NUMERICAL EXPERIMENTAL STUDY FOR THE EFFECT OF A BINARY TYPHOON SYSTEM ON THE EXTREME RAINFALL OF TYPHOON MORAKOT (0908)

A high-resolution numerical simulation of the extreme rainfall caused by typhoon Morakot (0908) over Taiwan Province, China, was made using the WRF-ARW/NCAR model (Version 3.2), ERA Interim reanalysis data (resolution: 1.5 degrees x1.5 degrees) from the European Centre for Medium-Range Weather Forecasts (ECMWF), and global real-time sea surface temperature analysis data (RTG_SST, 0.5 degrees x0.5 degrees) from NCEP/NOAA. The numerical simulation results showed that the extreme rainfall caused by Morakot over Taiwan was closely related to water vapor transport of the southwesterly flow. However, the effect of a binary typhoon system between Morakot and Severe Tropical Storm Goni (0907) was also an important factor. Goni strengthened the intensity of the southwesterly flow and the water vapor transport to Morakot and resulted in the heavy rainfall increasing over central and southern mountainous areas of Taiwan. Furthermore, the effect of the binary typhoon system increasing the northward component of the track of Morakot, and the typhoon's slow translation to Taiwan, caused the longtime and persistent severe rainfall over southern Taiwan. After removing Goni's circulation in the model initial field, the cumulative precipitation was greatly reduced by 35.78%, 33.03% and 31.5% in the 18-, 6-and 2-km resolution model results, respectively.