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This article deals with the detection of small-scale turbulence from in situ meteorological measurements performed under superpressure balloons (SPBs). These balloons allow long-duration flights (several months) at a prerequisite height level. The data set is gathered from the Strateole-2 probationary campaign during which eights SPBs flew in the tropical tropopause layer at altitudes of around 19 and 20.5 km from November 2019 to March 2020. Turbulence is not directly measured by the instrument set onboard the SPBs. Nonetheless, there is the potential to derive information about the occurrence of turbulence from the temporally well-resolved measurements of pressure, temperature, and position. It constitutes a challenge to extract the aforementioned information from a measurement set that was not designed for quantifying turbulence, and the paper explains the methodology developed to overcome this difficulty. It is observed that SPBs oscillate quasi-periodically around their equilibrium positions. The oscillation periods, which are 220 s on average with a range of 130 to 500 s, are close to but noticeably smaller than the Brunt–Väisälä period (∼300 s). The amplitude of these vertical motions is ∼±15 m, inducing large fluctuations in all quantities, whether measured (e.g., pressure, temperature and position) or inferred (e.g., density and potential temperature). The relationships between the changes in these quantities and the vertical displacements of the balloons are used to infer properties of the flow in which the SPBs drift. In the case of active turbulence, the vertical stratification as well as the wind shear are likely to be reduced by mixing. Hence, the increments of potential temperature, δθ, and of the vertical displacements of the balloon, δzB, are expected to be uncorrelated because ∂θ/∂z→0. Moreover, the local vertical gradients of measured quantities, temperature (T) and horizontal velocities (u and v), are estimated from the covariance of the increments of the considered quantity with δzB. The Richardson number of the flow is deduced. Several binary indexes (true or false) to describe the state of the flow, laminar or turbulent, are evaluated. These turbulence indexes, based either on correlations between δθ and δzB or on estimates of the local Richardson number, are found to be consistent, as they differ in less than 3 % of cases. The flow is observed to be turbulent for about 5 % of the time, with strong inhomogeneities along the longitude.

The role of gravity waves on microphysics of tropical cirrus clouds and air parcel dehydration was studied using the combination of Lagrangian observations of temperature fluctuations and a 1.5 dimension model. High frequency measurements during isopycnal balloon flights were used to resolve the gravity wave signals with periods ranging from a few days to 15 min. The detailed microphysical simulations with homogeneous freezing, sedimentation and a crude horizontal mixing represent the slow ascent of air parcels in the Tropical Tropopause Layer. A reference simulation describes the slow ascent of air parcels in the tropical tropopause layer, with nucleation occurring only below the cold point tropopause with a small ice crystals density. The inclusion of the gravity waves modifies drastically the low ice concentration vertical profile and weak dehydration found during the ascent alone, with the increased ice crystal number and size distribution agreeing better with observations. Numerous events of nucleation occur below and above the cold point tropopause, efficiently restoring the relative humidity over ice to equilibrium with respect to the background temperature, as well as increase the cloud fraction in the vicinity of the cold-point tropopause. The corresponding decrease in water vapor is estimated at 2 ppmv around the cold point tropopause.

The parameterization scheme that represents gravity waves due to convection in LMDz-6A, the atmospheric components of the IPSL coupled climate model (IPSLCM6), is directly compared to Strateole-2 balloon observations made in the lower tropical stratosphere from November 2019 to February 2020. The input meteorological fields necessary to run the parameterization offline are extracted from the ERA5 reanalysis and correspond to the instantaneous meteorological conditions found underneath the balloons. In general, we find a fair agreement between measurements of the momentum fluxes due to waves with periods less than 1hr and the parameterization. The correlation of the daily values between the observations and the results of the parameterization is around 0.4, which is statistically elevated considering that we analyse around 600 days of data and surprisingly good considering that the parameterization has not been tuned: the scheme is just the standard one that helps producing a Quasi-Biennial Oscillation in the IPSLCM6 model. Online simulations also show that the measured values of momentum fluxes are well representative of the zonally and averaged values of momentum fluxes needed in LMDz-6A to simulate a QBO. The observations also show that longer waves with periods smaller than a day carry about twice as much flux as waves with periods smaller than an hour, which is a challenge since the low period waves that make the difference are potentially in the “grey zone” of most climate models

