Numerical experiments investigating the mechanisms of a heavy rainfall event over northeastern Taiwan and a mesovortex during TAMEX |
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Authors: | C-S Chen C-H Lu W-C Chen |
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Institution: | (1) Institute of Atmospheric Physics, National Central University, Chung-Li, Taiwan |
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Abstract: | Summary This study analyzes the mechanisms of the development of a heavy rainfall event (17 June 1987) over the lee side of the Central
Mountain Range (CMR) in northeastern Taiwan during the southwesterly monsoon. This heavy rainfall event was examined using
gridded data from the European Centre for Medium-Range Weather Forecasts, surface rainfall data and numerical model results,
employing a non-hydrostatic fifth-generation mesoscale model (MM5) developed by the National Center for Atmospheric Research
and Pennsylvania State University. A tropical depression was simulated over the northern South China Sea on 16 June. Convergence,
resulting from the southeasterly winds associated with the circulation from the tropical depression, and northeasterly winds
over the Taiwan Strait, occurred over the northern Bashi Channel at 850 hPa. The convergence amplified planetary vorticity
and the vorticity associated with the intensifying tropical depression. Consequently, a mesovortex with low pressure over
the northeastern edge of the tropical depression near southern Taiwan was produced. Additional convergence over the ocean
adjacent to southern Taiwan caused by the interaction between the northeasterly flow, which was deflected over the southeastern
slope of the CMR, and the southeasterly flow of the tropical depression, also affected the intensity of the mesovortex. When
the mesovortex moved northward and reached southern Taiwan, the southeasterly flow associated with it interacted with an east-southeasterly
flow, which was related to the tropical depression, to form a mesoscale convective system (MCS) over the ocean adjacent to
southeastern Taiwan. As the mesovortex moved northward, the MCS, which was embedded in the southeasterly flow, also drifted
inland toward northeastern Taiwan. The orographic lifting and the ascending motion associated with the deceleration of the
easterly flow near the CMR enhanced the MCS over northeastern Taiwan and produced heavy rainfall.
To examine the role of Taiwan’s orography on the modelled rainfall, two simulations were conducted; one which included Taiwan’s
orography and one which excluded it. In both simulations, the mesovortex in the northern Bashi Channel and the MCS near southeastern
Taiwan were reproduced. However, in the simulation excluding the orography, the mesovortex was slightly less intense. In addition,
without the extra orographic lifting and the ascending motion caused by flow deceleration, rainfall over northeastern Taiwan
was weaker than in the simulation with the orography. |
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Keywords: | |
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