Observation have provided strong evidence for the role of low-level large-scale momentum forcing in triggering tropical cyclone formation (Love, 1985; Lee et. al.1986; Briegal and Frank, 1997). This study attempts to analyze the evolution of pre-cyclone disturbance under the influence of trade-wind surges and cross-equatorial surges. An examination of individual cases revealed that about 40% of all tropical cyclones had at least one of the above two low-level features during the formation stage. Results also showed that tropical cyclogenesis with trade-wind surges occurred primarily in the late season (October °V December). These systems experienced a stronger intensification rate and moved in a faster speed after reaching tropical storm intensity.

The PSU/NCAR MM5 was used to simulate the evolution of pre-cyclone disturbances with trade-wind surges or cross-equatorial surges. The model simulation started at about 36-48 hours before JTWC issued the TCFA (Tropical Cyclone Formation Alert) and ran for 72 hours. Results showed that the evolution of the large-scale circulation pattern as well as the intensity and motion of the system were simulated reasonably well for three trade-wind surge cases and two cross-equatorial surge cases. These cases were further analyzed to investigate the role of the low-level momentum forcing in triggering tropical cyclogenesis. Results showed that the horizontal vorticity advection associated with the strong trade wind to the north of the system played a key role in triggering the tropical cyclogenesis. For the cross-equatorial surge cases, there existed two vortices during the formation stage. The merging of these two vortices might be a key feature leading to tropical cyclogenesis.