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Tropical Cyclone Motion. Outline. Tropical Cyclone Motion Climatology Environmental Steering Flow The Beta Effect Additional Influences Trochoidal Motions The Fugiwhara Effect. TC Motion: Climatology. Typical Tracks Prevailing tracks are shown in white for each month

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Presentation Transcript
slide2

Outline

  • Tropical Cyclone Motion
    • Climatology
    • Environmental Steering Flow
    • The Beta Effect
    • Additional Influences
    • Trochoidal Motions
    • The Fugiwhara Effect

M. D. Eastin

slide3

TC Motion: Climatology

Typical Tracks

Prevailing tracks are

shown in white for

each month

Main Features

Bermuda High:

Note the west – east

shift and magnitude

changes during the

season

East U.S. Trough:

Note the northwest

to southeast shift

and magnitude

changes during the

season

L

L

H

H

L

L

H

H

L

L

H

H

M. D. Eastin

slide5

TC Motion: Steering Flow

  • Motion of Individual TCs:
  • The deep layer environmental
  • flow accounts for a large fraction
  • (up to 80%) of TC motion
  • Assumes the TC acts as a
  • passive vortex moving with the
  • speed and direction of the
  • mass-weighted deep layer flow
  • When a deep layer estimate is
  • unavailable use the following:
  • TD and TS: 700 mb flow
  • Hurricane: 500 mb flow

From Velden and Leslie (1991)

M. D. Eastin

slide6

TC Motion: The Beta Effect

  • Motion of Individual TCs:
  • The “beta effect” accounts for 15-20%
  • (up to 2 m/s) of TC motion
  • Results from quasi-symmetric cyclonic flow
  • superimposed on the north-south gradient
  • of the Coriolis force (β = df / dy)
  • “Simple” explanation from the Cartesian
  • non-divergent barotropic vorticity equation
  • Beta Contribution: An air parcel displaced
  • southward (northward) will acquire positive
  • (negative) relative vorticity
  • Results in an east-west dipole of maximum
  • negative-positive vorticity generation
  • across the cyclone

Local

Vorticity

Change

Advection

of Vorticity

Beta

Vorticity Generation via Beta

f3

-

+

f2

f1

Initially Symmetric Cyclonic Vortex

M. D. Eastin

slide7

TC Motion: The Beta Effect

  • Advection Contribution: The resulting
  • cyclonic advection of the Beta-generated
  • vorticity produces a north-south dipole of
  • local vorticity change
  • Their combination locally produces two
  • vorticity maxima, called “beta gyres”,
  • that induce a northwesterly component to
  • TC motion (in the northern hemisphere)

Local

Vorticity

Change

Advection

of Vorticity

Beta

Vorticity Generation via Beta

and Vorticity Advection

-

_

f3

-

+

f2

f1

+

+

Initially Symmetric Cyclonic Vortex

From Holland (1983)

M. D. Eastin

slide8

TC Motion: Additional Influences

  • Motion of Individual TCs:
  • Some storms tend to drift toward
  • their latent heating centroid (which
  • may be offset from the circulation
  • center due to vertical shear)
  • Some storms drift toward synoptic-
  • scale troughs (particularly if the
  • trough is deepening)
  • Many storms will move toward a
  • weakness in a ridge (a relative low
  • pressure in a high pressure system)
  • Common theme: TCs tend to drift
  • toward other areas of low pressure

Sea-Level Pressure 06Z 0914 2006

Formerly

Hurricane

Florence

L

Weakness

H

H

Hurricane

Gordon

Forecast Track

TS Helene

M. D. Eastin

slide9

TC Motion: Trochoidal Motions

  • Motion of Individual TCs:
  • Many hurricanes experience “wobbles”,
  • or oscillations, with respect to their time
  • averaged motion vector
  • This trochoidal motion is believed to result
  • from the co-rotation of the TC’s circulation
  • center with a smaller mesovortex (perhaps
  • generated by a deep convective burst)
  • Trochoidal motions are often removed
  • from the official ”best” track
  • Trochoidal motions are often misinterpreted
  • as “turns”…..forecasters beware

Hurricane Carla (1961)

Best Track

(offset)

Actual Track

(with trochoidal motions)

From Jarvinen et al. (1984)

M. D. Eastin

slide10

TC Motion: The Fugiwhara Effect

  • Motion of Two Neighboring TCs:
  • Occasionally two TCs in close
  • proximity will co-rotate (and in
  • some cases, they merge)
  • This process is superimposed on
  • the advection by the steering flow
  • and the beta effect
  • Named for Dr. S. Fujiwhara who
  • first studied the phenomenon

Earth Relative

Tracks

Centroid Relative Tracks

From Prieto et al. (2003)

M. D. Eastin

slide11

Tropical Cyclone Motion

  • Summary
  • TC Motion Climatology (seasonality, and large-scale forcing)
  • Deep layer steering flow (function of intensity, contribution to total)
  • Beta effect (physical processes, contribution to total)
  • Additional Influences
  • Thochoidal Motions (definition, possible causes)
  • Fujiwhara Effect (definition, net result)

M. D. Eastin

slide12

References

Holland, G. J., 1983: tropical cyclone motion: Environmental interaction plus a beta effect. J. Atmos. Sci.,

40, 328-342.

Jarvinen, B. R., C. J. Neumann, and M. A. S. Davis, 1984: A tropical cyclone data tape for the North

Atlantic basin, 1886-1983: Contents, limitations, and uses. NOAA Tech. Memo,

NWS-NHC-22, 21 pp.

Preito, R., B. D. McNoldy, S. R. Fulton, and W. H. Schubert, 2003: A classification of binary tropical

cyclone-like vortex interactions. Mon. Wea. Rev., 131, 2656-2666.

Velden, C. S., and L. L. Leslie, 1991: The basic relationship between tropical cyclone intensity and the

depth of the environmental steering layer in the Australian region. Wea. Forecasting, 6,

244-253.

M. D. Eastin

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