Characteristics of tropical cyclone tracks over western North Pacific - the case study of Typhoon Fengshen (2008)

This study quantitatively understands the key factors affecting the movement of typhoons over western North Pacific based on the potential vorticity (PV) diagnosis. The synoptic-scale environmental flow shows the reverse-oriented monsoon trough pattern during early period of Typhoon Fengshen (2008), and then gradually becomes the monsoon gyre pattern. The movement of Fengshen is different from the poleward movement shown by the conceptual model in Carr and Elsberry (2000). Instead, Fengshen moves to the west-northwest under the influence of the subtropical High during its early period. Due to the strengthening of Fengshen, the perturbation PV to the southwest of tropical cyclone (TC) increases, while the environmental flow associated with the monsoon trough steers Fengshen northward. From the PV diagnosis at different vertical levels, it can be shown that the steering flow in the middle and upper levels is more consistent with the motion of Fengshen than that in lower levels. During Fengshen's lifetime, its steering flow is mainly associated with the circulation of subtropical High, while the impact of the monsoon trough is secondary. However, the influence of the monsoon trough is critical when Fengshen sharply turns to the north. Several possible reasons for the forecast error associated with Typhoon Fengshen's movement in NCEP/GFS: a) The deviation of the initial vortex position; b) The intensity and structure of the initial vortex are too weak; c) The depth of steering flow is relatively shallow due to the weak vortex structure. Thus, the steering flow is mainly associated with the lower levels; d) The NCEP/GFS model underestimates (overestimates) the subtropical High (monsoon trough) strength. Thus, the northward environmental flow associated with the monsoon trough is the key factor to the early northward movement of TC.