The role played by turbulent mixing in the vertical transport of constituents in the UTLS is still poorly understood: there is a lack of knowledge of turbulence due to the limited number of observations in this region as well as to the limitations of current observation techniques.The first part of the present work deals with the detection of small-scale turbulence in the tropical upper troposphere - lower stratosphere from in-situ meteorological measurements collected under super-pressure balloons (SPBs). Eight SPBs were launched during the first Strateole-2 campaign, from November 2019 to March 2020 and flying for several weeks (∼ 3 months). Turbulence detection methods relies on the quasi-periodic vertical oscillations (∼ ±15 m) of the SPBs around their equilibrium positions, such oscillations inducing large fluctuations of measured quantities (pressure, temperature, positions) and inferred quantities (density, potential temperature). A first method of detection is based on correlations between the increments of potential temperature δθ and the vertical displacements of the balloons (i.e. of the sensors) δz. Such correlations are expected to be null as ∂θ/∂z → 0 in case of turbulent mixing. A second method relies on the Richardson number criterion, Ri < 0.25. Ri is deduced from the vertical gradients of measured quantities (T , u, v), estimated from covariances between the increments of the considered quantities and the vertical displacements δz. Turbulence indexes (true of false) to describe the different states of the flow encountered by the SPBs during their flights (laminar or turbulent), are evaluated. These different indexes, based on independent measurements and on various methods, correlations or linear regressions, are found to be consistent: they differ for less than 3% of the cases. The flow is observed to be turbulent for about 5% of the time, with strong inhomogeneities along the longitude.The second part of the present work aims to improve our understanding of turbulence, and its impacts, in the tropical UTLS by studying small- to meso-scale processes, i.e. atmospheric waves, deep convection and associated observed turbulence. These are all key processes of the dynamics of the equatorial UTLS. One can evaluate the probability of turbulence occurrences as a function of the distance to deep convection. Such a distance seems to be a good proxy of wave activity generated by deep convection. The occurrence frequency of turbulence is significantly larger when the distance to deep convection is small, i.e. smaller than ~ 200 km.This research should contribute to a better parametrization of these processes in climate models, and to a better estimation of their impact to vertical transport in the tropical UTLS.

L'éruption explosive du volcan Hunga Tonga - Hunga Ha’apai (HTHH), le 15 janvier 2022, a produit la plus puissante explosion enregistrée depuis les explosions du Krakatau et du Tambora dans les années 1800, libérant une énergie équivalente à 110 mégatonnes de TNT. Les ondes générées se sont propagées dans le sol, et dans l’atmosphère jusqu’à l’ionosphère. L'onde atmosphérique la plus énergétique observée sur les baromètres correspond au mode de Lamb. De période supérieure à 2000 s, son amplitude est comparable à celle observée lors de l’éruption du Krakatau en 1883. L’empreinte des perturbations atmosphériques a été caractérisée à l’échelle planétaire par des réseaux de mesures au sol, à bord de satellites ou de plateformes aéroportées. L’analyse combinée de ces observations a permis d’évaluer les conséquences à court terme de l'éruption du HTHH. Les méthodes d'investigation géophysiques présentées dans cette note montrent l’apport d’analyses interdisciplinaires pour caractériser la réponse impulsionnelle des enveloppes fluides planétaires (atmosphère, océans et mers) à une éruption d’une intensité exceptionnelle.

International audience; The 15 January 2022 eruption of the Hunga volcano (Tonga) generated a rich spectrum of waves, some of which achieved global propagation. Among numerous platforms monitoring the event, two stratospheric balloons flying over the tropical Pacific provided unique observations of infrasonic wave arrivals, detecting five complete revolutions. Combined with ground measurements from the infrasound network of the International Monitoring System, balloon-borne observations may provide additional constraint on the scenario of the eruption, as suggested by the correlation between bursts of acoustic wave emission and peaks of maximum volcanic plume top height. Balloon records also highlight previously unobserved long-range propagation of infrasound modes and their dispersion patterns. A comparison between ground- and balloon-based measurements emphasizes superior signal-to-noise ratios onboard the balloons and further demonstrates their potential for infrasound studies.

International audience; The ground movements induced by seismic waves create acoustic waves propagating upward in the atmosphere, thus providing a practical solution to perform remote sensing of planetary interiors. However, a terrestrial demonstration of a seismic network based on balloon-carried pressure sensors has not been provided. Here we present the first detection of seismic infrasound from a large magnitude quake on a balloon network. We demonstrate that quake's properties and planet's internal structure can be probed from balloon-borne pressure records alone because these are generated by the ground movements at the planet surface below the balloon. Various seismic waves are identified, thus allowing us to infer the quake magnitude and location, as well as planetary internal structure. The mechanical resonances of balloon system are also observed. This study demonstrates the interest of planetary geophysical mission concepts based on seismic remote sensing with balloon platforms, and their interest to complement terrestrial seismic networks.

