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.