### Hurricane dynamics: on the role of Vortex Rossby Waves (VRWs)

Brunet, G *(Weather Science, Met Office)*

Monday 28 October 2013, 11:00-12:00

Seminar Room 1, Newton Institute

#### Abstract

Despite the fact that asymmetries in hurricanes, such as spiral rainbands, polygonal eyewalls and mesovortices, have long been observed in radar imagery, many aspects of their dynamics still remain unsolved, particularly in the formation of the secondary eyewall. To fill this gap, a simple 2D barotropic model (Martinez et al., 2010) and the high-resolution PSU-NCAR non-hydrostatic mesoscale model (MM5) are used to study hurricane asymmetries (Chen et al., 2003; Martinez et al., 2011).The Empirical Normal Mode (ENM) diagnostics (Brunet, 1994), together with the Eliassen-Palm (EP) flux calculations are used to isolate wave modes from the model datasets to investigate their impact on the changes in the structure and intensity of the simulated hurricanes (Chen et al., 2003).
The ENMs are obtained in a similar manner as Empirical Orthogonal Functions (EOFs) but with the use of a quadratic form instead of Euclidean norm. The quadratic forms are global invariants, the pseudo-momentum and pseudo-energy wave activities, of the linearized equations about a basic state (Brunet and Vautard, 1996). ENM theory bridges two important diagnostic tools of geophysical fluid dynamics: principal component analysis and normal mode theory.
The role of internal dynamics on concentric eyewall genesis is further evaluated using the full physics MM5 simulation. The leading modes of the ENM diagnostics exhibit mainly characteristics of VRWs and their contribution to the EP flux divergence induced two regions of maximum tangential wind acceleration; one inside the primary eyewall which accounts for eyewall contraction and the other outside the primary eyewall which explains the development of the secondary eyewall (Martinez et al., 2011).
We will point out the expected implication of these results in the context of numerical weather prediction at different space-time resolutions for intensifying and mature hurricanes of different strengths.

#### Presentation

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