This article deals with the detection of small-scale turbulence from in-situ meteorological measurements performed under superpressure balloons (SPBs). These balloons allow long duration flights (several months) at a prerequisite height level. The dataset is gathered from the Strateole-2 probationary campaign during which eights SPBs flew in the tropical tropopause layer at around 19 and 20.5 km altitudes, from November 2019 to March 2020. Turbulence is not directly measured by the instrument set onboard the SPBs. Nonetheless, there is a potential to derive information about the occurrence of turbulence from the well-resolved in time measurements of pressure, temperature and position. It constitutes a challenge to extract that information from a measurement set that was not designed for quantifying turbulence, and the manuscript explains the methodology developed to overcome this difficulty. It is observed that SPBs oscillate quasi-periodically around their equilibrium positions. The oscillation periods, 220 s in the average, range from 130 to 500 s, close to, but noticeably smaller than, the Brunt-Väisälä period (∼ 300 s). The amplitude of these vertical motions is ∼ ±15 m, inducing large fluctuations in all quantities, whether measured (pressure, temperature, positions) of inferred (density, potential temperature). The relationships between the changes in these quantities and vertical displacements of the balloons are used to infer properties of the flow in which the SPBs drift. In case of active turbulence, the vertical stratification as well as the wind shear are likely to be reduced by mixing. Hence, the increments of potential temperature, δθ, and of the vertical displacements of the balloon, δzB, are expected to be uncorrelated since ∂θ / ∂z → 0. Also, the local Richardson number is expected to be less than ∼ 0.25. Several binary indexes (true of false) to describe the state of the flow, laminar or turbulent, are evaluated. They are based either on correlations between δθ and δzB, or on estimations of the local Richardson number. Correlation coefficients are computed and compared, by using different measures of δθ and δzB and by estimating either the Pearson or the Spearman coefficients. Turbulence indexes based on a null-correlation are built using a randomisation test to check whether these correlations are significantly non-positive. It is also shown than a linear regression between the increments of a quantity and the increments of vertical displacements allows to estimate the vertical gradient of this quantity. Least square fit and Theil-Sen fit are used to estimate time series of vertical gradients ∂T / ∂z, ∂u / ∂z, ∂v / ∂z. Related quantities such as the Brunt-Väisälä frequency, or the local Richardson number, Ri, are inferred, allowing to establish turbulence indexes from Ri. These different indexes, based on independent measurements and on various methods, correlations or linear regressions, are found to be consistent: they differ for less than 3 % of the cases. The flow is observed to be turbulent for about 5 % of the time, with strong inhomogeneities along the longitude.

Current climate models have difficulty representing realistic wave–mean flow interactions, partly because the contribution from waves with fine vertical scales is poorly known. There are few direct observations of these waves, and most models have difficulty resolving them. This observational challenge cannot be addressed by satellite or sparse ground-based methods. The Strateole-2 long-duration stratospheric superpressure balloons that float with the horizontal wind on constant-density surfaces provide a unique platform for wave observations across a broad range of spatial and temporal scales. For the first time, balloon-borne Global Navigation Satellite System (GNSS) radio occultation (RO) is used to provide high-vertical-resolution equatorial wave observations. By tracking navigation signal refractive delays from GPS satellites near the horizon, 40–50 temperature profiles were retrieved daily, from balloon flight altitude (∼20 km) down to 6–8 km altitude, forming an orthogonal pattern of observations over a broad area (±400–500 km) surrounding the flight track. The refractivity profiles show an excellent agreement of better than 0.2 % with co-located radiosonde, spaceborne COSMIC-2 RO, and reanalysis products. The 200–500 m vertical resolution and the spatial and temporal continuity of sampling make it possible to extract properties of Kelvin waves and gravity waves with vertical wavelengths as short as 2–3 km. The results illustrate the difference in the Kelvin wave period (20 vs. 16 d) in the Lagrangian versus ground-fixed reference and as much as a 20 % difference in amplitude compared to COSMIC-2, both of which impact estimates of momentum flux. A small dataset from the extra Galileo, GLONASS, and BeiDou constellations demonstrates the feasibility of nearly doubling the sampling density in planned follow-on campaigns when data with full equatorial coverage will contribute to a better estimate of wave forcing on the quasi-biennial oscillation (QBO) and improved QBO representation in models.

