Interaction of a tropical depression with Taiwan topography and its Impacts on Producing Heavy Orographic Rainfall

A tropical depression approached Taiwan from the southwest has caused heavy rainfall and flash flood in southern Taiwan in the early morning of 7 August 1999. It appeared that the interaction of the tropical depression and the Central Mountain Range (CMR) of Taiwan was responsible for causing this heavy rainfall. In this study, we use a mesoscale model to simulate the synoptic environment with a relatively coarse resolution as well as the mesoscale environment by nested simulations with finer resolutions. The initial fields were initialized by the NCEP AVN global model. From the numerical simulations, we found that the movement of the tropical depression was influenced by Taiwan s CMR, the pre-existing typhoon Paul located to the northeast of it at the lower troposphere and the cold-core low located over the east coast of central China at upper troposphere. The CMR tended to slow down the movement of this tropical depression and deflected its track further to the west near surface. The tropical depression appeared to be influenced by typhoon Paul, which slowed down its northeastward movement. The strength of the cold-core low decreases both upward and downward, but was able to penetrate to the 500 mb level. 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, which forced the tropical depression to become more stationary over Taiwan as well as to provide upward motion at lower levels. These helped produce the heavy rainfall. The low-level jet associated with the tropical depression has a significant component perpendicular to the southern CMR, which might serve as a conveyor for the high equivalent potential temperature air to impinge on the mountains and trigger instability over the upslope.