Riwal Plougonven

Internal gravity waves from atmospheric jets and fronts

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.

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

Abstract 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...

On the Intermittency of Gravity Wave Momentum Flux in the Stratosphere

AbstractIn this article, long-duration balloon and spaceborne observations, and mesoscale numerical simulations are used to study the intermittency of gravity waves in the lower stratosphere above Antarctica and the Southern Ocean; namely, the characteristics of the gravity wave momentum-flux probability density functions (pdfs) obtained with these three datasets are described. The pdfs consistently exhibit long tails associated with the occurrence of rare and large-amplitude events. The pdf tails are even longer above mountains than above oceanic areas, which is in agreement with previous studies of gravity wave intermittency in this region. It is moreover found that these rare, large-amplitude events represent the main contribution to the total momentum flux during the winter regime of the stratospheric circulation. In contrast, the wave intermittency significantly decreases when stratospheric easterlies develop in late spring and summer. It is also shown that, except above mountainous areas in winter, ...

Observations and Numerical Simulations of Inertia–Gravity Waves and Shearing Instabilities in the Vicinity of a Jet Stream

The characteristics and dynamics of inertia‐gravity waves generated in the vicinity of an intense jet stream/ upper-level frontal system on 18 February 2001 are investigated using observations from the NOAA GulfstreamIV research aircraft and numerical simulations. Aircraft dropsonde observations and numerical simulations elucidate the detailed mesoscale structure of this system, including its associated inertia‐gravity waves and clearair turbulence. Results from a multiply nested numerical model show inertia‐gravity wave development above the developing jet/front system. These inertia‐gravity waves propagate through the highly sheared flow above the jet stream, perturb the background wind shear and stability, and create bands of reduced and increased Richardson numbers. These bands of reduced Richardson numbers are regions of likely Kelvin‐Helmholtz instability and a possible source of the clear-air turbulence that was observed.

Inertia–Gravity Waves Generated within a Dipole Vortex

Abstract Vortex dipoles provide a simple representation of localized atmospheric jets. Numerical simulations of a synoptic-scale dipole in surface potential temperature are considered in a rotating, stratified fluid with approximately uniform potential vorticity. Following an initial period of adjustment, the dipole propagates along a slightly curved trajectory at a nearly steady rate and with a nearly fixed structure for more than 50 days. Downstream from the jet maximum, the flow also contains smaller-scale, upward-propagating inertia–gravity waves that are embedded within and stationary relative to the dipole. The waves form elongated bows along the leading edge of the dipole. Consistent with propagation in horizontal deformation and vertical shear, the waves’ horizontal scale shrinks and the vertical slope varies as they approach the leading stagnation point in the dipole’s flow. Because the waves persist for tens of days despite explicit dissipation in the numerical model that would otherwise damp th...

A Baroclinic Instability that Couples Balanced Motions and Gravity Waves

Normal modes of a linear vertical shear (Eady shear) are studied within the linearized primitive equations for a rotating stratified fluid above a rigid lower boundary. The authors’ interest is in modes having an inertial critical layer present at some height within the flow. Below this layer, the solutions can be closely approximated by balanced edge waves obtained through an asymptotic expansion in Rossby number. Above, the solutions behave as gravity waves. Hence these modes are an example of a spatial coupling of balanced motions to gravity waves. The amplitude of the gravity waves relative to the balanced part of the solutions is obtained analytically and numerically as a function of parameters. It is shown that the waves are exponentially small in Rossby number. Moreover, their amplitude depends in a nontrivial way on the meridional wavenumber. For modes having a radiating upper boundary condition, the meridional wavenumber for which the gravity wave amplitude is maximal occurs when the tilts of the balanced edge wave and gravity waves agree.