We present first results aiming at understanding the impact of gravity waves on homogeneous ice nucleation in a simplified microphysics set-up. We use a 1D model of homogeneous ice nucleation, growth, sedimentation, and mixing due to wind shear. This model describes the evolution of ice crystals number and mass with 36 bins of size, after freezing on particles of ammonium sulfate. 420 supersaturated air parcels of 20m thickness and 200m length, distributed in a 7 columns grid, are simulated simultaneously, allowing exchanges of ice crystals between the air masses by sedimentation and horizontal mixing. A first simple set-up represents the large-scale ascending motion of air parcels in the tropics, with a vertical speed of 0.5 mm/s. Air parcels follow a typical tropical temperature profile, between 16400 and 17700 m of altitude. The maximum of nucleated ice crystals is found just below the cold point and nucleation occurs in a layer of 400m, whereas sedimented ice crystals are found down to the bottom of the columns of air masses, showing that the width of cirrus clouds is different from the nucleation layer. The majority of nucleated ice crystals is small enough to be carried up with the air parcels. Yet the fall streaks of a few bigger crystals deplete the humidity of air parcels beneath, preventing new nucleation events from happening. We find that more than half of the total ice mass in air parcels is from the sedimented crystals only, even though they represent less than a third of the number of crystals counted in our system. These few crystals are responsible for more than half of the diminution of the humidity within the air parcels. A second experiment is designed to take into account smaller scale perturbations induced by gravity waves, by coupling the microphysics model with lagrangian temperature measurements from superpressure balloons of the first Stratéole-2 campaign. Gravity waves are found to create more nucleation events, in time and at all altitude levels of our experiment, expanding the nucleation layer up to one kilometer. The larger cooling rates create more small crystals, but the growth of ice is slowed down by the waves’s warming phases and the decreased humidity from more nucleation events. Furthermore, gravity waves prevent the biggest ice crystals from appearing. Last, the addition of gravity waves removes on average less humidity from the air parcels than the sedimentation of ice crystals nucleated during the slow unperturbed ascent.

Atmospheric waves in the tropical tropopause layer are recognized as a significant influence on processes that impact global climate. For example, waves drive the quasi-biennial oscillation (QBO) in equatorial stratospheric winds and modulate occurrences of cirrus clouds. However, the QBO in the lower stratosphere and thin cirrus have continued to elude accurate simulation in state-of-the-art climate models and seasonal forecast systems. We use first-of-their-kind profile measurements deployed beneath a long-duration balloon to provide new insights into impacts of fine-scale waves on equatorial cirrus clouds and the QBO just above the tropopause. Analysis of these balloon-borne measurements reveals previously uncharacterized fine-vertical-scale waves (1000km) and multiday periods. These waves affect cirrus clouds and QBO winds in ways that could explain current climate model shortcomings in representing these stratospheric influences on climate. Accurately simulating these fine-vertical-scale processes thus has the potential to improve sub-seasonal to near-term climate prediction.

Tropical gravity waves that are generated by convection are generally too small in scale and too high in frequency to be resolved in global climate models, yet their drag forces drive the important...

ESA supported airborne and ground-based campaigns constitute an essential element in the development and operation of satellite missions, providing the opportunity to test novel observation technologies, acquire representative data for the development of the mission concepts, processors and use cases, as well as in their calibration and validation phases once in orbit.For the Aeolus Doppler Wind Lidar satellite mission, ESA has implemented a campaign programme that started in 2007 and has continued beyond the launch of the mission on 22. August 2018. Building on the successful WindVal-I and –II campaigns with DLR’s A2D and 2µm Doppler Wind Lidar systems on-board the DLR Falcon aircraft, a number of validation campaigns have been successfully implemented: WindVal-III in November 2018, AVATAR-E in May 2019, and AVATAR-I in September 2019. In addition, ESA supported the CNES pre-Stratéole-2/TAPAPA campaign with eight stratospheric balloons having been launched from the Seychelles in November/December 2019 providing unique upper level wind data in the Tropics. The validation by stratospheric balloons has been extended in the frame of a collaboration with Loon LLC for a test case covering the months August and September 2019.As the largest impact of the Aeolus observations is expected in the Tropics, and in particular over the Tropical oceans, ESA, in close collaboration with NASA and European partners, is currently implementing a Tropical campaign in July 2021. With its base in Cape Verde the activity comprises both airborne and ground-based activities addressing the tropical winds and aerosol validation, as well as a wide range of science objectives. The location is unique as it allows the study of the Saharan Aerosol layer, African Easterly Waves and Jets, the Tropical Easterly Jet, as well as deep convection in ITCZ and tropical cyclogenesis, with a focus on the impact of Saharan dust on micro-physics in tropical cloud systems. The campaign builds on remote and in-situ observations from aircraft (DLR Falcon-20, the Safire Falcon-20, the NASA DC-8 and an Aerovizija/UNG light aircraft) and drone systems, as well as an extensive aerosol and cloud measurement programme with a range of lidar, radar and radiometer systems coordinated by NOA.This paper will provide a summary of the Aeolus campaign activities, focussing on the completed and planned post launch campaigns.

