Heavy rainfall induced by a tropical depression over a mesoscale mountain range

A tropical depression approached Taiwan from the southwest has produced heavy rainfall and flash floods in southwestern Taiwan in the early morning of 7 August 1999. By performing numerical simulations, we found that the complicated interactions among the tropical depression, the pre-existing Typhoon Paul located to the northeast of Taiwan and south of Korea and Japan, the upper-tropospheric cold-core low over the east coast of China, and the Central Mountain Range (CMR) of Taiwan played essential roles in producing the heavy orographic rainfall in the southwest Taiwan. The northeastward movement of the tropical depression has been slowed down by Typhoon Paul, and its track has been deflected to the west by CMR. As normally observed, the strength of the cold-core low decreases both upward and downward, but was able to penetrate downward to the mid-troposphere. At 500 mb, the cyclonic circulation associated with the cold-core low interacts strongly with the cyclonic circulation associated with the tropical depression. At later times, this interaction forces the tropical depression to become a cut-off low and then an embedded trough of the cold-core low circulation in mid-troposphere, which was able to trigger upward motion near the upslope of southern CMR at lower levels. These interactions helped produce the heavy rainfall. The low-level jet associated with the tropical depression has a significant component perpendicular to the southern CMR, which appears to serve as a conveyor in transporting the high equivalent potential temperature air toward the mountains and triggered instability and strong convection over the upslope. A historical event has also been analyzed and compared with the present case to help identify some essential mesoscale environments conducive to heavy orographic rainfall or flooding.