This study reviews the spontaneous balance adjustment (SBA) hypothesis that is developed over a series of studies over the past decade (e.g., Zhang 2004 JAS; Wang and Zhang 2007 MWR; Plougonven and Zhang 2007 JAS; Lin and Zhang 2008 JAS; Wang et al. 2009 JAS, 2010 QJ; Wang and Zhang 2010 JAS) through examining gravity waves initiation from idealized simulations of baroclinic life cycles and vortex-jet dipoles with both high-resolution complex non-hydrostatic mesoscale models and/or linear forcing or ray tracing models. More specifically, it is hypothesized that within the developing baroclinic jet-front system, the large-scale background flow can continuously produce flow imbalance while the gravity waves are continuously generated from flow imbalance through spontaneous balance adjustment [Zhang 2004 JAS]. A framework to describe this emission mechanism was proposed by Plougonven and Zhang [2007 JAS] through scale analysis and analytical derivation of a wave equation linearized on the balanced background flow that is forced by synoptic-scale flow imbalance. This was implemented and expanded to explain gravity waves emitted in dipoles [Wang and Zhang, 2010], and has recently been used to explain at least some of the jet-exit region gravity waves found in baroclinic life cycles. Ongoing study is also extend this framework to examine the gravity waves in moist baroclinic life cycles.
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