Near-inertial waves (NIWs) with intrinsic frequency close to the local Coriolis parameter f constitute a striking component of the kinetic energy spectrum in both the atmosphere and the ocean. However, contrary to the oceanic case, the strong and variable background atmospheric winds tend to shift the frequency of the waves (Doppler effect). As a consequence, atmospheric NIWs cannot generally be observed directly as a kinetic energy peak at ground-based frequency f but are instead diagnosed indirectly (e.g. using the polarisation and dispersion relations). This complication does not appear when analyzing quasi-lagrangian observations from superpressure balloons (SPB), which drift together with the flow and are thus exempt from Doppler shift. Past SPB observations in the lower stratosphere have revealed the magnitude of the kinetic energy peak associated with NIWs and it was recently shown that state-of-the-art reanalyses partly represent this feature.In this presentation, we will investigate the variability of NIWs using ECMWF (re)analysis products (the operational analysis and ERA5) and balloon observations from recent CNES campaigns (2005, 2010 and 2019-2020) at various latitudes ranging from the equator to the pole (and hence different inertial frequencies). As in Podglajen et al. (2020), NIWs are extracted from the (re)analyses by computing Lagrangian trajectories using the analyzed wind and temperature fields. We will illustrate the remarkable realism of model NIWs, both statistically and for specific case studies. Then, we will characterize the geographic and seasonal variability of NIW properties. In light of those results, possible factors influencing the near-inertial energy peak (horizontal wave propagation, refraction near critical levels, tide interactions) and the parallel with the oceanic situation will be discussed, as well as the ability of the model and data assimilation system to simulate them.Reference :Podglajen, A., Hertzog, A., Plougonven, R., and Legras, B.: Lagrangian gravity wave spectra in the lower stratosphere of current (re)analyses, Atmos. Chem. Phys., 20, 9331–9350, https://doi.org/10.5194/acp-20-9331-2020, 2020.

Strateole-2 is a project aimed at studying the coupling between the troposphere and the stratosphere in the deep tropics. The project originality pertains to the use of long-duration ballons, which can fly for several months at 18 or 20 km altitude. The first Strateole-2 campaign took place from November 2019 to February 2020: 8 balloons with various instrumental configurations were released in the lower stratosphere from Seychelles Islands, in the Indian Ocean.This first campaign was primarily devoted to testing all systems (balloons, gondolas, and instruments) developed for the project, and was very successful: the balloons flew for 85 days onaverage over the whole tropical band, and most instruments performed nominally. In-situ meteorological measurements performed every 30-s on each flight provide a unique description of gravity-wave activity in the tropics and its relation to deep convection. The first observations of aerosols and water vapor onboard long-duration balloons were also achieved, which e.g. highlighted the tape recorder signal in the tropical lower stratosphere. Very innovative instruments also premiered during the campaign: RACHuTS, a light reeled payload, for instance performed 50 high-resolution vertical profiles of temperature, aerosols and water vapor down to 2km below the balloon, crossing several times the cold-point tropopause. ROC collected hundreds of temperature profiles down to the middle troposphere through GPS radio-occultations. Last, one balloon also carried a nadir-pointing backscatter lidar, which has described the underlying convection at unprecedented temporal resolution. An overview of the flights and first results will be presented.Two forthcoming balloon campaigns are planned within Strateole-2, in 2021-22 and 2024-25. Each will release 20 balloons.

Tropical gravity wave activity is investigated using measurements of momentum flux obtained by superpressure balloons. The dataset contains 8 balloons that flew in the equatorial band from November 2019 to February 2020, for 2 to 3 months each, collecting data every 30s. The relation between gravity waves and deep convection was investigated using geostationary satellite data from the NOAA/NCEP GPM\_MERGEIR satellite data product, at 1 hour resolution. The amplitude of gravity wave momentum fluxes shows a clear dependence on the distance to the nearest convection site, with a strong decay as distance to convection increases. The largest values of momentum flux (more than 5 mPa) are only found in the vicinity of deep convection (< 200 km). The sensitivity to distance from convection is stronger for high frequency gravity waves (periods shorter than 30 minutes). Lower frequency waves tend to a non-zero, background value away from convection, supporting some background value in gravity-wave drag parameterizations. On the other hand, the wide range of momentum flux values close to the convection sites emphasizes the intermittent nature of gravity waves. This intermittency was also studied on a larger scale, using a 20° longitudinal grid of the recorded momentum flux in the deep tropics. The results highlight spatial variations of gravity wave activity, with the highest momentum flux recorded over the continent, and associated to higher intermittency.

