Transport Reversal Due to Typhoon Krosa in the Taiwan Strait
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The Open Ocean Engineering Journal, 2010, 3, 143-157 143 Open Access Transport Reversal Due to Typhoon Krosa in the Taiwan Strait Yu-Hsien Lin1, Ming-Chung Fang 2 and Hwung-Hweng Hwung3* 1Fishing Boat and Marine Engineeirng Research Center, National Cheng-Kung University, Tainan City 70101, Taiwan 2Department of Systems and Naval Mechatronic Engineering, National Cheng-Kung University, Tainan City 70101, Taiwan 3Tainan Hydraulics Laboratory, & Senior Executive Vice President, National Cheng-Kung University, Tainan City 70101, Taiwan Abstract: A comprehensive hydrodynamic model, COMCOT-S, has been aimed at transport reversal in the Taiwan Strait during the period of Typhoon KROSA in 2007. The numerical model is originally based on COMCOT, and a parametric wind model with atmospheric sub-model is imbedded in COMCOT-S for simulating the typhoon-induced currents. The transport characteristics through the Taiwan Strait under the influence of Typhoon KROSA were analyzed using both field observations and numerical simulations during the period from 05 October to 08 October 2007 (GMT+8). The re- sults show that typhoon-induced currents did not fully response to the cease of the northward winds after the flow rever- sal, representing the influence of other dynamical forces on the flow orientation in the Taiwan Strait. Typhoon-induced wind shear stress and the water level gradient are found to be the direct driving forces of the southward transport events, whereas the atmospheric pressure gradient is dominant after the southward transport peak. By comparing with six numeri- cal experiments, the result of finer grid resolution is in good agreement with the observed current data at THL1 station near Mailiao Harbor. According to the transport simulation of numerical experiments, the Coriolis force is found to play a dominant role in the transport reversal as well, especially in the north Taiwan Strait. Besides, it is more relevant in the Penghu Channel than in the Taiwan Banks that the Coriolis force makes a negative contribution to the southward transport events. Finally, it’s verified that the generation of an anticyclonic eddy near the Changyun Rise would not be reached without the contribution of the Coriolis force to the northward transport in the Penghu Channel. Keywords: Typhoon-induced current, typhoon, volume transport, taiwan strait, storm surge, COMCOT-S. 1. INTRODUCTION Strait is scanty. Zhang et al. [7] implied that the strong southward transport would occur temporarily during the pas- Taiwan, which is located on the west side of the Pacific sage of typhoons over the Taiwan Strait. They also con- Ocean, is usually invaded by typhoons during summer and cluded that the typhoon-generated wind stress and the along- autumn seasons. A long, shallow channel, which connects strait water level gradient were the main driving forces in the East China Sea (ECS) and South China Sea (SCS) on the these temporal southward events. Moreover, the contribution continental Shelf of the West Pacific Ocean, is Taiwan of the momentum gradient term to the transport events is less Strait. It is about 350 km long, 180 km wide with the aver- compared with that of the nonlinear advection term. How- aged water depth of 60 m except over the deep Penghu ever, the characteristics of transport reversal as typhoons Channel next to the middle-west side of Taiwan Island [1]. enter the Taiwan Strait have been neglected in these litera- The historical records from Central Weather Bureau (CWB) tures. of Taiwan indicate that there were totally 393 typhoons strik- ing Taiwan from 1897 to 2007. The recurrence probability of To shed more light on the transport reversal [8] induced typhoon events invading Taiwan is about 3.96 per year. In by typhoons in the Taiwan Strait, a case study of typhoon 2007, there were totally up to five typhoons striking Taiwan KROSA in 2007 including the field observations near during the period from August to October. Mailiao Harbor and model results with systematic numerical experiments were considered in this research. In this paper, Although a lot of investigations of seasonal volume the numerical model, COMCOT-S (COrnell Multi-grid Cou- transport through the Taiwan Strait from the ECS to the SCS pled Tsunami model for Storm-surge) adopts explicit stag- have been conducted by various authors [2-6], the discussion gered leap-frog finite difference scheme in a multi-grid sys- of the influence of typhoons on transport through the Taiwan tem to solve the nonlinear shallow water wave equation in a spherical coordinate. Since this study was focused on the typhoon-induced transport through the Taiwan