Tropical Cyclone Motion
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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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Tropical Cyclone Motion

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Tropical cyclone motion

Tropical Cyclone Motion

M. D. Eastin


Tropical cyclone motion

Outline

  • Tropical Cyclone Motion

    • Climatology

    • Environmental Steering Flow

    • The Beta Effect

    • Additional Influences

    • Trochoidal Motions

    • The Fugiwhara Effect

M. D. Eastin


Tropical cyclone motion

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


Tropical cyclone motion

TC Motion: An Atypical Track

M. D. Eastin


Tropical cyclone motion

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


Tropical cyclone motion

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


Tropical cyclone motion

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


Tropical cyclone motion

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


Tropical cyclone motion

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


Tropical cyclone motion

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


Tropical cyclone motion

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


Tropical cyclone motion

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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