Barotropic vortex adjustment to asymmetric forcing with application to tropical cyclone motion
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Authors
Carr, Lester E.
Subjects
Tropical cyclone motion
Vortex stability
Vortex propagation
Vortex stability
Vortex propagation
Advisors
Elsberry, R.L.
Date of Issue
1989-09
Date
Publisher
Monterey, California. Naval Postgraduate School
Language
en_US
Abstract
A nondivergent. barotropic analytical model to predict steady tropical cyclone (TC)
propagation relative to the large-scale environment is developed in terms of a "selfadvection"
process in which the TC is advected by an azimuthal wavenumber one gyre
flow that results from TC-environment interaction. The model is comprehensive in that
it includes the first-order effects of all of the dynamical influences that are presently understood
to be important to barotropic propagation: gradients of planetary and environmental
vorticity, changes in TC wind structure, and environmental windshear. An
unforced version of the model is used to show that angular windshear in the symmetric
TC circulation acts to damp perturbations from axisymmetry by tilting the perturbations
downshear. The resultant transfer of kinetic energy from perturbation to symmetric
circulation thus tends to restore axisymmetry. Thus, steady propagation of TC-like
barotropic vortices is a manifestation of a stable response to asymmetric forcing. To
predict both the asymmetric gyre flow and the propagation it induces, the forced
Barotropic Self-Advection Model (BSAM) is closed by seeking a particular pattern in the
vorticity tendencies of the TC-cnvironmental interaction flow. For realistic combinations
of environmental vorticity gradients and linear windshear, the BSAM predicts
propagation speeds and directions that are consistent with TC propagation characteristics
obsened in composite data. The capability of the BSAM to account for variable
TC structure is used to show that errors in determining TC outer wind strength of ±1
m s can result in an 85 km forecast error at 48 h. Finally, and most importantly, the
capability of the BSAM to initialize a barotropic numerical model so that quasi-steady
TC propagation occurs almost immediately is demonstrated for several simple dynamical
situations.
Type
Thesis
Description
Series/Report No
Department
Meteorology
Organization
Identifiers
NPS Report Number
Sponsors
Funder
Format
143 p.
Citation
Distribution Statement
Approved for public release; distribution is unlimited.