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26/5/2016 à 11h30: Séminaire de Gilbert BRUNET (LMD/ENS)

On the role of breaking African easterly waves and critical layers in hurricane genesis
Quand ? Le 26/05/2016,
de 11:30 à 12:30
Où ? salle 235 B, 29 Rue d'Ulm
S'adresser à François Gay-Balmaz
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This study bring new understanding on the decades-old hurricane genesis problem that starts with westward travelling African easterly waves that can evolve into coherent cyclonic vortices depending on their strength and other nonlinear wave breaking processes. To better understand the dynamics involved in hurricane genesis, the flow characteristics and the physical and dynamical mechanisms by which easterly waves form cat's eyes are investigated with the help of atmospheric reanalyzes and numerical simulations. We perform a climatological study of developing easterly waves covering the 1998-2001 hurricane seasons using ERA-Interim 6-hourly reanalysis data. Composite analyses for all named storms show a monotonic potential vorticity (PV) profile with weak meridional PV gradient and a cyclonic (i.e. south of the easterly jet axis) critical line for time periods of several days preceding the cat's eye formation. In addition, the developing PV anomaly composite shows a statistically significant companion wave-packet of non-developing easterly waves. The latter permit to develop a geometrical criteria to distinguish developing versus non-developing easterly waves (EWs) suggesting that only developing waves (~25% of the total) are associated with a nonlinear critical layer adjacent to a region of weak meridional PV gradient. A shallow water model is used to study the initial value and forced problems of disturbances. The results highlight the synergy of the dynamical mechanisms and the thermodynamical mechanisms. These findings are consistent with the analytical theory of free and forced disturbances to an easterly parabolic jet (Brunet and Warn, 1990; Brunet and Haynes, 1995; Choboter et al., 2000).

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