Three-dimensional wind fields derived from dual-Doppler radar analyses of
Hurricane Olivia (1994) permit investigation of kinematic asymmetries in
the vicinity of the eyewall. While early studies using airborne pseudo
dual-Doppler techniques provided insight into the symmetric and wavenumber one
structures of the hurricane inner core, the 0.5-2 hour time required to composite
the three-dimensional wind field precluded accurate resolution of higher
wavenumber asymmetries. This study utilizes data obtained via the actual
dual-Doppler procedure using two NOAA/HRD research aircraft. The compositing
time of the inner-core winds is reduced to 10-15 minutes which decreases the
aliasing of higher wavenumbers and allows for successive "snapshots" of the
hurricane as it evolves.
On the second day of observation of Hurricane Olivia the lower fuselage
reflectivity field was found to transition from a highly symmetric distribution
to one in which the eyewall reflectivity was concentrated in one quadrant of the
storm. The degree of asymmetry is quantified through a Fourier decomposition
of the reflectivity. The evolution of the wavenumber components of reflectivity
is then related to the asymmetric development of the wind, vorticity, and
divergence fields. The source of the increased asymmetry of Olivia may in part be
a consequence of increased vertical shear and tilting of the vortex as discussed
by Jones (1995), although the non-unidirectional nature of the vertical shear
observed in this case provides additional complication. Further insight into the
nature of the asymmetries is obtained through a budget analysis of perturbation
vorticity over the period of observation.
Recent work has suggested the importance of non-axisymmetric vorticity dynamics
on the development of tropical storms (e.g., Montgomery and Kallenbach, 1997;
Montgomery and Enagonio, 1998; Moeller and Montgomery, 1998; Shapiro, 1998).
Vorticity asymmetries in the inner-core region produced via convection or
interaction of the hurricane with its environment will propagate on the background
vorticity gradient of the hurricane and interact with the vortex, producing
changes in its symmetric structure. If time permits, results demonstrating
the effects of eddy processes on the symmetric vorticity distribution in the
inner-core region of Olivia will be presented. Mechanisms responsible for the
observed eddy transports of vorticity are suggested.