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 the 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.
Accomplishment:
When a semispectral model of a barotropic, but otherwise hurricane-like, vortex is initialized from
axisymmetry and rest on a beta plane, the vortex motion is slower and more poleward when the
swirling wind decreases more sharply with radius. This result is consistent with propagation due to a
wavenumber 1 asymmetry with forcing proportional to the product of the vortex relative angular
momentum with the environmental potential vorticity (PV) gradient.
In a uniform zonal environmental current, the vortex propagation and the evolution of axisymmetric
structure are faster in a westerly current than in an equally strong easterly current because the
environmental PV gradient is stronger. The rate of change is less than directly proportional to the
gradient, however, because the vortex relative angular momentum decreases more rapidly with time
when the PV gradient is stronger. For the same reason, the total propagation relative to a current on a
beta plane is less than the sum of the propagation in the same current on an f plane and that 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, the asymmetry
draws energy from the current; if the shear is cyclonic it gives energy up. In anticyclonic shear, the
forcing seen in moving coordinates is closer to resonance with a linear normal mode. Thus, the vortex
propagation is faster in the former case despite weaker linear forcing caused by weaker PV gradient
and smaller angular momentum.
Direct advection of the shear also forces wavenumber 2. The asymmetries
are Rossby waves that propagate upon the radial gradient of axisymmetric PV. The phase lines are
fixed to the forcing so that they remain stationary by upstream propagation against the mean swirling
flow. In the cyclonically sheared environment, the eddy transports of angular momentum and wave
energy are inward and outward, respectively. Although the asymmetries arise in the barotropic model,
they have exactly the properties expected of the upper tropospheric eddies hypothesized to modulate
tropical cyclone intensity changes in nature.
Key reference:
Willoughby, H. E., 1996: Nonlinear
motion of a barotropic vortex in an environmental zonal flow. J. Atmos.
Sci.,
53, (in press).
Last modified: 8/9/96