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| Titel |
Asymmetric and axisymmetric dynamics of tropical cyclones |
| VerfasserIn |
J. Persing, M. T. Montgomery, J. C. McWilliams, R. K. Smith |
| Medientyp |
Artikel
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| Sprache |
Englisch
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| ISSN |
1680-7316
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| Digitales Dokument |
URL |
| Erschienen |
In: Atmospheric Chemistry and Physics ; 13, no. 24 ; Nr. 13, no. 24 (2013-12-18), S.12299-12341 |
| Datensatznummer |
250085889
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| Publikation (Nr.) |
 copernicus.org/acp-13-12299-2013.pdf |
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| Zusammenfassung |
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. |
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