Tropical Cyclone Motion

Tropical M. D. Eastin

Tropical Cyclone Motion


Outline


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


TC Motion: Climatology

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Typical Tracks

Prevailing tracks areshown in white foreach month

Main Features

Bermuda High:

Note the west – eastshift and magnitudechanges during theseason

East U.S. Trough:

Note the northwestto southeast shiftand magnitudechanges during theseason


TC Motion: An Atypical Track


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)


TC Motion: The Beta Effect


• 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

BetaAdvection of Vorticity

Local VorticityChange

Initially Symmetric Cyclonic Vortex

f1

f2

f3

+ -

Vorticity Generation via Beta

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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)

Initially Symmetric Cyclonic Vortex

f1

f2

f3

+ -

Vorticity Generation via Betaand Vorticity Advection

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From Holland (1983)

BetaAdvection of Vorticity

Local VorticityChange

v

yv

xu

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TC Motion: Additional Influences


• 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

HH

TS Helene

FormerlyHurricane Florence

Hurricane Gordon

Forecast Track

WeaknessL


TC Motion: Trochoidal Motions


• 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

From Jarvinen et al. (1984)

Hurricane Carla (1961)

Best Track(offset)

Actual Track(with trochoidal

motions)


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

From Prieto et al. (2003)

Earth RelativeTracks

Centroid Relative Tracks


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)



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 NorthAtlantic 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 tropicalcyclone-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 thedepth of the environmental steering layer in the Australian region. Wea. Forecasting, 6,244-253.

Tropical Cyclone Motion

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