Analysis of the intensity, structure and precipitation in high-resolution numerical simulations of Typhoon Morakot

Freshwater flooding is the most prolific danger to life and property posed by tropical cyclones in many parts of the world. This was demonstrated in the case of Typhoon Morakot, which made landfall and crossed Taiwan during August 2009. Although Morakot was only a moderate intensity typhoon at landfall, it was a large and relatively slow-moving storm, and the most damaging aspect of the system's impact was the extreme rainfall it produced, reaching 1404 mm in 24 hours at Weiliao Mountain in Pingtung County, and a three day total of 2777 mm at Alishan, both of which are records for Taiwan and relatively close to world record values. The subsequent flooding and landslides caused about 650 deaths or missing persons, and were responsible for economic losses estimated at around NT$110 billion (US$3.3 billion). This case emphasizes the need to better understand, and eventually improve predictions of, the structure of landfalling tropical cyclones in complex terrain.

Morakot was embedded in a moderately sheared environment and had become highly asymmetric in structure as it approached Taiwan, with virtually all the deep convection confined to the southern semi-circle. Little significant rainfall was recorded to the north of the typhoon's track. The changes in vertical structure of the cyclone before and during its passage over the island, combined with upslope flow along the westward slopes of the mountains, were critical in the development of the intense, prolonged rainfall.

Operational forecast models displayed mixed results in the relatively complex synoptic environment during the landfall phase of Morakot. Here, we present the results of simulations of Morakot utilising a high resolution mesoscale model, with finest grid scale of 1 km. The results of these simulations, together with direct observations, are analysed to investigate the structural aspects of the cyclone and their predictability. Further, the effects of the environment and complex topography on intensity and motion in the landfall phase, and mechanisms behind the development and prolonged maintenance of the intense rainfall, will be investigated through a series of sensitivity experiments. The model results will be compared with a wide range of observational datasets.