Wave Packets and Life Cycles of Troughs in the Upper Troposphere: Examples from the Southern Hemisphere Summer Season of 1984/85

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Abstract
In this paper, reanalysis data for the Southern Hemisphere summer season of 1984/85, produced by the European Centre for Medium-Range Weather Forecasts, have been analyzed to examine wave packets and life cycles of baroclinic waves. A semiobjective algorithm has been devised to track wave packets. It is found that the perturbations in the upper troposphere are dominated by wave packets. Some of these wave packets can be tracked for several weeks, during which they can make up to two trips around the latitude circle. The energy life cycles of waves that make up several of these wave packets were analyzed, and apart from the incipient waves that started the wave packets, most of the subsequent wave development was found to be dominated by convergence and divergence of energy fluxes rather than due to baroclinic or barotropic conversions, demonstrating that these wave packets are indeed coherent entities that propagate due to downstream development (or dispersion). The energy life cycle of the most significant waves in terms of total energy or height fall during the entire season have also been examined, and the results show that during that season, approximately two-thirds of the most significant wave cases had developed due to downstream development, with the remaining ones developed due to baroclinic or barotropic growth. In addition, during the decay phase of these waves, again about two-thirds of the cases are dominated by downstream development rather than barotropic decay or dissipation. These results show that a majority of the most significant waves in that season were associated with coherent wave packets. In nearly all cases of upper-level trough development associated with these downstream developing wave packets that were studied, a surface cyclone can be identified developing just to the east of the upper-level trough, suggesting that surface cyclogenesis is frequently caused by the approach of an upper-level wave packet.

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