Topographic Effects on Typhoon Toraji 2001

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Topographic Effects on Typhoon Toraji (2001)

• Yang, M.-J. and L. Ching, 2004 A Modeling Study
  of Typhoon Toraji (2001) Physical
  Parameterization Sensitivity and Topographic
  Effect. Terr. Atmos. Ocean. Sci., 16, 177-213.
• Lin, Y.-L., D. B. Ensley, S. Chiao, and C.-Y.
  Huang, 2002 Orographic Influences on Rainfall
  and Track Deflection Associated with the Passage
  of a Tropical Cyclone. Mon. Wea. Rev., 130,
  2929-2950.
• Lin, Y.-L., S. Y. Chen, C. M. Hill, and C.-Y.
  Huang, 2005 Control Parameters for the
  Influences of a Mesoscale Mountain Range on
  Cyclone Track Continuity and Deflection. J.
  Atoms. Sci., 62, 1849-1866.

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Introduction

• In this study, we will discuss the topographic
  effects on Toraji, which include the generation
  of heavy orographic rainfall and the degree of
  deflection of storms track by Taiwans
  topography.

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Case Description
24 hours accumulated rainfall on 30 July 2001
(LST).
The JTWC best track
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Model Setting

• The MM5 model configuration includes three nested
  domains with grid size of 60, 20, and 6.67 km,
  respectively.

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Physics schemes used by each numerical
experiment.
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Ensemble analysis
Ensemble-analyzed tracks
Cumulus
Microphysic
PBL
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The simulated 24-h accumulated rainfall (in mm)
of the ensemble average plus one stand deviation.
Cumulus
Microphysic
PBL
The maximum rainfall around 664 mm over the south
peak of CMR
346.5
430.3
544
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Orographic Rainfall

• Follow the flux method of Lin et al. (2002) to
  see how much Torajis precipitation over Taiwan
  is related to topography-enhanced moisture flux
  (Wu and Kuo 1999 Lin et al. 2001 Wu et al.
  2002).

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Orographic Rainfall

• Lin et al. (2001) proposed the orographic
  precipitation approximated by this formula
• the three-dimensional
  orographic moisture flux
• the general vertical moisture flux

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before landfall
Simulated 1-h accumulated rainfall valid for
1200-1300 UTC 29 July.
General vertical moisture flux valid at 7/29/12
UTC.
Orographically induced vertical moisture flux
valid at 7/29/12 UTC.
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passed land
Simulated 1-h accumulated rainfall valid for
0000-0100 UTC 30 July.
General vertical moisture flux valid at 7/30/00
UTC.
Orographically induced vertical moisture flux
valid at 7/30/00 UTC.
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Track Deflection

• A momentum budget is conducted to understand the
  physical mechanisms responsible for the track
  turning right before Torajis landfall on Taiwan.

obs
model
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Analyzed sea level pressure at 7/29/14 UTC. (Wang
et al. 2001)
Simulated sea level pressure at 7/29/12 UTC.
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• The u- and v-momentum equations in the MM5 (Grell
  et al. 1994) are

ADV advection
PGF pressure-gradient force
COR Coriolis force
TUB turbulent mixing and friction
TEN local tendency
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Vertical profile of (a) u-momentum and (b)
v-momentum budget terms averaged over a square
area around Torajis center and over a 1-h period
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velocity vectors of the time- (1-h) and
tropospheric-averaged steering flows around
Typhoon Toraji
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Control parameter analysis

• Lin et al. (2002) indicated that when Vmax/Nh gt
  1.6, Vmax/U gt 7.0, and Vmax/Rf gt 4.0, the TC
  track would be continuous.
• Lin et al. (2004) showed that when R/Ly lt 0.35
  with strong blocking, the TC track would be
  discontinuous. The TC would be deflected
  anticyclonically.

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Vmax/Nh

• The nondimensional parameter may be regarded as a
  vortex Froude number of the typhoon tangential
  circulation.
• When Vmax/Nh is large, it is easier for the TC
  vortex to pass over the mountain because the
  kinetic energy is more than enough to overcome
  the work required to lift the stratified flow
  against gravity and the orographic potential
  energy barrier.

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Vmax/U

• Vmax/U may also be viewed as the ratio of the
  vortex Froude number to the basic-flow of Froude
  number, (Vmax/Nh) / (U/Nh).
• A large Vmax/U represents a stronger vortex and,
  based on the energy arguments above, makes it
  easier for the vortex to pass over the mountain.

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Vmax/Rf

• Vmax/Rf is a measure of the ratio of vortex
  vorticity (Vmax/R) to planetary vorticity (f).
• When Vmax/Rf is large, the TC vortex is more
  stable and it is easier to pass over the mountain.

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R/Ly

• R/Ly is the ratio of the size of the TC to the
  length of the mountain chain.
• When R/Ly is small, the TC vortex is more
  difficult to climbing over the mountain. The TC
  track would be discontinuous, and the track
  deflection is large.

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When TC is easy to pass over the mountain
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When TC is difficult to pass over the mountain
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lt1.6 lt7.0 gt4.0 lt0.35
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Control parameter analysis
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Conclusions

• A combination of the topographically- and
  environmentally-induced vertical moisture fluxes,
  calculated based on the flux model of Lin et al.
  (2001), corresponded well to the hourly surface
  rainfall distribution.
• The positive area of orographically induced
  vertical moisture flux cloud be used to help
  predict the following rainfall distribution.
• The general vertical moisture flux can predict
  rainfall over a flat surface, which is not
  predicted by the orographically induced moisture
  flux.

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Conclusions

• The westward turning of Torajis track right
  before the landfall may be caused by horizontal
  advection process, on the basis on a momentum
  budget analysis.
• An analysis of nondimensional parameters for
  typhoons track continuity over the Taiwan island
  shows that Typhoon Torajis track discontinuity
  is consistent with the control parameter analysis
  proposed by Lin et al. (2002).

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Thanks
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