The tropical tropopause layer (TTL; 14–18.5 km) is the gateway for most air entering the stratosphere, and therefore processes within this layer have an outsized influence in determining global stratospheric ozone and water vapor concentrations. Despite the importance of this layer there are few in situ measurements with the necessary detail to resolve the fine-scale processes within this region. Here, we introduce a novel platform for high-resolution in situ profiling that lowers and retracts a suspended instrument package beneath drifting long-duration balloons in the tropics. During a 100 d circumtropical flight, the instrument collected over a hundred 2 km profiles of temperature, water vapor, and aerosol at 1 m resolution, yielding unprecedented geographic sampling and vertical resolution. The instrument system integrates proven sensors for water vapor, temperature, pressure, and cloud and aerosol particles with an innovative mechanical reeling and control system. A technical evaluation of the system performance demonstrated the feasibility of this new measurement platform for future missions with minor modifications. Six instruments planned for two upcoming field campaigns are expected to provide over 4000 profiles through the TTL, quadrupling the number of high-resolution aircraft and balloon profiles collected to date. These and future measurements will provide the necessary resolution to diagnose the importance of competing mechanisms for the transport of water vapor across the TTL.

International audience; Tropical gravity wave activity is investigated using measurements of momentum fluxes gathered during Strateole-2 superpressure balloon flights. The data set consists of eight balloon flights performed in the deep tropics from November 2019 to February 2020. The flights lasted for 2–3 months each, and in-situ meteorological data were collected every 30 s. The relation between gravity waves and deep convection is investigated using geostationary satellite data from the NOAA/NCEP GPM_MERGIR satellite data product, at 1 h resolution. The amplitude of gravity wave momentum fluxes shows a clear dependence on the distance to the nearest convective system, with a strong decay as distance to convection increases. The largest momentum-flux values (>urn:x-wiley:2169897X:media:jgrd57204:jgrd57204-math-0001mPa) are only found less than 200 km away from deep convection. The sensitivity of the wave flux to distance from convection is stronger for high frequency gravity waves (periods shorter than 60 min). Lower frequency waves tend to a non-zero, background value away from convection, supporting some background value in gravity-wave drag parameterizations. On the other hand, the wide range of momentum flux values observed close to the convection emphasizes the intermittent nature of the gravity-wave source. The large scale variation of gravity-wave intermittency within the equatorial belt is also studied. The results highlight spatial variations of gravity wave activity, with the highest momentum flux recorded over land.

The Tropical Tropopause Layer (14–18.5 km) is the gateway for most air entering the stratosphere, and therefore processes within this layer have an outsized influence in determining global stratospheric ozone and water vapor concentrations. Despite the importance of this layer there are few in situ measurements with the necessary detail to resolve the fine scale processes within this region. Here, we introduce a novel platform for high resolution in situ profiling that lowers and retracts a suspended instrument package beneath drifting long duration balloons in the tropics. During a 100-day circumtropical flight, the instrument collected over 100 two-kilometer profiles of temperature, water vapor and aerosol at one-meter resolution, yielding unprecedented geographic sampling and vertical resolution. The instrument system integrates proven sensors for water vapor, temperature, pressure and cloud and aerosol particles with an innovative mechanical reeling and control system. A technical evaluation of the system performance demonstrated the feasibility of this new measurement platform for future missions with minor modifications. Six instruments planned for two upcoming field campaigns are expected to provide over 4000 profiles through the TTL, quadrupling the number of high-resolution aircraft and balloon profiles collected to date. These and future measurements will provide the necessary resolution to diagnose the importance of competing mechanisms for the transport of water vapor across the TTL.

Due to their increasing spatial resolution, numerical weather prediction (NWP) models and the associated analyses resolve a growing fraction of the gravity wave (GW) spectrum. However, it is unclear how well this “resolved” part of the spectrum truly compares to the actual atmospheric variability. In particular, the Lagrangian variability, relevant, for example, to atmospheric dispersion and to microphysical modeling in the upper troposphere–lower stratosphere (UTLS), has not yet been documented in recent products. To address this shortcoming, this paper presents an assessment of the GW spectrum as a function of the intrinsic (air parcel following) frequency in recent (re)analyses (ERA-Interim, ERA5, the ECMWF operational analysis and MERRA-2). Long-duration, quasi-Lagrangian balloon observations in the equatorial and Antarctic lower stratosphere are used as a reference for the atmospheric spectrum and are compared to synthetic balloon observations along trajectories calculated using the wind and temperature fields of the reanalyses. Overall, the reanalyses represent realistic features of the spectrum, notably the spectral gap between planetary and gravity waves and a peak in horizontal kinetic energy associated with inertial waves near the Coriolis frequency f in the polar region. In the tropics, they represent the slope of the spectrum at low frequency. However, the variability is generally underestimated even in the low-frequency portion of the spectrum. In particular, the near-inertial peak, although present in the reanalyses, has a reduced magnitude compared to balloon observations. We compare the observed and modeled variabilities of temperature, zonal momentum flux and vertical wind speed, which are related to low-, mid- and high-frequency waves, respectively. The probability density function (PDF) distributions have similar shapes but show increasing disagreement with increasing intrinsic frequency. Since at those altitudes they are mainly caused by gravity waves, we also compare the geographic distribution of vertical wind fluctuations in the different products, which emphasizes the increase of both GW variance and intermittency with horizontal resolution. Finally, we quantify the fraction of resolved variability and its dependency on model resolution for the different variables. In all (re)analysis products, a significant part of the variability is still missing, especially at high frequencies, and should hence be parameterized. Among the two polar balloon datasets used, one was broadcast on the Global Telecommunication System for assimilation in NWP models, while the other consists of independent observations (unassimilated in the reanalyses). Comparing the Lagrangian spectra between the two campaigns shows that the (re)analyses are largely influenced by balloon data assimilation, which especially enhances the variance at low GW frequency.

