Orographic Influence on Typhoon Tracks over the Central Mountain Range of Taiwan Yuh-Lang Lin North Carolina State University Collaborators: C.-Y. Huang, J. Han, D. Hamilton S.-Y. Chen, C. M. Hill, C. Savage, N. C. Witcraft, Y.-H. Kuo, S.-T. Wang

2 Continuous Track strong (V max >50m/s) deep (D>10.7 km) Discontinuous Track weak (V max : 25-50m/s) shallow (D<6 km) 1. Effects of CMR on typhoon tracks (Wang 1980)

3 2. Control Parameters for track deflection Based on previous studies (e.g., Wang 1980; Brand and Blelloch 1974; Chang 1982; Yeh and Elsberry 1993a,b; Zehnder and Reeder 1997; Huang and Lin 1997; Lin et al. 1999), we identified 8 key parameters: V max, U, h, R, L x, L y, f, N V max, U, h, R, L x, L y, f, N which controls the track deflection.

4 Based on the Buckingham-pi theorem, we may choose the following 6 independent nondimensional parameters: (8-2=6) U/Nh, V max /Nh, h/L x, R/L y, U/fL x & V max /fR Physical meanings: U/Nh: basic-flow Froude number U/Nh: basic-flow Froude number V max /Nh: vortex Froude number V max /Nh: vortex Froude number h/L x : steepness of the mountain h/L x : steepness of the mountain R/L y : ratio of the cyclone and mountain sizes R/L y : ratio of the cyclone and mountain sizes U/fL x : basic flow Rossby number U/fL x : basic flow Rossby number V max /fR: vortex Rossby number V max /fR: vortex Rossby number

5 Effects of orography on TC may be linearly decomposed into: (a) Influence on the basic flow (b) Influence on the cyclone circulation Physical meaning of U/Nh and V max /Nh (Lin, Chen, Hill, Huang 2005 JAS) (Lin, Chen, Hill, Huang 2005 JAS)

6 V max /Nh U/Nh V max /fR h/L x Based on previous studies, it was found: Track is continuous when V max /Nh > 1.5; Otherwise it is discontinuous. It appears that track continuity is mainly controlled by V max /Nh (at least for CMR) SN CD CD CD

7 Verification of previous results by systematic idealized numerical simulations (Lin et al. 2005) U/Nh h/L x V max /fR R/L y Vmax/NhVmax/Nh CD CD Track is continuous (discontinuous) when V max /Nh>1.6 ( 1.6 (<1.2) Track continuity is mainly controlled by V max/ Nh, and is less Track continuity is mainly controlled by V max/ Nh, and is less sensitive to other parameters (for CMR) sensitive to other parameters (for CMR)

8 Discontinuous V max /Nh=0.8 U/Nh=0.13 R/L y =0.75 relative vorticity streamlines Continuous V max /Nh=2.0 U/Nh=0.5 R/L y =1.042

9 With a very small R/Ly, the track becomes discontinuous and the cyclone is even deflected southward O b s. 18/00Z (STY) 18/12Z (TY) 17/12Z (STY) 19/00Z (TY) 12 0E 12 1E 12 2E 25 N 24 N 23 N 22 N 12 3E ★ ★ Wu- Fen- Shan Hu a- Lie n (Jian and Wu 2006) CTR L NT Our study helps explain the curving of Tphoon Haitang (2005)

10 Degree of Track Deflection When V max /Nh or U/Nh is smaller, the track deflection is larger V max /Nh U/Nh

11 (a) Weak blocking deflected slightly northward upstream deflected slightly northward upstream continuous track continuous track (b) Moderate blocking deflected northward upstream deflected northward upstream a secondary vortex forms on the lee -> discontinuous track a secondary vortex forms on the lee -> discontinuous track (c) Strong blocking deflected southward upstream deflected southward upstream a secondary vortex forms on the lee -> discontinuous track a secondary vortex forms on the lee -> discontinuous track

12 U/Nh Vmax/Nh B T Application of the control parameter hypothesis to Supertyphoon Bilis (2000) and typhoon Toraji (2001) (Lin, Witcraft, Kuo 2006 MWR)

13 3. Effects of Lanfalling locations: R/Ly comes into play

14 4. Effects of impinging angles

15 Upstream of the mountain: vorticity advection dominates Vorticity advection Vorticity stretching Local vorticity tendency Vorticitytilting

16 Over the mountain: vorticity stretching dominates Vorticity advection Vorticity stretching

17 On the lee side: vorticity advection regains the control Vorticity stretching Vorticity advection 18

18 5. Summary Track continuity is more sensitive to V max /Nh and R/L y. Track continuity is more sensitive to V max /Nh and R/L y. A combination of small V max /Nh, R/L y, U/Nh, U/fL x, and V max /fR and large (small) h/L x will give larger degree of track deflection. A combination of small V max /Nh, R/L y, U/Nh, U/fL x, and V max /fR and large (small) h/L x will give larger degree of track deflection. (Lin, Chen, Hill, Huang 2005 JAS) Blocking plays an essential role in dictating the track deflection Blocking plays an essential role in dictating the track deflection Tracks are also influenced by landfalling location and impinging angle Tracks are also influenced by landfalling location and impinging angle

19 Thank You!

20 The deflection of a cyclone encountering a mountain range is largely controlled by vorticity advection and stretching, depending upon the landfall location and approach angle of the cyclone. The deflection of a cyclone encountering a mountain range is largely controlled by vorticity advection and stretching, depending upon the landfall location and approach angle of the cyclone. The deflection of a cyclone encountering a mountain range is largely controlled by vorticity advection and stretching, depending upon the landfall location and approach angle of the cyclone. Deflection is controlled by vorticity advection and stretching Deflection is controlled by vorticity advection and stretching Upstream: Vorticity advection Upstream: Vorticity advection Crossing over: Vorticity stretching Crossing over: Vorticity stretching Downstream: Vorticity advection Downstream: Vorticity advection

21 3. Application to Track Deflection of Bilis and Toraji (Lin, Witcraft and Kuo 2006 MWR) Use MM5 to investigate the dynamics of track deflection for typhoons crossing over the CMR Examination of 2 storms: Super Typhoon Bilis (2000) Fast-moving; Category 5; Continuous track Typhoon Toraji (2001) Slowly-moving; Category 2/3; Discontinuous track

22 Observed and Simulated Bilis Tracks TC symbols – Observed Stars – Surface Squares – 700 mb Circles – 500 mb Triangles – 300 mb Max wind: 160 knots

23 Observed and Simulated Toraji Tracks TC symbols – Obs Stars – Surface Squares – 700 mb Circles – 500 mb Triangles – 300 mb *open symbols – coexisting centers Max winds: 95 knots

24 Bilis: Backward trajectories (8/22/17Z)

25 Toraji: Backward trajectories (7/29/21Z) Most low-level air parcels are blocked by CMR and go around the mountain

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27 Obs. 18/00Z (STY) 18/12Z (TY) 17/12Z (STY) 19/00Z (TY) 120E121E122E 25N 24N 23N 22N 123E ★ ★ Wu-Fen-Shan Hua-Lien CTRL NT Tracks: Obs. CTRL NT (Jian and Wu 2006)