Dryline, lee vortex, and local circulation in Taiwan
Sun and Ogura (1978) proposed "Inland-sea-breeze hypothesis" for the formation of dryline in the Great Plains. Sun and Wu (1992) proved that the formation and propagation (eastward propagation during day time and retreat at night) of the dryline depend on: (a) strong soil moisture gradient along the W-E direction (i.e., inland-sea-breeze theory); (b) vertical wind shear; and (c) slope of terrain. Their results also show that dryline can develop within a day in fair weather in the late spring and the early summer in Oklahoma and Texas.
Sun et al. (1991) also successfully simulated: (a) the lee-vortex located off the southeastern coast of Taiwan under a southwesterly wind in summer; (b) and the vortex toward northwestern Taiwan under a northeasterly wind in winter, as observed. From the numerical results, they found that the formation of lee vortex is a combination of the effects of baroclinicity, tilting, and stretching in the vorticity equation. Using the Purdue Regional Model, Sun and Chern (1993) reproduced the cloud distribution, land-sea breeze, mountain waves, downslope wind, and the formation and the diurnal propagation of the lee-vortex during the Taiwan Area Mesoscale Experiment in 1987. They also provided the physical explanation that a surface low pressure formed to the south of the vortex center, as observed, due to adiabatic warming of the down-slope wind over the Central Mountain Range.
Illustrations
Play the slideshow, or click a thumbnail image to view the full-size illustration.
 |
Lee vortex and land-sea breeze in Taiwan during TAMEX in 1987. Observation (a) and model simulation (b) at 900 mb during TAMEX at 0200 LST 17 (1800 UTC 16) May 1987. (Sun and Chern 1993) |
Publications
Sun, W. Y. and Y. Ogura, 1979: Boundary-layer forcing as a possible trigger to a squall-line formation. J. Atmos. Sci. 36, 235–254.
Sun, W. Y. and C. C. Wu, 1992: Formation and diurnal oscillation of dryline. J. Atmos. Sci. 49, 1606–1619.
Wu, C. C. Y. C. Yu, W. R. Hsu, K. J. Hsu, W. Y. Sun, 2004: Numerical study on the wind fields and atmospheric transports of a typical winter case in Taiwan and surrounding area. (appeared in Atmopheric Sciences) (in Chinese)
Hsu, W. R., C. C. Wu, and W. Y. Sun, 2003: A three-dimensional simulation of moist airflow over an isolated mountain. International conference on Alpine meteorology and the Mesoscale Alpine Programme Meeting 2003; May 19–23, 2003; Brig, Switzerland.
Sun, W. Y., J. D. Chern, C.C. Wu, and W. R. Hsu., 1991: Numerical simulation of mesoscale circulation in Taiwan and surrounding area. Mon. Wea. Rev. 119, 2558–2573.
Sun, W. Y. and J. D. Chern, 1993: Diurnal variation of lee-vortexes in Taiwan and surrounding area. J. Atmos. Sci. 50, 3404–3430.
Sun, W. Y. and J. D. Chern, 1994: Numerical experiments of vortices in the wake of idealized large mountains. J. Atmos. Sci. 51, 191–209.
Sun, W. Y. and J. D. Chern, 2004: Numerical study of influence of mountain ranges on Mei-Yu front. (under revision)