This paper investigates the accuracy of simulated long-duration super-pressure balloon trajectories in the lower stratosphere. The observed trajectories were made during the (tropical) Pre-Concordiasi and (polar) Concordiasi campaigns in 2010, while the simulated trajectories are computed using analyses and forecasts from the European Centre for Medium-Range Weather Forecasts (ECMWF) Integrated Forecast System model. In contrast with the polar stratosphere situation, modelling accurate winds in the tropical lower stratosphere remains challenging for numerical weather prediction systems. The accuracy of the simulated tropical trajectories are quantified with the operational products of 2010 and 2016 in order to understand the impact of model physics and vertical resolution improvements. The median errors in these trajectories are large (typically ≳ 250 km after 24 h), with a significant negative bias in longitude, for both model versions. In contrast, using analyses in which the balloon-borne winds have been assimilated reduces the median error in the balloon position after 24 h to &sim, 60 km. For future campaigns, we describe operational strategies that take advantage of the geographic distribution and the episodic nature of large error events to anticipate the amplitude of error in trajectory forecasts. We finally stress the importance of a high vertical resolution in the model, given the intense shears encountered in the tropical lower stratosphere.

Safety compliance issues for operational studies of the atmosphere with balloons require quantifying risks associated with descent and developing strategies to reduce the uncertainties at the location of the touchdown point. Trajectory forecasts are typically computed from weather forecasts produced by an operational center, for example, the European Centre for Medium-Range Weather Forecasts. This study uses past experiments to investigate strategies for improving these forecasts. Trajectories for open stratospheric balloon (OSB) short-term flights are computed using mesoscale simulations with the Weather and Research Forecasting (WRF) Model initialized with ECMWF operational forecasts and are assimilated with radio soundings using the Data Assimilation Research Testbed (DART) ensemble Kalman filter, for three case studies during the Strapolété 2009 campaign in Sweden. The results are very variable: in one case, the error in the final simulated position is reduced by 90% relative to the forecast using the ECMWF winds, while in another case the forecast is hardly improved. Nonetheless, they reveal the main source of forecasting error: during the ceiling phase, errors due to unresolved inertia–gravity waves accumulate as the balloon continuously experiences one phase of a wave for a few hours, whereas they essentially average out during the ascent and descent phases, when the balloon rapidly samples through whole wave packets. This sensitivity to wind during the ceiling phase raises issues regarding the feasibility of such forecasts and the observations that would be needed. The ensemble spread is also analyzed, and it is noted that the initial ensemble perturbations should probably be improved in the future for better forecasts.

The austral stratospheric final warming date is often predicted with substantial delay in several climate models. This systematic error is generally attributed to insufficient parameterized gravity wave (GW) drag in the stratosphere around 60°S. A simulation with a general circulation model [Laboratoire de Météorologie Dynamique zoom model (LMDZ)] with a much less pronounced bias is used to analyze the contribution of the different types of waves to the dynamics of the final warming. For this purpose, the resolved and unresolved wave forcing of the middle atmosphere during the austral spring are examined in LMDZ and reanalysis data, and a good agreement is found between the two datasets. The role of parameterized orographic and nonorographic GWs in LMDZ is further examined, and it is found that orographic and nonorographic GWs contribute evenly to the GW forcing in the stratosphere, unlike in other climate models, where orographic GWs are the main contributor. This result is shown to be in good agreement with GW-resolving operational analysis products. It is demonstrated that the significant contribution of the nonorographic GWs is due to highly intermittent momentum fluxes produced by the source-related parameterizations used in LMDZ, in qualitative agreement with recent observations. This yields sporadic high-amplitude GWs that break in the stratosphere and force the circulation at lower altitudes than more homogeneously distributed nonorographic GW parameterizations do.

International audience; Wave-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.

