A Numerical Simulation of the Environmental Momentum Influences on Typhoon Formation

Abstract During the formation of typhoon Mike (1990), the trade wind to the north of the system increased significantly. However, the cross-equatorial flows were apparent to the south of typhoon Betty (1987) during its formation stage. The intensities of both systems increased greatly right after being named a storm. To help understand the influences of the environmental momentum field on the formation of typhoon, we used the PSU/NCAR MM5 to simulate the formation process of Betty and Mike. The ECMWF grid point data were used as the model initial field. Results show that the trade wind burst associated with Betty's formation and the cross-equatorial flows associated with Mike's formation were simulated reasonably well. The intensities of model typhoons were also closed to the observations.

Both angular momentum and vorticity budgets were analyzed using model-simulated data. Results showed that the eddy horizontal angular momentum transport caused by the trade wind burst played a significant role to the spin-up of the model vortex thus was favorable for the formation of Mike. The horizontal vorticity advection caused by the cross-equatorial flow associated with Betty's formation did not contribute significantly to the vorticity generation of model vortex. However, the low-level convergence enhanced by the cross-equatorial flow did play an important role on the vorticity generation near system center.