Jets, vortex et ondes d'inertie-gravité: séparation dynamique et émission

Nonlinear geostrophic adjustment of rectilinear jets is examined analytically. It is shown that the Lagrangian formalism is best suited to describe unambiguously the splitting between a balanced part and a fast, gravity wave part of the flow. Modifications to the standard scenario of geostrophic adjustment are highlighted. Data analysis of FASTEX radiosoundings is carried out to show that atmospheric jets and fronts are a significant source of inertia-gravity waves. A mechanism for the radiation of gravity waves from non-stationary vortical motions is illustrated theoretically by considering the waves excited by an ellipsoidal vortex in a stratified fluid.L'ajustemnt géostrophique nonlinéaire de fronts et de jets est analysé. Il est démontré que le formalisme lagrangien est le mieux adapté pour décrire sans ambiguité ce problème. Des modifications du scenario classique d'ajustement de Rossby sont mises en évidence. L'analyse de radiosondages de la campagne FASTEX montre que les jets et les fronts atmosphériques aux moyennes latitudes sont des sources importantes d'ondes de gravité. Un mécanisme d'excitation d'ondes de gravité à partir de mouvements vorticaux est analysé théoriquement dans le cadre d'un fluide stratifié, pour les ondes excitées par un tourbillon ellispoïdal

Gravity Waves Generated by Sheared Three-Dimensional Potential Vorticity Anomalies

International audienceThe gravity waves (GWs) produced by three-dimensional potential vorticity (PV) anomalies are examined under the assumption of constant vertical shear, constant stratication, and unbounded domain. As in the two-dimensional case analyzed in an earlier paper, the disturbance near the PV anomaly is well modeled by quasi-geostrophic theory. At larger distances the nature of the disturbance changes across the two inertial layers that are located above and below the anomaly, and it takes the form of a vertically propagating GW beyond these. For a horizontally monochromatic PV anomaly of innitesimal depth, the disturbance is described ana-lytically using both an exact solution and a WKB approximation; the latter includes an exponentially small term that captures the change of the solution near the PV anomaly induced by the radiation boundary condition in the far eld. The analytical results reveal a strong sensitivity of the emission to the Richardson number and to the orientation of the horizontal wavenumber: the absorptive properties of the inertial layers are such that the emission is maximized in the Northern Hemisphere for wavenumbers at negative angles to the shear. For localized PV anomalies, numerical computations give the temporal evolution of the GW eld. Ana-lytical and numerical results are also used to establish an explicit formfor the Eliassen-Palmux that could be used to parameterize GW sources in GCMs. The properties of the Eliassen-Palm ux vector imply that in a westerly shear, the GWs exert a drag in a southwest direction in the upper inertial layer, and in a northwest direction at the altitudes where the GWs dissipate aloft. © 2012 American Meteorological Society

Gravity waves generated by sheared potential vorticity anomalies

International audienceThe 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-Väisälä 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. © 2010 American Meteorological Society

Lagrangian temperature and vertical velocity fluctuations due to gravity waves in the lower stratosphere

International audienceWave-induced Lagrangian fluctuations of temperature and vertical velocity in the lower stratosphere are quantified using measurements from superpressure balloons (SPBs). Observations recorded every minute along SPB flights allow the whole gravity wave spectrum to be described and provide unprecedented information on both the intrinsic frequency spectrum and the probability distribution function of wave fluctuations. The data set has been collected during two campaigns coordinated by the French Space Agency in 2010, involving 19 balloons over Antarctica and 3 in the deep tropics. In both regions, the vertical velocity distributions depart significantly from a Gaussian behavior. Knowledge on such wave fluctuations is essential for modeling microphysical processes along Lagrangian trajectories. We propose a new simple parameterization that reproduces both the non-Gaussian distribution of vertical velocities (or heating/cooling rates) and their observed intrinsic frequency spectrum

A barycenter-based approach for the multi-model ensembling of subseasonal forecasts

Ensemble forecasts and their combination are explored from the perspective of a probability space. Manipulating ensemble forecasts as discrete probability distributions, multi-model ensembles (MMEs) are reformulated as barycenters of these distributions. Barycenters are defined with respect to a given distance. The barycenter with respect to the L2-distance is shown to be equivalent to the pooling method. Then, the barycenter-based approach is extended to a different distance with interesting properties in the distribution space: the Wasserstein distance. Another interesting feature of the barycenter approach is the possibility to give different weights to the ensembles and so to naturally build weighted MME. As a proof of concept, the L2- and the Wasserstein-barycenters are applied to combine two models from the S2S database, namely the European Centre Medium-Range Weather Forecasts (ECMWF) and the National Centers for Environmental Prediction (NCEP) models. The performance of the two (weighted-) MMEs are evaluated for the prediction of weekly 2m-temperature over Europe for seven winters. The weights given to the models in the barycenters are optimized with respect to two metrics, the CRPS and the proportion of skilful forecasts. These weights have an important impact on the skill of the two barycenter-based MMEs. Although the ECMWF model has an overall better performance than NCEP, the barycenter-ensembles are generally able to outperform both. However, the best MME method, but also the weights, are dependent on the metric. These results constitute a promising first implementation of this methodology before moving to combination of more models.Comment: 24 pages, 9 figure

