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Impacts of Air-Sea Interaction on Tropical Cyclone Track and Intensity

This study examines the effects of air-sea interaction on the track and intensity of tropical cyclones. It explores the role of surface wind stress, turbulent mixing, and ocean responses in modifying cyclone behavior. The findings highlight the importance of symmetric and asymmetric SST anomalies and their impact on TC motion and intensity weakening.

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Impacts of Air-Sea Interaction on Tropical Cyclone Track and Intensity

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  1. Impacts of Air-Sea Interaction on Tropical Cyclone Track and Intensity Liguang Wu, Bin Wang, and Scott A. Braun Submitted to Mon. Wea. Rev. June 2004

  2. Introduction • A tropical cyclone (TC) develops and maintains itself by drawing its primary energy from the underlying ocean surface. • The surface wind stress associated with a TC can generate strong turbulent mixing, deepening of the ocean mixed layer (ML) and entrainment of cooler water to the surface that can lead to significant SST decreace.

  3. Introduction • Khain and Ginis (1991) found that westward-moving (eastward-moving) TCs in coupled experiments were displayed farther to the south (north) than in the corresponding experiments without air-sea interaction. • They attributed these track differences to asymmetric precipitation patterns which were shifted azimuthally because of air-sea interaction.

  4. Model and experimental design • Hurricane model is designed by Wang (1998). • Grid size: 25km. • 16 vertical layers. • Two and one half layer ocean model is developed by Wang et al. (1995). • The SST and wind stress are passed between the hurricane and ocean models every 3 minutes.

  5. Modified ocean model for TC simulation • The Kraus-Turner scheme to parameterize the vertical turbulent mixing (entrainment) is replaced by the Deardorff’s scheme to include the important shear instability. • The downward heat flux decreases exponentially in the mixed layer and the turbulent momentum and heat fluxes are not allowed to penetrate below the ML base in the modified ocean model.

  6. Modified ocean model for TC simulation • The vertical temperature gradient in the entrainment layer is proportional to the mean vertical temperature gradient in the thermocline layer.

  7. Model and experimental design The maximum wind (Vm) is 25 ms-1 at rm=100 km. b is set to 0.5

  8. Ocean response in the coupled model (E2C) Quasi-steady state 1.2ms-1 1.1ms-1 Maximum current speeds 3.5oC Minimum ML temperature 4

  9. 96h ocean responses in the coupled experiment of E2 (E2C) rightward bias • SST anomaly • ML depth anomaly • entrainment rate • thermocline depth anomaly • currents in the ML • currents in the thermocline layer Function of 1.Wind stress 2.Velocity shear at the base of ML 3.Convective overturning due to the surface buoyancy fluxes 4

  10. Time series of the maximum wind speed and minimum central pressure in experiments E2, E1 and E3. ○ fixed SST ● asymmetric SST □ coupled ■ symmetric SST 12ms-1 14ms-1 10ms-1 24mb 18mb 32mb

  11. Decomposition of the SST anomalies at 96 h resulting from hurricane-ocean interaction in the coupled experiment of E2 (E2C) (a) symmetric component (b) asymmetric component

  12. (a) The lowest-level asymmetric wind field in the fixed SST experiment of E2 (E2F). (b) The wind difference between the asymmetric forcing (E2A) and fixed SST (E2F) experiments of E2 at 96 h.

  13. TC tracks in the fixed SST (dashed) and coupled (solid) experiments for (a) E1, (b) E2, (c) E3, (d) B2, (e) B1, and (f) B3. β-effect

  14. Rainfall rate in • the fixed SST • asymmetric forcing • symmetric forcing • experiments of E1

  15. TC tracks in the fixed (solid circles), symmetric (open squares) and asymmetric (open circles) SST experiments.

  16. Rainfall rate of E2. • fixed SST • coupled

  17. Rainfall rate of E3. • fixed SST • coupled

  18. Conclusions • The coupled model can reasonably produce the major features of the ocean responses to moving TC forcing. ML deepening, SST cooling, ML and thermocline layer currents • The influence of the asymmetric anomalies is insignificant while the resulting symmetric cooling plays a decisive role in the weakening of TC intensity.

  19. Conclusions • The symmetric and asymmetric SST anomalies modify the asymmetry of the diabatic heating with respect to the TC center, thus affecting TC motion. • When the vortex strength is relatively weak (strong), a TC in the coupled model moves to the north (south) of the TC in the corresponding fixed SST experiments.

  20. THANK YOU

  21. β-effect BACK • f↑ ∵ζa=const. • ∴ζ↓ • f↓ ∵ζa=const. • ∴ζ↑ toward west-northwest ζ+ ζ- • ζ decrease 2

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