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Tropical Cyclone Motion

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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  1. Tropical Cyclone Motion M. D. Eastin

  2. Outline • Tropical Cyclone Motion • Climatology • Environmental Steering Flow • The Beta Effect • Additional Influences • Trochoidal Motions • The Fugiwhara Effect M. D. Eastin

  3. 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

  4. TC Motion: An Atypical Track M. D. Eastin

  5. 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

  6. 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

  7. 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

  8. 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

  9. 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

  10. 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

  11. 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

  12. 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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