and rain gauge observations. These include the storm track, the contraction and sizes of the eye and eyewall, the spiral rainbands, the rapid pressure rise (ϳ1.67 hPa h Ϫ1 ) during landfall, and the nearly constant intensity after landfall. In addition, the model captures the horizontal rainfall distribution and some local rainfall maxima associated with Taiwan's orography.A series of sensitivity experiments are performed in which Taiwan's topography is reduced 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 but less rainfall in lower-terrain runs. In contrast, changing the terrain heights produces nonlinear tracks with circular shapes and variable movements associated with different degrees of blocking effects. Parameter and diagnostic analyses reveal that the nonlinear track dependence on terrain heights results from the complex interactions between the environmental steering flow, Nari's intensity, and Taiwan's topography, whereas the terraininduced damping effects balance the intensifying effects of latent heat release associated with the torrential rainfall in maintaining the near-constant storm intensity after landfall.

Extreme Rainfall Mechanisms Exhibited by Typhoon Morakot (2009)

“…Another example was the severe rainfall of more than 1000 mm associated with the moderate southwest bound Typhoon Nari (2001). This typhoon has been well simulated by MM5 at a high horizontal resolution (e.g., Huang et al 2005;Yang et al 2008Yang et al , 2011. Recently, Wu et al (2009) successfully simulated an intense rainfall event associated with the northeast bound Typhoon Babs (1998) as its outer cyclonic flow confronted the winter monsoon flow in north Taiwan.…”

“…The interaction between the typhoon vortex and the CMR which cause track deviations and different rainfall activities has long been a challenging problem tackled by numerous studies (e.g., Chang 1982;Bender et al 1987;Yeh and Elsberry 1993a, b;Lin et al 2002Lin et al , 2005Wu et al 2002Wu et al , 2009Chiao and Lin 2003;Huang et al 2005;Jian and Wu 2008;Yang et al 2008Yang et al , 2011Chien and Kuo 2011). The intense flow associated with a typhoon striking Taiwan tends to produce persistent heavy rains over the steep windward slopes, a situation favorable for a possible "terrain-locking" mechanism that somewhat reduces the difficulty in obtaining rainfall predictability (Wu et al 2002Lin et al 2005).…”

Mesoscale processes for super heavy rainfall of Typhoon Morakot (2009) over Southern Taiwan

“…It is well known that damages are strongly related to the location of the typhoon's landfall and interactions between its circulation and the CMR (Brand and Blelloch 1974;Wu and Kurihara, 1996;Wu and Kuo, 1999;Lin et al, 2001;Chien et al, 2008;Ge et al, 2010). Previous studies pointed out that enormous amounts of rainfall occur and are enhanced due to mountain lifting when a typhoon passes over the CMR (Wu and Kuo, 1999;Lin et al, 2001Wu et al, 2002;Yang et al, 2008). Moreover, it is common that the strong southwesterly flow follows the departure of an invading typhoon and brings heavy rainfall over central or Southern Taiwan Chien et al, 2008).…”

Orographic Effect on the Precipitation with Typhoon Washi in the Mindanao Island of the Philippines

“…For the sensitivity experiments, the surface topographic data were changed as in Yang et al (2008). The input surface topo- ysis: TMPA-RT) for the period during the typhoon hit Mindanao Island, and Figs.…”

“…According to Wu et al (2002), both of 6.7 km and 2.2 km grid spacing resolution is similarly sufficient to reproduce the rainfall distribution by the orographic effect in Taiwan. As Yang et al (2008) showed, sensitivity experiments with different terrains are effective to investigate topographic influences on land-falling typhoons.…”