Principal Investigator: H. E. Willoughby
Collaborating scientist(s):
R. W. Jones
Objective: Understanding of synoptic--scale environmental forcing of hurricanes.
Rationale: Although hurricanes draw their energy from thermodynamic disequilibrium between the tropical atmosphere and the sea, their synoptic--scale surroundings control motion and intensification. Advection by the steering flow is the dominant factor in motion, but propagation across the surrounding flow is also important. Typical hurricanes intensify slowly, but the most dangerous ones deepen rapidly in a day or two from category one or two on the Saffir--Simpson scale to category four or five. A substantial body of observational evidence shows that upper tropospheric eddy influxes of angular momentum are essential to initiation of "rapid deepening." What are the dynamics of these eddies and generally of wave--like motions that mediate environmental forcing of tropical cyclones?
Method: Semispectral modeling of asymmetric waves on balanced vortices that evolve and move through complicated environmental flows on a rotating earth (Figure).
Accomplishment: When a barotropic, but otherwise hurricane--like, vortex is initialized from axisymmetry and rest in a uniform zonal environmental current on a beta plane, evolution of the vortex propagation and axisymmetric structure is faster in a westward current than in an equally strong eastward current because the environmental potential vorticity gradient is stronger. The tempo of the change is less than directly proportional to the gradient, however, because as the vortex evolves, its relative angular momentum becomes more anticyclonic. For the same reason, the total propagation relative to the advecting current on a beta plane is less than the sum of the propagation in a current on an f--plane and the propagation through still surroundings on a beta plane. In a shearing environmental current, the wavenumber 1 asymmetry can exchange energy with the environmental current through wave--wave interaction. If the shear is anticyclonic, it draws energy from the current; if the shear is cyclonic it gives energy up. The vortex propagation is faster in the former case despite weaker linear forcing caused by both a weaker potential vorticity gradient and more anticyclonic relative angular momentum. Direct advection of the shear also forces wavenumber 2. The asymmetries are Rossby waves that propagate upon the radial gradient of axisymmetric potential vorticity. The waves move against the mean swirling flow so that their phase lines remain stationary. In the cyclonically sheared environment, the eddy transports of angular momentum and wave energy are inward and outward, respectively. Although they arise in a barotropic model, these asymmetries have exactly the properties expected of the upper tropospheric eddies hypothesized to initiate rapid deepening.
Key reference: Willoughby, H. E. 1996: Nonlinear motion of a barotropic vortex in an environmental zonal flow. J. Atmos. Sci., 53, (in press).

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