Strait, which *Address correspondence to this author at the Director, Tainan Hydraulics is the depth-integrated result of volume fluxes, the depth- Laboratory, & Senior Executive Vice President, National Cheng-Kung averaged shallow water equations were used in the numerical University, Tainan City 70101, Taiwan;Tel: 886-6-2741475; Fax: 886-6- 2366265; E-mail: [email protected]; Working E-mail: simulations. Although the seasonal variation of monsoon [email protected] winds (e.g. winter wind bursts) plays an important part in 1874-835X/10 2010 Bentham Open 144 The Open Ocean Engineering Journal, 2010, Volume 3 Lin et al. determining the orientation of ocean currents in the Taiwan P : ambient pressure, Strait, only the contribution of transient extreme weather of a typhoon events to strait transport would be taken into ac- f represents Coriolis parameter, equaling to 2 sin , count. whereas is the angular speed while the earth rotates. 2. MODEL DESCRIPTION The bottom shear stress terms are modeled by using the The hydrodynamic model implemented in this study Manning’s formula as follows, adopts a spherical coordinate in the multi-grid system. In COMCOT-S, the Non-Linear Shallow water Wave equation 2 1/2 (4) b gn 2 2 (NLSW, hereinafter) can be assigned to each specific sub- F = PP+ Q , 7/3 () level grid and be solved by the staggered leap-frog finite H difference scheme. The sub-level grid regions are dynami- cally connected. In the largest grid (main layer), the earth 2 1/2 (5) b gn 2 2 F = QP+ Q , rotation is relatively prominent and the local water depth in 7/3 () the open ocean is much larger than the propagating wave H amplitude. Thus, the nonlinear advective terms and the bot- In the present study, the Manning’s relative roughness tom friction terms can be neglected in this situation. How- coefficient, n, is given as bottom friction coefficient. ever, as the wave propagates over a continental shelf and approaches coastal regions, the shoaling effect would be The surface wind shear stress is computed by conven- increasingly significant. In that case, the nonlinear advective tional bulk formula as: term and bottom frictional term become prominent during (6) s uru uru the simulation procedure, while the significance of the Cori- F = aCd Vw Vw , olis force diminishes. The governing equations, dynamic forcing of the model and the numerical scheme will be de- uru where V is wind speed at height of 10 m, is air density scribed in this section. w a and the drag coefficient, C , would be introduced in the next 2.1. Governing Equations d section. The governing equations, including the depth-averaged continuity equation and the momentum equations, are util- 2.2. Modeling of Typhoon Winds ized in a spherical coordinate system as follows. The C coefficient in Eq. (6) is assumed to vary with d (1) 1 P uru (7) + { + ()cos Q }= 0, 2.16 V 26ms1 t Rcos w uru uru 0.49+ 0.065 V 10ms1 V < 26ms1 (2) w w P 1 P2 1 PQ gH 1 uru 1 + + + fQ 3 1.14 3ms Vw <10ms t Rcos H R H Rcos 10 Cd = uru 1.56 1 1 h P 1 s b 0.62+ 1ms V < 3ms a F F , uru w = + () Vw Rcos uru 2.18 V <1ms1 , (3) w Q 1 PQ 1 Q2 gH + + + + fP t Rcos H R H R The C formula follows the result by Large and Pond [9] d when the local wind speed is less than 26 ms-1, otherwise, it h P 1 s b = a + F F , would be assumed as a constant value suggested by Powell R () et al. [10]. where The Holland model [11] provides a wind field directly from input values of Rmax (radius of the maximum winds) (,) : longitude and latitude of the earth, and P (the atmospheric pressure in the storm center), based c R: earth radius, on the exponential distribution of the atmospheric pressure H: total water depth (= æ+h), with a peakness parameter B. æ: free surface elevation, (8) B R h : still water depth, P P P P exp max , = c + ()n c r P, Q: volume fluxes in longitude and latitude, g: gravitational acceleration, Transport Reversal Due to Typhoon Krosa The Open Ocean Engineering Journal, 2010, Volume 3 145 where P is the atmospheric pressure at radius r (radius 2.3. Numerical Scheme starting from the storm center) and P is the ambient pres- n Explicit leap-frog finite difference scheme has been sure. The resulting wind speed suggested by Phadke et al. adopted to solve the NLSW in the previous literatures [16, [12] is called the gradient wind speed and is given by 17]. The nonlinear advective terms are discretized with an upwind scheme. In general, the upwind scheme is considera- (9) B B bly stable and produces some numerical dissipation. But if BP P R R r 2 f 2 rf V = ()n c max exp max + , the velocity gradient in the fluid field is not too steep and if g R r 4 2 a the stability criterion, gh t / x 1 , is satisfied, upwind < formulation is preferred for nonlinear advective terms [18].