For several decades, jets and fronts have been known from observations to be significant 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 different 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 reason is that the waves constitute small deviations from the balanced dynamics of the flow generating them; i.e., they arise beyond our fundamental understanding of jets and fronts based on approximations that filter out gravity waves. Over the past two decades, the pressing need for improving parameterizations of nonorographic gravity waves in climate models that include a stratosphere has stimulated renewed investigations. The purpose of this review is to present current knowledge and understanding on gravity waves near jets and fronts from observations, theory, and modeling, and to discuss challenges for progress in coming years.

/pdf/internal-gravity-waves-from-atmospheric-jets-and-fronts-22oa2c4omk.pdf

Internal gravity waves from atmospheric jets and fronts

The spontaneous generation of inertia–gravity waves in idealized life cycles of baroclinic instability is investigated using the Weather Research and Forecasting Model. Two substantially different life cycles of baroclinic instability are obtained by varying the initial zonal jet. The wave generation depends strongly on the details of the baroclinic wave’s development. In the life cycle dominated by cyclonic behavior, the most conspicuous gravity waves are excited by the upper-level jet and are broadly consistent with previous simulations of O’Sullivan and Dunkerton. In the life cycle that is dominated by anticyclonic behavior, the most conspicuous gravity waves even in the stratosphere are excited by the surface fronts, although the fronts are no stronger than in the cyclonic life cycle. The anticyclonic life cycle also reveals waves in the lower stratosphere above the upper-level trough of the baroclinic wave; these waves have not been previously identified in idealized simulations. The sensiti...

Inertia–Gravity Waves Spontaneously Generated by Jets and Fronts. Part I: Different Baroclinic Life Cycles

[1] Turbulence is a well-known hazard to aviation that is responsible for numerous injuries each year, with occasional fatalities, and is the underlying cause of many people's fear of air travel. Not only are turbulence encounters a safety issue, they also result in millions of dollars of operational costs to airlines, leading to increased costs passed on to the consumer. For these reasons, pilots, dispatchers, and air traffic controllers attempt to avoid turbulence wherever possible. Accurate forecasting of aviation-scale turbulence has been hampered in part by a lack of understanding of the underlying dynamical processes. However, more precise observations of turbulence encounters together with recent research into turbulence generation processes is helping to elucidate the detailed dynamical processes involved and is laying the foundation for improved turbulence forecasting and avoidance. In this paper we briefly review some of the more important recent observational, theoretical, and modeling results related to turbulence at cruise altitudes for commercial aircraft (i.e., the upper troposphere and lower stratosphere), and their implications for aviation turbulence forecasting.

/pdf/sources-and-dynamics-of-turbulence-in-the-upper-troposphere-4ot1i6tief.pdf

Sources and dynamics of turbulence in the upper troposphere and lower stratosphere: A review

Radiosonde observations made from Davis station, Antarctica, (68.6°S, 78.0°E) between 2001 and 2012 are used to compile a climatology of lower stratosphere inertial gravity wave characteristics. Wavelet analysis extracts single wave packets from the wind and temperature perturbations. Wavelet parameters, combined with linear gravity wave theory, allow for the derivation of a wide range of wave characteristics. Observational filtering associated with this analysis preferentially selects inertial gravity waves with vertical wavelengths less than 2–3 km. The vertical propagation statistics show strong temporal and height variations. The waves propagate close to the horizontal and are strongly advected by the background wind in the wintertime. Notably, around half of the waves observed in the stratosphere above Davis between early May and mid-October propagate downward. This feature is distributed over the observed stratospheric height range. Based on the similarity between the upward and downward propagating waves and on the vertical structure of the nonlinear balance residual in the polar winter stratosphere, it is concluded that a source due to imbalanced flow that is distributed across the winter lower stratosphere best explains the observations. Calculations of kinetic and potential energies and momentum fluxes highlight the potential for variations in results due to different analysis techniques.

/pdf/radiosonde-observations-of-gravity-waves-in-the-lower-28773qffzc.pdf

Radiosonde observations of gravity waves in the lower stratosphere over Davis, Antarctica

A new method of clear-air turbulence (CAT) forecasting based on the Lighthill–Ford theory of spontaneous imbalance and emission of inertia–gravity waves has been derived and applied on episodic and seasonal time scales. A scale analysis of this shallow-water theory for midlatitude synoptic-scale flows identifies advection of relative vorticity as the leading-order source term. Examination of leading- and second-order terms elucidates previous, more empirically inspired CAT forecast diagnostics. Application of the Lighthill–Ford theory to the Upper Mississippi and Ohio Valleys CAT outbreak of 9 March 2006 results in good agreement with pilot reports of turbulence. Application of Lighthill–Ford theory to CAT forecasting for the 3 November 2005–26 March 2006 period using 1-h forecasts of the Rapid Update Cycle (RUC) 2 1500 UTC model run leads to superior forecasts compared to the current operational version of the Graphical Turbulence Guidance (GTG1) algorithm, the most skillful operational CAT forecasting method in existence. The results suggest that major improvements in CAT forecasting could result if the methods presented herein become operational.

Application of the Lighthill-Ford Theory of Spontaneous Imbalance to Clear-Air Turbulence Forecasting

Studies on the spontaneous emission of gravity waves from jets, both observational and numerical, have emphasized that excitation of gravity waves occurred preferentially near regions of imbalance. Yet a quantitative relation between the several large-scale diagnostics of imbalance and the excited waves is still lacking. The purpose of the present note is to investigate one possible way to relate quantitatively the gravity waves to diagnostics of the large-scale flow that is exciting them. Scaling arguments are used to determine how the large-scale flow may provide a forcing on the right-hand side of a wave equation describing the linear dynamics of the excited waves. The residual of the nonlinear balance equation plays an important role in this forcing.

/pdf/notes-and-correspondence-on-the-forcing-of-inertia-gravity-ai3r7gscz3.pdf

NOTES AND CORRESPONDENCE On the Forcing of Inertia-Gravity Waves by Synoptic-Scale Flows

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NOTES AND CORRESPONDENCE On the Forcing of Inertia-Gravity Waves by Synoptic-Scale Flows