Departures from axisymmetric balance dynamics during secondary eyewall formation
Abstract
Departures from axisymmetric balance dynamics are quantified during a case of secondary eyewall
formation. The case occurred in a three-dimensional mesoscale convection-permitting numerical
simulation of a tropical cyclone, integrated from an initial weak mesoscale vortex in an idealized
quiescent environment. The simulation exhibits a canonical eyewall replacement cycle. Departures
from balance dynamics are quantified by comparing the azimuthally averaged secondary circulation
and corresponding tangential wind tendencies of the mesoscale integration with those diagnosed as
the axisymmetric balanced response of a vortex subject to diabatic and tangential momentum forcing.
Balance dynamics is defined here, following the tropical cyclone literature, as those processes
that maintain a vortex in axisymmetric thermal wind balance.
The dynamical and thermodynamical fields needed to characterize the background vortex for the
Sawyer-Eliassen inversion are obtained by azimuthally averaging the relevant quantities in the
mesoscale integration and by computing their corresponding balanced fields. Substantial differences
between azimuthal averages and their homologous balance-derived fields are found in the boundary
layer. These differences illustrate the inappropriateness of the balance assumption in this region
of the vortex (where the secondary eyewall tangential wind maximum emerges). Although the balance
model does broadly capture the sense of the forced transverse (overturning) circulation, the
balance model is shown to significantly underestimate the inflow in the boundary layer. This
difference translates to unexpected qualitative differences in the tangential wind tendency. The
main finding is that balance dynamics does not capture the tangential wind spinup during the
simulated secondary eyewall formation event.
Description
The article of record as published may be found at http://dx.doi.org/10.10.1175/JAS-D-14-0018.1
Rights
This publication is a work of the U.S. Government as defined in Title 17, United States Code, Section 101. Copyright protection is not available for this work in the United States.Collections
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