ENVIRONMENTAL FORCING OF HURRICANE MOTION AND INTENSIFICATION.
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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