The Palette of Fronts and Cyclones within a Baroclinic Wave Development

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Abstract
A study is undertaken of the concurrent development of fronts and cyclones within a growing baroclinic wave. The dynamical framework for the study is that of the semigeostrophic, adiabatic flow of a uniform potential vorticity atmosphere that is sandwiched between two horizontal plates on an f plane. A three-dimensional spectral model is used to examine the growth to finite amplitude of the normal modes of this system for basic states that comprise of a symmetric jetlike baroclinic flow with (and without) a superposition of a uniform barotropic shear of either cyclonic or anticyclonic vorticity. It is demonstrated that the presence of such lateral shear in the ambient environment exerts a profound influence upon the ensuing palette of fronts and cyclones. On the synoptic scale this flow component influences the scale and strength of the individual surface highs and lows, and also modifies the extent of their lateral drift away from, and their relative movement along, the baroclinic zone. Aloft, the development of the trough–ridge system is altered appreciably with positive (negative) shear favoring the formation of cutoff cold (warm) air pools. On the subsynoptic scale the lateral shear induces a selective frontogenetic effect; e.g., positive shear sustains surface warm fronts and suppresses surface cold frontogenesis. The range of simulated frontal phenomena includes frontal fracture, warm air seclusion and a bent-back warm front. In particular for a weak ambient positive barotropic shear the evolving frontal palette acquires the distinctive and characteristic λ-shaped configuration. Theoretical considerations and diagnostic analyses provide insight upon the sensitivity of the flow response to the lateral shear. At root it is shown to be related to two intrinsic symmetry features of the semigeostrophic system. In the quasi-linear phase of the development the shear induces a displacement of the critical-surface of the wave relative to the main baroclinic zone, a concomitant asymmetric modification in the structure of the normal modes, and a substantial positive feedback upon the shear of the mean flow. The latter changes effectively determine the lateral and relative movement of the pressure systems as well as their spatial scale. The diagnostic analyses also serve to pinpoint, for the nonlinear phase of the development, the salient single level and upper level–lower level interactions that drain and reverse the zonal-mean surface baroclinicity in midstream, force the perturbations to overshoot the baroclinic zone, and engender the rich subsynoptic scale features. Consideration is given to the relationship with observed weather systems and to the physical and mathematical limitations of the simulations.

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