Observed frontal cyclones form and develop in a complex flow that includes other troughs and ridges of varying size and intensity. Because individual lows appear to operate somewhat autonomously, many synopticians view the interaction between upper and lower troughs to be somewhat isolated from the larger environment. For example, "type B cyclogenesis" is usually described as a single upper level trough approaching a single lower level trough.
In contrast, it is mathematically simpler to use non-isolated states in theoretical studies. For example, a single wavenumber in the along-flow direction is often used. This is particularly common in studies of flows that do not have downstream variation. In studies with zonally varying flows, the eigensolutions tend to be more isolated, but still consist of a chain of highs and lows. In initial value studies such as Grotjahn et al (1995), the lack of localized initial structure leads to rather ambiguous initial conditions where the "upstream" trough appears closely "downstream" in a periodic domain. This study aims to remove that ambiguity.
This presentation explores results using initial conditions that are more localized in space. Some care is needed in defining the isolated initial features. Simply prescribing an isolated PV maximum using the Eady model (say) will not remain sufficiently isolated due to the (eigen) structures inherent in that model and their dispersion. Structures that have greater vertical localization are found when compressibility is included. A model with compressibility, variable Coriolis, and realistic vertical static stability is used for this reason. To maintain coherence, the initial condition is constructed primarily from those eigenmodes having very similar phase speeds plus similar localized vertical structure. The result is an initial condition with structures that are localized in the vertical and horizontal.
Having created stable, isolated initial features, one can more directly study how and when growth commences as an isolated upper level feature approaches an isolated lower level feature. In addition, these parameters can be varied: tracks followed by, sizes of, and shapes of the features. Varying these parameters mimics the mixture seen in nature. To understand the onset and the mechanisms of the growth, time series of various diagnostics will be shown.
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12th Conference on Atmospheric and Oceanic Fluid Dynamics