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Tropical M. D. Eastin
Tropical Cyclone Motion
Tropical M. D. Eastin
Outline
Tropical Cyclone Motion
• Climatology• Environmental Steering Flow• The Beta Effect• Additional Influences• Trochoidal Motions• The Fugiwhara Effect
Tropical M. D. Eastin
TC Motion: Climatology
L L
L
H
L
L L
H
H
H
H
H
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
Tropical M. D. Eastin
TC Motion: An Atypical Track
Tropical M. D. Eastin
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)
Tropical M. D. Eastin
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
BetaAdvection of Vorticity
Local VorticityChange
Initially Symmetric Cyclonic Vortex
f1
f2
f3
+ -
Vorticity Generation via Beta
v
yv
xu
t
Tropical M. D. Eastin
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
+
-
+
_
From Holland (1983)
BetaAdvection of Vorticity
Local VorticityChange
v
yv
xu
t
Tropical M. D. Eastin
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
HH
TS Helene
FormerlyHurricane Florence
Hurricane Gordon
Forecast Track
WeaknessL
Tropical M. D. Eastin
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 From Jarvinen et al. (1984)
Hurricane Carla (1961)
Best Track(offset)
Actual Track(with trochoidal
motions)
Tropical M. D. Eastin
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
Tropical M. D. Eastin
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)
Tropical Cyclone Motion
Tropical M. D. Eastin
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.