Impacts of AirSea Interaction on Tropical Cyclone Track and Intensity

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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

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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.

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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.

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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.

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Modified ocean model for TC simulation

• The Kraus-Turner scheme to parameterize the
  vertical turbulent mixing (entrainment) is
  replaced by the Deardorffs 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.

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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.

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Model and experimental design
The maximum wind (Vm) is 25 ms-1 at rm100 km. b
is set to 0.5
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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
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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
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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
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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
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(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.
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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.
ß-effect
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• Rainfall rate in
• the fixed SST
• asymmetric forcing
• symmetric forcing
• experiments of E1

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TC tracks in the fixed (solid circles), symmetric
(open squares) and asymmetric (open circles) SST
experiments.
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Rainfall rate of E2.

• fixed SST
• coupled

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Rainfall rate of E3.

• fixed SST
• coupled

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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.

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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.

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THANK YOU
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ß-effect
BACK

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toward west-northwest
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