Numerical Simulation of Mesoscale Circulation in Taiwan and Surrounding Area

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  • 1 Department of Earth and Atmospheric Sciences, Purdue University, West Lafayette, Indiana
  • | 2 Department of Atmospheric Sciences, National Taiwan University, Taipei, Taiwan
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Abstract

Mesoscale circulation around Taiwan and the surrounding area has been investigated using the Purdue mesoscale model. The numerical results generated in an inviscid atmosphere show:

(a) A cyclonic vortex forms in the southeast and a slightly weaker anticyclonic vortex forms in the northeast of Taiwan uner a westerly or southwesterly wind. Subsidence warming also generates a relative low pressure on the southeastern coast.

(b) A low pressure associated with a cyclonic flow forms in the northwest and a slightly weaker anticyclonic flow forms in the southwest of Taiwan under an easterly mean flow. The easterly wind tends to turn northeasterly over the Taiwan Strait, with a stronger wind speed, due to the blocking effects of the mountains in Taiwan and along the Chinese coast.

(c) Under the existence of an easterly surface wind with a reverse shear, the horizontal temperature advection is not important in the formation of low pressure on the leeside, due to the small length scale of the island of Taiwan.

(d) The Froude number is an important parameter to estimate the blocking effect of the central mountain range; however, the flow pattern also depends on other parameters, such as the shape of mountains, the terrain of the surrounding areas, and other meteorological parameters.

(e) The budget study of the vorticity equation shows that stretching, tilting, and friction are important for the formation of lee vortices in our results.

These results may provide some physical explanations for the observed mesolow and cyclonic flow in the southeast and northwest of Taiwan during the late spring and early summer—a transitional period of the winter and summer monsoons in Taiwan.

Abstract

Mesoscale circulation around Taiwan and the surrounding area has been investigated using the Purdue mesoscale model. The numerical results generated in an inviscid atmosphere show:

(a) A cyclonic vortex forms in the southeast and a slightly weaker anticyclonic vortex forms in the northeast of Taiwan uner a westerly or southwesterly wind. Subsidence warming also generates a relative low pressure on the southeastern coast.

(b) A low pressure associated with a cyclonic flow forms in the northwest and a slightly weaker anticyclonic flow forms in the southwest of Taiwan under an easterly mean flow. The easterly wind tends to turn northeasterly over the Taiwan Strait, with a stronger wind speed, due to the blocking effects of the mountains in Taiwan and along the Chinese coast.

(c) Under the existence of an easterly surface wind with a reverse shear, the horizontal temperature advection is not important in the formation of low pressure on the leeside, due to the small length scale of the island of Taiwan.

(d) The Froude number is an important parameter to estimate the blocking effect of the central mountain range; however, the flow pattern also depends on other parameters, such as the shape of mountains, the terrain of the surrounding areas, and other meteorological parameters.

(e) The budget study of the vorticity equation shows that stretching, tilting, and friction are important for the formation of lee vortices in our results.

These results may provide some physical explanations for the observed mesolow and cyclonic flow in the southeast and northwest of Taiwan during the late spring and early summer—a transitional period of the winter and summer monsoons in Taiwan.

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