Generation and enhancement mechanisms for extreme orographic rainfall associated with Typhoon Morakot (2009) over the Central Mountain Range of Taiwan

The generation and enhancement mechanisms and essential ingredients for the extreme rainfall associated with Typhoon Morakot (2009) passing over Taiwan's Central Mountain Range (CMR) were investigated by using the Advanced Research Weather Research and Forecasting (WRF) model. First, we found that the extreme rainfall was due to the essential orographic rain ingredients: high precipitation efficiency, strong low-level jet, steep terrain, high moisture upstream, large convective system, and slower movement of the typhoon. Second, the Orographic Rain Index (ORI) proposed by Rostom (2015) was modified and tested by twelve (12) TC cases and found it has a good correlation with daily rainfall. Thus it can be used to help the prediction of extreme orographic TC rainfall. Third, we found there are two types of orographic rainfall mechanisms associated with Morakot, namely initiation and enhancement. These rainfall mechanisms may occur simultaneously M different locations, such as in northeast and southwest CMR. The orographic rain in the northeastern CMR was initiated by the high and steep CMR while the typhoon was still 200 km away. The rainfall was enhanced gradually and continually by the CMR when Morakot was approaching it, which produced a maximum rainfall of 265 mm/3 h starting at 1800 UTC August 7. The rain then decreased when the typhoon's eyewall structure and convection were damaged and weakened, respectively, when it invaded the high and steep CMR. Orographic rain was generated in the southwestern CMR initially when the conditionally unstable, southwesterly monsoonal flow impinged on the southern CMR. It was enhanced afterward by the merged monsoonal flow and Morakot's rainband leading to an extreme maximum rainfall of 311 mm/3 h starting 0000 UTC August 8. Later, this region of orographically enhanced TC rain moved northward along the western flank of the CMR. At last, we presented a conceptual model summarizing four key processes associated with the generation and enhancement of orographic TC rain over the southwest of CMR during the passage of typhoon Morakot (2009).