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International audience; Two case studies of nonorographic gravity waves are carried out for wave events that occurred over the Southern Ocean in November 2005. Mesoscale simulations were carried out with the Weather and Research Forecast model. The simulated waves were compared to observations from superpressure balloons of the Vorcore campaign and from the High Resolution Dynamic Limb Sounder satellite. Satisfactory agreement is found, giving confidence in the estimations of wave parameters and amplitudes. For the amplitudes, both the model and observations provide a lower bound, for different reasons. Waves are found in the lower stratosphere with horizontal wavelengths of the order of 150-200 km in the horizontal, 5-8 km in the vertical, corresponding to intrinsic frequencies between 5 and 10 f, where f is the Coriolis parameter. Although the tropospheric flow is very different between the two cases, there are features which are common and appear significant for the gravity waves: these include intense localized updrafts associated with convection in the troposphere and a displaced polar vortex inducing strong winds in the stratosphere above the frontal region. Relative to theoretical expectations, the simulations emphasize the role of moisture. Intrinsic frequencies are significantly higher than those expected for waves produced by dry spontaneous generation from jets. To quantify the contribution of moisture, dry simulations were carried out, yielding momentum fluxes over oceanic regions that were 2.5 times weaker. Identification of the generation mechanisms in these complex flows calls for further study, and these should include moisture and a realistic stratospheric jet.

[1] Convective gravity waves in the Tropics are studied by analyzing in situ measurements from long-duration stratospheric balloons launched during the PreConcordiasi campaign (2010) and mesoscale simulations. An improved temporal resolution of the observations as well as the balloon quasi-Lagrangian behavior allow an unprecedented investigation of the whole gravity wave spectrum. First, a case study of gravity waves generated by a developing cyclone, Tropical Storm Gelane (February 2010), is carried out using observations complemented by numerical simulations with the Weather Research and Forecast model, with a resolution down to 1 km. Distributions of momentum fluxes obtained from both data show reasonable agreement and emphasize waves with short wavelengths (

Recent studies have revealed that strong sea surface temperature (SST) fronts, on the scale of a Western Boundary Current, significantly affect not just the Marine Boundary Layer but the entire troposphere. This has aroused renewed interest in air-sea interactions. The present study investigates the atmospheric response to fixed SST anomalies associated with mesoscale oceanic eddies and submesoscale filaments, using idealized simulations. Our main result is that in weak wind conditions, the vertical velocity in the planetary boundary layer (PBL) is linearly proportional to the SST Laplacian. This is established by a quantitative analysis in the spatial space as well as in the spectral space. Comparing the responses to two different SST fields shows that vertical velocities are much more intense when the submesoscales are more energetic. These results hold for different configurations of the atmospheric large-scale state and for different PBL parameterizations. Surface winds play the role of low-pass filter and reduce the response at the smaller scales. To our knowledge, this study is the first to clearly reveal the high impact of oceanic submesoscales on the atmospheric boundary layer at midlatitudes, as well as the direct link between the vertical velocity and the SST Laplacian.

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 greater than or similar to 10 m/s disagreement 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.

[1] Generation of inertia gravity waves by the midlatitude tropospheric jet is studied on the basis of the data obtained from the radio soundings over the North Atlantic during the Fronts and Atlantic Storm-Track Experiment campaign. A sample of 224 radio soundings is used to analyze the wave activity as a function of the distance to the jet. It is shown that radio soundings displaying the most intense gravity wave activity, both in the stratosphere and in the troposphere, are the ones closest to the jet axis. Thus the jet region is the dominant source of gravity waves in this region far from orography. Further examination allows for identification of two specific regions of the flow that are associated with intense gravity wave activity: the vicinity of the maximum of the jet velocity and the regions of strong curvature of the jet. The detailed case studies we provide suggest that geostrophic adjustment is the dynamical mechanism responsible for the generation of large-amplitude inertia gravity waves in the regions of the strong curvature of the wind. The generation of waves in the vicinity of the regions where the wind veers, in the deep troughs of the geopotential, appears to be systematic.