Tropical Cyclone Motion 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 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 L L H H L L H H L L H H Tropical M. D. Eastin

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

From Velden and Leslie (1991) 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

Vorticity Generation via Beta Initially Symmetric Cyclonic Vortex 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 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) Local Vorticity Change Advection of Vorticity Beta Vorticity Generation via Beta and Vorticity Advection - _ f3 + - f2 f1 + + Initially Symmetric Cyclonic Vortex From Holland (1983) 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 Formerly Hurricane Florence L Weakness H H Hurricane Gordon Forecast Track TS Helene Tropical M. D. Eastin

(with trochoidal motions) 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) Tropical M. D. Eastin

Centroid Relative Tracks 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) Tropical M. D. Eastin

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) 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 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. Tropical M. D. Eastin