Spontaneous balance adjustment and gravity wave generation from atmospheric jets and fronts

For several decades, jets and fronts have been known from observations to be signicant sources of internal gravity waves in the atmosphere. Motivations to investigate these waves have included their impact on tropospheric convection, their contribution to local mixing and turbulence in the upper-troposphere, their vertical propagation into the middle atmosphere and the forcing of its global circulation. While many dierent studies have consistently highlighted jet exit regions as a favored locus for intense gravity waves, the mechanisms responsible for their emission had long remained elusive: one reason is the complexity of the environment in which the waves appear, another is that the waves constitute small deviations from the balanced dynamics of the flow generating them, i.e. they arise beyond our fundamental, balanced understanding of jets and fronts. Over the past two decades, the pressing need for improving parameterizations of non-orographic gravity waves in climate models that include a stratosphere has stimulated renewed investigations.

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.