On tropical cyclogenesis: climatology and a dynamical investigation

In general, observation has indicated that only a small fraction of the easterly waves occur in a single hurricane season contribute to tropical cyclogenesis. However, this small fraction includes a large portion of named storms. In addition, a recent study by Dunkerton et al. (2009) has shown that named storms in the Atlantic and eastern Pacific basins are almost all associated with a cyclonic Kelvin cat's eye of a tropical easterly wave critical layer, located equatorward of the easterly jet axis. Therefore, to better understand the dynamics involved in tropical cyclogenesis, it is desirable to investigate the flow characteristics and the physical mechanism for an easterly wave to form a cat's eye.

Our methodology involves performing a climatological study of developing easterly waves covering the 1998-2001 hurricane seasons using the ECMWF ERA-Interim 6-hourly reanalysis data. Spatial and temporal filtering was applied to decompose the desired fields, and time-lagged composites were obtained in a translating reference frame following the disturbances. Statistical analysis is also used to determine the levels of confidence in the obtained pattern from the composite fields to assess the reliability of the results. Composite perturbation PV showed a wave-like pattern in a 90-degree zonal domain with wave number 4-5 (i.e., wavelengths between ~2000 to 2500 km) which is in agreement with typical wavelengths for the easterly waves. Analysis showed that at cat's eye formation region, the basic state potential vorticity (PV) profiles are nearly constant with latitude. The associated meridional PV gradients are weak for time periods of several days before the formation of the cat's eye. Also, the composite basic state zonal wind profiles indicate a cyclonic critical layer near the same region. These structures show a linearly stable flow and highlight the possible role of nonlinear critical layer in the cat's eye formation as suggested by Brunet and Haynes (1995).

In addition, the total PV contours show a closed pattern only for one of the easterly wave troughs within the domain, the one associated with the tropical storms. It takes 4-5 days for the open total PV contours to transform to a coherent closed structure and form the cat's eye. To determine the dynamics underlying the formation of the cat's eye, a nonlinear shallow water model is used to examine the evolution of a PV perturbation in the context of the climatological basic state. It is found that the nonlinear evolution of instabilities associated with critical layers plays a significant role in generating coherent cyclonic vortices with spatio-temporal structures consistent with the climatological analysis.

References Dunkerton, T. J., M. T. Montgomery, and Z. Wang, 2009: Tropical cyclogenesis in a tropical wave critical layer: Easterly waves. Atmos. Chem. Phys., 9, 5587–5646. Brunet, G., P. H. Haynes, 1995: The Nonlinear Evolution of Disturbances to a Parabolic Jet. J. Atmos. Sci., 52, 464–477.