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Lagrangian Trajectories at the Outflow of Tropical Cyclones

Cohen, Yair and Paldor, Nathan (2020) Lagrangian Trajectories at the Outflow of Tropical Cyclones. Quarterly Journal of the Royal Meteorological Society . ISSN 0035-9009. (In Press)

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Recent observations from the upper levels of tropical cyclones show the development of a Low engulfed by a larger High. The impact of such geopotential map on the dynamics of the outflow is studied by analyzing the Lagrangian trajectories of air parcels. The geopotential is modeled by the hydrostatic sum of two, opposite sign, Gaussians: a warm core and a near surface Low, capturing relevant features of the observed tropical cyclones. Assuming a time independent and axisymmetric geopotential map the dynamics is described by an integrable, two degrees‐of‐freedom, angular momentum conserving, Hamiltonian system with a single nondimensional parameter. The steady states of this system bifurcate when the geopotential amplitude increases above a threshold as the single elliptic fixed‐point becomes hyperbolic surrounded by a pair of elliptic‐fixed points located on its sides. A gradient balance prevails near the elliptic points while an inertial balance occurs at the radius of maximum geopotential, where the hyperbolic (unstable) point is located. The divergence of trajectories around this hyperbolic point implies the existence of strong horizontal wind divergence. A gradient non‐balance region emerges in the steady state (radius, geopotential amplitude) map and this region expands with the increase of geopotential amplitude requiring higher angular momentum values for trajectories to transition from High to Low. A transformation of the (radius, radial velocity) variables to action‐angle variables yields an approximate expression for the tangential drift (long‐time average of the tangential speed) that estimates the tangential winds. The analysis is extended numerically to the case of warm core offset, where the two Gaussians have different centers.

Item Type:Article
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Paldor, Nathan0000-0002-4419-4201
Additional Information:© 2020 Wiley. Accepted manuscript online: 09 September 2020; Manuscript accepted: 17 August 2020; Manuscript revised: 12 August 2020; Manuscript received: 10 October 2019.
Subject Keywords:gradient non‐balance; hurricane outflow; Lagrangian trajectories; tropical cyclone structure; warm core
Record Number:CaltechAUTHORS:20200929-095552059
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Official Citation:Cohen, Y. and Paldor, N. (2020), Lagrangian Trajectories at the Outflow of Tropical Cyclones. Q J R Meteorol Soc. Accepted Author Manuscript. doi:10.1002/qj.3904
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:105626
Deposited By: Tony Diaz
Deposited On:29 Sep 2020 17:27
Last Modified:29 Sep 2020 17:27

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