Asymmetric and axisymmetric dynamics of tropical cyclones
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Author
Persing, J.
Montgomery, M.T.
McWilliams, J.C.
Smith, R.K.
Date
2013Metadata
Show full item recordAbstract
We present the results of idealized numerical experiments
to examine the difference between tropical cyclone
evolution in three-dimensional (3-D) and axisymmetric (AX)
model configurations. We focus on the prototype problem
for intensification, which considers the evolution of an initially
unsaturated AX vortex in gradient-wind balance on an
f plane. Consistent with findings of previous work, the mature
intensity in the 3-D model is reduced relative to that in
the AX model. In contrast with previous interpretations invoking
barotropic instability and related horizontal mixing
processes as a mechanism detrimental to the spin-up process,
the results indicate that 3-D eddy processes associated
with vortical plume structures can assist the intensification
process by contributing to a radial contraction of the maximum
tangential velocity and to a vertical extension of tangential
winds through the depth of the troposphere. These
plumes contribute significantly also to the azimuthally averaged
heating rate and the corresponding azimuthal-mean
overturning circulation.
The comparisons show that the resolved 3-D eddy momentum
fluxes above the boundary layer exhibit counter-gradient
characteristics during a key spin-up period, and more generally
are not solely diffusive. The effects of these eddies
are thus not properly represented by the subgrid-scale parameterizations
in the AX configuration. The resolved eddy
fluxes act to support the contraction and intensification of the
maximum tangential winds. The comparisons indicate fundamental
differences between convective organization in the 3-
D and AX configurations for meteorologically relevant forecast
timescales. While the radial and vertical gradients of the
system-scale angular rotation provide a hostile environment
for deep convection in the 3-D model, with a corresponding
tendency to strain the convective elements in the tangential
direction, deep convection in the AX model does not suffer
this tendency. Also, since during the 3-D intensification
process the convection has not yet organized into annular
rings, the azimuthally averaged heating rate and radial gradient
thereof is considerably less than that in the AX model.
This lack of organization results broadly in a slower intensification
rate in the 3-D model and leads ultimately to a weaker
mature vortex after 12 days of model integration. While azimuthal
mean heating rates in the 3-D model are weaker than
those in the AX model, local heating rates in the 3-D model
exceed those in the AX model and at times the vortex in the
3-D model intensifies more rapidly than AX. Analyses of the
3-D model output do not support a recent hypothesis concerning
the key role of small-scale vertical mixing processes
in the upper-tropospheric outflow in controlling the intensification
process.
In the 3-D model, surface drag plays a particularly important
role in the intensification process for the prototype intensification
problem on meteorologically relevant timescales
by helping foster the organization of convection in azimuth.
There is a radical difference in the behaviour of the 3-D
and AX simulations when the surface drag is reduced or increased
from realistic values. Borrowing from ideas developed
in a recent paper, we give a partial explanation for this
difference in behaviour.
Our results provide new qualitative and quantitative insight
into the differences between the asymmetric and symmetric
dynamics of tropical cyclones and would appear
to have important consequences for the formulation of a
fluid dynamical theory of tropical cyclone intensification
and mature intensity. In particular, the results point to some
fundamental limitations of strict axisymmetric theory and
modelling for representing the azimuthally averaged behaviour
of tropical cyclones in three dimensions.
Description
The article of record as published may be found at http://dx.doi.org/10.5194/acp-13-12299-2013
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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