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Wen-Yih Sun and Ching-Chi Wu

Abstract

The formation and diurnal evolution of the dryline during fair weather have been investigated through the use of a two-dimensional mesoscale model that includes condensation/evaporation, budget equations of surface energy and moisture field, as well as turbulence and radiation parameterizations.A moderately strong, vertical wind shear was introduced on a sloping terrain, where the soil is very dry on the west side but moist on the east. Initially, a weak easterly geostrophic wind exists to the east but a weak westerly geostrophic wind to the west of the dryline. During daytime, deepening of the mixed layer due to vertical mixing, especially on the west side, forces the dryline to advance eastward. The westerly wind on the west side is maintained by downward transport of westerly momentum due to strong vertical mixing; the easterly wind on the east side is strengthened due to the inland sea-breeze circulation. Therefore, the resulting low-level convergence sustains a strong moisture gradient along the dryline.During the night, convection near the ground ceases, and a rapid surface cooling to the west of the dryline changes the pressure gradient force near the surface, which is able to maintain a weak westerly wind there. In contrast, a strong easterly wind persists on the east side and forces the dryline to retrograde until sunrise. The strong moisture gradient was also maintained by the low-level convergence.Sensitivity tests show that the most important factors in sustaining a strong moisture gradient along the dryline are the low-level wind shear, the sloping terrain, and the horizontal soil moisture gradient, in that order. Under favorable conditions, a very weak moisture gradient zone can develop into an intensive dryline within 36 hours.

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Wen-Yih Sun, Jiun-Dar Chern, Ching-Chi Wu, and Wu-Ron Hsu

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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