Numerical Study of the Influence of Central Mountain Ranges in Taiwan on a Cold Front

Abstract

The numerical simulations were performed to study the intrusion of a shallow cold front along the China coastal mountain range, and the deformation of the front by the Central Mountain Range (CMR) in Taiwan during the IOP-9 (1600 UTC June 14-1700 UTC June 15, 1987) of the Taiwan Area Mesoscale Experiment (TAMEX). The essential features of the observed front, such as the faster movement of the eastern part of the front than the western part, were well reproduced. The control and sensitivity simulations suggested that the frontal deformation in Taiwan was caused by the dynamical interaction between the front and the underlying topography. As the cold air approached northern Taiwan from the north, a relative high pressure with anticyclonic circulation built up on the windward side. More oncoming flow was diverted to the northeast of Taiwan than to the northwest and caused the difference of the frontal speed between the eastern and western parts of the front. The dynamics of the frontal movement along southeast China was similar to the dynamics of coastally trapped mesoscale ridges, or orographical jet. The momentum budget equation showed that the contribution of the nonlinear advection, ageostrophic forcing, and friction were equally important to the local change of the wind just behind the front. This might indicate that the dynamics of the front was more complicated than that described by the density current theory derived from an irrotational fluid. The invicid sensitivity simulation revealed a faster propagating front, and a stronger wind behind its leading edge in comparison with the control simulation. The front bears no dynamical resemblance to either an orographically trapped density current or a Kelvin wave, because the major forces in the momentum budget are the pressure gradient force, Coriolis force and advection terms.

Vortex Merger in Shallow Water Model

“…In a linearized axisymmetric flow, the speed of Kelvin wave is U(r) + √ gh(r), where U(r) and h(r) are the velocity and height at distance r from the center of rotation. Our simulated vortices advect with U(r) along h-contours, which is like the observed cold front advected southward along the eastern coasts of Taiwan and China with the observed wind U instead of U + √ gh (Sun and Chern 2006). We do not see any instability, because shear is not strong in our simulations.…”

The vortex moving toward Taiwan and the influence of the central mountain range

“…The block of the CMR and the NNE-SSW orientation of the Island introduce a weak anticyclonic circulation, similar to a high pressure on the inward side of an isolated island in the atmosphere model with the earth rotation (Sun et al 1991;Sun and Chern 1993, 1994, 2006. Because of small .…”

“…The CMR has a significant impact on the movement of the front (Sun and Chern 2006) and typhoons near Taiwan (Wang 1980;Shieh et al 1998, etc.). The vortex passed an isolated island, has been widely investigated using observations and numerical models (Brand and Blelloch 1974;Wang 1980;Chang 1982;Bender et al 1987;Yeh and Elsberry 1993;Lin et al 2005;Huang and Lin 2008;Huang et al 2011;Tang and Chan 2013, etc.).…”

“…Surface friction is important to the flow over mountains (Peng and Thompson 2003;Sun and Hsu 2005;Sun and Chern 2006;Sun 2013;Sun and Sun 2015). Previously, it is also believed that surface friction is secondary; therefore, it is not included in Lin et al (2005); Huang and Lin (2008), and other studies.…”

Numerical Simulations of Asian Dust-Aerosols and Regional Impact on Weather and Climate- Part I: Control Case-PRCM Simulation without Dust-Aerosols

“…We are incorporating the mass-conserved, positive-definite semiLagrangian scheme (Sun et al, 1996;Sun and Yeh, 1997;Oh, 2007;Sun, 2007), the sea-ice-mixed layer ocean module (Sun and Chern, 2010), and a new semi-implicit scheme (Sun, 2010(Sun, , 2011 into the PRCM. The model has been applied to study the observed cyclogenesis and winter storms over the complex terrain in the western USA (Chern, 1994); formation and propagation of leevortices in Taiwan (Sun et al, 1991, Sun and Chern, 1993, 1994; winter storms and vortices in the Rocky Mountains (Haines et al, 1997); interactions between cold front and mountains in Taiwan and China (Sun and Chern, 2006), etc. It has also been integrated from a few weeks to a few months continuously without nudging to study the East Asia climate for 10 summers (from May 1 to August 31) between 1991-2000, the bias of the mean-sea-level pressure is -0.15 hPa, and the root-mean-square-error (RMSE) is 0.6 hPa; the bias of the surface temperature is 0.47 K, and RMSE is 0.72 K Yu et al, 2004;Yu, 2007).…”