Propagating and stationary spiral bands are ubiquitous asymmetric features
of a hurricane, whose dynamics is central to understanding the storm's
development. Recently, Montgomery and Kallenbach (1997; hereafter MK) and
Moeller and Montgomery (1998; hereafter MM) investigated the characteristics
of spiral bands by studying the properties of outward-propagating Rossby waves
whose restoring mechanism is associated with the radial gradient of the storm
vorticity. These waves provide a mechanism for transferring energy from the
hurricane's inner core to larger radius. Thus, they are fundamental to the
process by which a hurricane responds to a changing environment, and may play
an important role in hurricane intensification as well as the creation of
secondary eyewalls. A hurricane is essentially a thermodynamic engine, and
its eyewall and spiral band features are convective in nature. Therefore,
any complete understanding of the role of asymmetric wave disturbances in a
storm's evolution must include the influence of cumulus convection on both the
symmetric vortex and the asymmetries. The approach of the present study
is to use a three-layer model to couple the waves investigated by MK and MM
in a dry context to the boundary layer and convection.
In a benchmark experiment, a symmetric vortex is first spun up on an
f-plane for 24 h. A weak azimuthal-wavenumber two PV asymmetry confined to the
middle layer of the model is then added to the vortex near its radius of
maximum wind (RMW). After an additional 6 h (for a total 30-h simulation) the
change in the azimuthally-averaged tangential wind in the middle layer due to
the asymmetry is qualitatively the same as in a dry simulation, with a maximum
acceleration inside the RMW, a maximum deceleration outside, and a net
intensification of the vortex. The changes in the tangential wind induce
corresponding changes in the boundary layer convergence and convective mass
flux, with a maximum increase inside and a maximum decrease near the RMW.
Later evolution of the symmetric vortex in this experiment leads to a net
weakening within ~24h.
A diagnosis of the contributions to changes in the symmetric wind tendency
due to the asymmetry confirm the dominance of horizontal eddy momentum fluxes
in the very early evolution of the vortex (to ~1h). At later times the
contribution from the eddy fluxes decreases, and that from changes in the
symmetric momentum fluxes and horizontal diffusion dominate.
Additional experiments with an imposed isolated double-PV anomaly instead
of wavenumber-two have been completed. When the double-anomaly is placed near
the RMW, the evolution of the symmetric vortex is similar to that with the
wavenumber two asymmetry. Contrary to expectation, moving the anomaly closer
to the center of the vortex or further out does not change the overall
evolution of the symmetric vortex, which strengthens to ~6h and weakens
within ~24h. The physical mechanism responsible for the robustness of this
result is being investigated using a budget for the asymmetric
vorticity.
Results of these experiments and diagnostics will be reported upon
as available.