Uncertainties in using the hodograph method to retrieve gravity wave characteristics from individual soundings

reality, the radiosonde takes time to ascend and drifts horizontally as it ascends. The uncertainty associated with the finite ascent rate has been addressed in several previous studies [e.g., Guest et al., 2000] and will not be addressed further. [3] In this study, we will try to access potential uncertainties in using the hodograph method to retrieve gravity wave characteristics even if the wave packet can be reasonably approximated by linear analysis. These uncertainties may be due to: (1) the separation into a background flow and a perturbation, since there is unavoidably some arbitrariness in the choice of the filter or polynomial fit used for that purpose; (2) the geographical variation of the wave packet; and (3) the presence of multiple waves. We estimate the importance of these uncertainties in a case study, using high-resolution numerical simulations in which a coherent quasi-monochromatic low-frequency wave is present. The reference characteristics of the wave are obtained from the overall 4-dimensional (4D) output of the simulations. Characteristics retrieved from individual profiles of the simulated wind by the hodograph method are then compared with those from 4D output.

Gravity Waves Generated by Sheared Potential Vorticity Anomalies

The gravity waves (GWs) generated by potential vorticity (PV) anomalies in a rotating stratified shear flow are examined under the assumptions of constant vertical shear, two-dimensionality, and unbounded domain. Near a PV anomaly, the associated perturbation is well modeled by quasigeostrophic theory. This is not the case at large vertical distances, however, and in particular beyond the two inertial layers that appear above and below the anomaly; there, the perturbation consists of vertically propagating gravity waves. This structure is described analytically, using an expansion in the continuous spectrum of the singular modes that results from the presence of critical levels. Several explicit results are obtained. These include the form of the Eliassen-Palm (EP) flux as a function of the Richardson number N2/?2, where N is the Brunt-Vaisala frequency and L the vertical shear. Its nondimensional value is shown to be approximately exp(-N/L)/8 in the far-fieldGWregion, approximately twice that between the two inertial layers. These results,which imply substantialwave-flowinteractions in the inertial layers, are valid for Richardson numbers larger than 1 and for a large range of PV distributions. In dimensional form they provide simple relationships between the EP fluxes and the large-scale flow characteristics. As an illustration, the authors consider a PV disturbance with an amplitude of 1 PVU and a depth of 1 km, and estimate that the associated EP flux ranges between 0.1 and 100 mPa for a Richardson number between 1 and 10. These values of the flux are comparable with those observed in the lower stratosphere, which suggests that the mechanism identified in this paper provides a substantial gravity wave source, one that could be parameterized in GCMs.

Assessment of the accuracy of (re)analyses in the equatorial lower stratosphere

The accuracy of horizontal winds and temperature in the equatorial lower stratosphere is evaluated in different (re)analyses (European Centre for Medium-Range Weather Forecasts (ECMWF) operational analysis, ERA Interim, and Modern-Era Retrospective Analysis for Research and Applications) using an independent data set collected at low latitudes during long-duration balloon flights in early 2010. The three analyzed wind products are found significantly less accurate than in the extratropics, with periods of ≳10m/sdisagreement with the observations lasting several days. To highlight the dynamical context in which the major disagreement events occur, case studies are carried out. The events are shown to be related to an improper representation of large-scale equatorial Kelvin and Yanai wave packets with vertical wavelengths smaller than 5 km. Such events can induce large errors on trajectories computed with analyzed winds relatively to the actual (balloon) trajectory: 4000 km separation after 5 days of calculation. Reasons for analyses inaccuracy are discussed. The vertical resolution of the underlying model likely plays a role, but the main factor responsible for deficiencies appears to be the lack of wind observations. Indeed, errors in analyzed winds during the campaign have a strong longitudinal structure, with root-mean-square errors twice as large over the Indian Ocean and western Pacific, poorly covered by radiosounding stations, as over the Maritime Continent or South America. For the ECMWF analysis, this structure mirrors that of the analysis increments, which have largest amplitudes over observed regions. We argue that the reported events are more likely to happen during maximum shear phases of the quasi-biennial oscillation.

On the Forcing of Inertia–Gravity Waves by Synoptic-Scale Flows

Abstract 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.