A numerical study on the slow translation speed of typhoon morakot (2009)

Typhoon Morakot (2009), a devastating tropical cyclone (TC) that made landfall in Taiwan in August 2009, produced the highest recorded rainfall in southern Taiwan in 50 years. The slow translation speed of Morakot, among many other factors, was found to play an important role in heavy rainfall. Using the WRF model, this study examined the causes of the slow TC translation speed in relation to the interaction of Morakot with Typhoon Goni (2009) and Typhoon Etau (2009). The simulated track of Morakot was relatively consistent with the observation in the control run. However, Morakot deviated more westward and moved slower in a sensitivity simulation without Goni, compared with that in the control run, and had a similar but faster track in another simulation without Etau. Comparisons also show that Goni helped to increase southerly to southwesterly steering flow to Morakot, while Etau's circulation helped to produce slightly weaker northerly to northwesterly steering flow. These two opposite forces counteracted partly the south-southeasterly steering of the Pacific high, resulting in a slowly north-northwestward tracking of Morakot.