During the past few years, baroclinic waves have been shown to exhibit strong tendencies to exist in the form of coherent, downstream developing wave packets. While modelling studies have suggested that baroclinic waves organize themselves readily into wave packets, we still do not have a clear understanding about the physical processes that lead to such organization. In this study, ECMWF reanalysis data were analyzed to examine the structure of observed wave packets, as well as their interactions with the large scale basic state flow, in order to provide more insight on the dynamics governing the evolution of these wave packets.
Energy as well as wave activity composites of wave packets were obtained based on three DJF seasons of ECMWF reanalysis data. The composites of the energetics support earlier individual case studies that mature eddies in the middle and upstream part of the wave packet convert energy from the basic state flow baroclinically and feeds the developing eddies towards the downstream end via ageostrophic energy fluxes, and barotropic conversion is strongest towards the downstream end where eddies are growing. The wave activity composites (based on the wave activity fluxes defined in Plumb 1986) shows a wave source at the lower boundary towards the upstream part of the wave packet, with wave activity fluxes suggesting upward, then rearward propagation in the lower troposphere, but turning towards the downstream side above 500 hPa before turning equatorwards at the front part of the wave packet. Such a flux configuration is consistent with the picture obtained from the energetics consideration. However, since eddies are growing towards the front end of wave packets and decaying towards the rear, when viewed in terms of individual eddy life-cycles, the 'saturation-propagation-saturation' paradigm suggested by Thorncroft et. al. (1994), as well as the earlier baroclinic growth/barotropic decay paradigm suggested by Simmons and Hoskins (1978) are not valid for waves evolving within a downstream developing wave packet, and a revised paradigm is proposed.
By following the wave packets, we have also been able to isolate the correction to the mean flow due to the wave packets, and the stability properties of the three dimensional flow within which the wave packet is embedded has been examined. Currently we are conducting experiments to investigate the temporal evolution (both linear and nonlinear) of a localized wave packet within this mean flow to study their dynamics.
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12th Conference on Atmospheric and Oceanic Fluid Dynamics