Gravity-wave characteristics derived from quasi-Lagrangian balloon flights in the stratosphere

Most of our observational knowledge of gravity waves in the atmosphere comes from vertical profiles performed by ground-based instruments (radar, lidar), radiosoundings or space-borne instruments. Superpressure balloon flights on the other hand provide the opportunity to sample the atmosphere along quasi-Lagrangian trajectories, like drifters in the oceans

Wind power predictions from nowcasts to 4-hour forecasts: a learning approach with variable selection

We study the prediction of short term wind speed and wind power (every 10 minutes up to 4 hours ahead). Accurate forecasts for those quantities are crucial to mitigate the negative effects of wind farms' intermittent production on energy systems and markets. For those time scales, outputs of numerical weather prediction models are usually overlooked even though they should provide valuable information on higher scales dynamics. In this work, we combine those outputs with local observations using machine learning. So as to make the results usable for practitioners, we focus on simple and well known methods which can handle a high volume of data. We study first variable selection through two simple techniques, a linear one and a nonlinear one. Then we exploit those results to forecast wind speed and wind power still with an emphasis on linear models versus nonlinear ones. For the wind power prediction, we also compare the indirect approach (wind speed predictions passed through a power curve) and the indirect one (directly predict wind power)

Observed and Modeled Mountain Waves from the Surface to the Mesosphere Near the Drake Passage

Four state-of-the-science numerical weather prediction (NWP) models were used to perform mountain wave- (MW) resolving hind-casts over the Drake Passage of a 10-day period in 2010 with numerous observed MW cases. The Integrated Forecast System (IFS) and the Icosahedral Nonhydrostatic (ICON) model were run at Δx ≈ 9 and 13 km globally. TheWeather Research and Forecasting (WRF) model and the Met Office Unified Model (UM) were both configured with a Δx = 3 km regional domain. All domains had tops near 1 Pa (z ≈ 80 km). These deep domains allowed quantitative validation against Atmospheric InfraRed Sounder (AIRS) observations, accounting for observation time, viewing geometry, and radiative transfer. All models reproduced observed middle-atmosphere MWs with remarkable skill. Increased horizontal resolution improved validations. Still, all models underrepresented observed MW amplitudes, even after accounting for model effective resolution and instrument noise, suggesting even at Δx ≈ 3 km resolution, small-scale MWs are under-resolved and/or over-diffused. MWdrag parameterizations are still necessary in NWP models at current operational resolutions of Δx ≈ 10 km. Upper GW sponge layers in the operationally configured models significantly, artificially reduced MW amplitudes in the upper stratosphere and mesosphere. In the IFS, parameterized GW drags partly compensated this deficiency, but still, total drags were ≈ 6 time smaller than that resolved at Δx ≈ 3 km. Meridionally propagating MWs significantly enhance zonal drag over the Drake Passage. Interestingly, drag associated with meridional fluxes of zonal momentum (i.e. u'v') were important; not accounting for these terms results in a drag in the wrong direction at and below the polar night jet

Constraining parameterizations of gravity waves in climate models.

Non UBCUnreviewedAuthor affiliation: Ecole PolytechniqueFacult

Generation and backreaction of spontaneously emitted inertia-gravity waves

International audienceSpontaneous generation of inertia-gravity waves from balanced flows is investigated in idealized simulations of dipoles. Long integrations are performed for dipoles with different Rossby numbers (Ro) to identify the backreaction of the waves. Emission of waves is detected only for large enough Ro (>0.15), and it then leads to a slow decay of the dipole's kinetic energy. A major finding is that this decay is well captured by the simulations, although positions of the waves appear still sensitive to the resolution, and their maximum vertical velocity increases linearly with resolution. The interpretation is that the emission process is well resolved and fairly insensitive to resolution, while the propagation and dissipation at small scales remains sensitive to resolution. The implication is that the simulations yield an estimate of the leakage of energy from balanced motions to gravity waves, providing a useful estimate of a poorly constrained flux in the ocean's energy budget