RESUME / ABSTRACT  




Simulations à haute résolution d'ouragans et diagnostiques d'activité ondulatoires

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Gilbert Brunet





The theory of empirical normal modes (ENMs) was applied to diagnose inner spiral bands formed in an explicitly simulated hurricane using the high-resolution PSU-NCAR nonhydrostatic mesoscale model MM5 in Chen et al. 2003 (J. Atmos. Sci. - JAS). The ENM method has the capability to decompose simultaneously wind and thermal fields into dynamical consistent and orthogonal modes with respect to wave-activities (Brunet 1994 JAS; Brunet and Vautard 1996 JAS; Charron and Brunet 1999 JAS; Zadra et al. 2002 JAS).

For wavenumber one and two anomalies, it was found that the leading modes are vortex Rossby waves that explain 40% to 50% of the statistical variances in a period of 24 hours. The Eliassen-Palm (EP) flux and its divergence show that the vortex Rossby waves are concentrated in the inner-core region where the radial gradient of the basic state potential vorticity is large. The vortex Rossby waves lead to, through wave-mean-flow interaction indicated by the divergence of the EP flux, the acceleration of the mean tangential wind in the lower and middle troposphere inside and outside the eyewall and the deceleration aloft in the eyewall region.

The vortex Rossby waves show also characteristics typical of sheared flow with critical level. One of the difficulties encountered when interpreting these results was that the model lacks of spatial resolution in the critical level. A long term collaborative effort between scientists from the Earth Simulator Center (ESC), McGill University and RPN is now focusing on simulating the full life cycle of hurricane Earl (September 1998) with the Canadian MC2 Community Model. We propose to revisit this critical layer problem in the context of these numerical experiments on the Earth Simulator.


References:

Brunet, G., 1994: Empirical normal mode analysis of atmospheric data. J. Atmos. Sci., 51, 932-952.

Brunet, G. and R. Vautard, 1996: Empirical normal modes versus empirical orthogonal functions for statistical prediction. J. Atmos. Sci., 53, 3468-3489.

Charron, M. and G. Brunet, 1999: Gravity Wave Diagnosis Using Empirical Normal Modes. J. Atmos. Sci., 56, 2706-2727.

Zadra, A., G. Brunet and J. Derome 2002 An empirical normal mode diagnostics algorithm applied to NCEP reanalyses J. Atmos. Sci., 59, 2811-2829.

Chen, Y., G. Brunet and P. Yau 2003 Spiral bands in a simulated hurricane PART II: Wave activity diagnostics J. Atmos. Sci. , 60, 1239-1256.