2004: Potential vorticity diagnosis of the key factors affecting the motion of Typhoon Sinlaku

Potential vorticity diagnosis is applied to study the factors contributing to the decrease of the forward motion of Typhoon Sinlaku (2002) and is used to evaluate the reason why the operational Aviation (AVN) model failed to predict this slowdown and had a southward track bias as Sinlaku approached the area offshore northeastern Taiwan. The analysis indicates that the initial deceleration was mainly associated with the retreat of the Pacific subtropical high (SH) under the influence of the deepening midlatitude trough (TR). The upper-level cold-core low (CCL) played only a minor role in impeding Sinlaku from moving northward, while the continental high (CH) over mainland China strongly steered Sinlaku westward. Because the steering effect from the above four systems (SH, TR, CCL, and CH) tend to cancel one another out, the subtle interaction therein makes it difficult to make a precise track forecast. The analyses of the AVN model forecasts show that overestimating the CH and underestimating the SH are the main causes for the forecast bias. It is believed that results from this work can provide a useful method to quantitatively determine the key factors affecting the typhoon track. Such an analysis should be able to guide future observing and modeling strategies to improve the forecasts of typhoons. 1