Superpressure balloons (SPB), which float on constant density (isopycnic) surfaces, provide a unique way of measuring the properties of atmospheric gravity waves (GW) as a function of wave intrinsic frequency. Here we devise a quasi-analytic method of investigating the SPB response to GW motions. It is shown that the results agree well with more rigorous numerical simulations of balloon motions and provide a better understanding of the response of SPB to GW, especially at high frequencies. The methodology is applied to ascertain the accuracy of GW studies using 12 m diameter SPB deployed in the 2010 Concordiasi campaign in the Antarctic. In comparison with the situation in earlier campaigns, the vertical displacements of the SPB were measured directly using GPS. It is shown using a large number of Monte Carlo-type simulations with realistic instrumental noise that important wave parameters, such as momentum flux, phase speed and wavelengths, can be retrieved with good accuracy from SPB observations for intrinsic wave periods greater than ca. 10 min. The noise floor for momentum flux is estimated to be ca. 10−4 mPa.

In the Stratéole 2 program, set to launch in November 2018, instruments will ride balloons into the stratosphere and circle the world, observing properties of the air and winds in fine detail.

The French metrological laboratory LNE-CNAM, in the framework of a study commissioned by the Laboratory of Dynamic Meteorology LMD, has carried out a preliminary metrological characterization of a device based on submillimetre thermistors, to be used for atmospheric temperature and humidity measurements from the ground up to the lower stratosphere. The device is a measuring system intended to perform temperature measurements between 173 K (around -100 °C) and 233 K (around −40 °C). It is composed of a self-manufactured electronic device, in which submillimetre thermistors measure both air temperature and the temperature of a hygrometer sensor, in order to determine accurately water vapour concentration. Since the needed final measurement uncertainty is below 0.05 K, a comprehensive metrological characterization must be performed. One instrument prototype was supplied to the LNE-CNAM, to be characterized between 173 K and 273 K (around 0 °C). Measurements were carried out in the quasi-adiabatic calorimeter used by the LNE-CNAM to realize the national temperature standard below 84 K. The calorimeter was adapted to work at temperatures up to 273 K. At any setpoint between 173 K and 273 K, the stability of the temperature was within 5 mK for several hours. The thermistors were calibrated by comparison to a calibrated reference thermometer. Preliminary results showed that calibrations with an uncertainty of around 0.02 K are possible. Tests are going to be performed in situ by the LMD, and further characterizations will be carried out to assess the obtained uncertainties and check long-term reproducibility.

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.

Gravity waves are an ubiquitous feature of the atmosphere and influence clouds in multiple ways. Regarding cirrus clouds, many studies have emphasized the impact of wave-induced temperature fluctuations on the nucleation of ice crystals. This paper investigates the impact of the waves on the motion and distribution of ice particles, using the idealized 2-D framework of a monochromatic gravity wave. Contrary to previous studies, a special attention is given to the impact of the wind field induced by the wave. Assuming no feedback of the ice on the water vapor content, theoretical and numerical analyses both show the existence of a wave-driven localization of ice crystals, where some ice particles remain confined in a specific phase of the wave. The precise location where the confinement occurs depends on the background relative humidity, but it is always characterized by a relative humidity near saturation and a positive vertical wind anomaly. Hence, the wave has an impact on the mean motion of the crystals and may reduce dehydration in cirrus by slowing down the sedimentation of the ice particles. The results also provide a new insight into the relation between relative humidity and ice crystals presence. The wave-driven localization is consistent with temperature-cirrus relationships recently observed in the tropical tropopause layer (TTL) over the Pacific during the Airborne Tropical Tropopause EXperiment (ATTREX). It is argued that this effect may explain such observations. Finally, the impact of the described interaction on TTL cirrus dehydration efficiency is quantified using ATTREX observations of clouds and temperature lapse rate.

The contribution of turbulent mixing to heat and tracer transport in the tropical tropopause layer (TTL) is poorly constrained, partly owing to a lack of direct observations. Here, the authors use high-resolution (20 Hz) airborne measurements to study the occurrence and properties of small-scale (

In this study we validated temperatures and horizontal winds of meteorological analyses in the Antarctic lower stratosphere, a region of the atmosphere that is of major interest regarding chemistry and dynamics of the polar vortex. The validation was performed with long-duration observations from 19 superpressure balloon flights during the Concordiasi field campaign in September 2010 to January 2011. Our intercomparison covers the European Centre for Medium-Range Weather Forecasts (ECMWF) operational analysis, the ERA-Interim reanalysis, the Modern-Era Retrospective analysis for Research and Applications (MERRA), and the National Centers for Environmental Prediction and National Center for Atmospheric Research (NCEP/NCAR) reanalysis. We found that large-scale temperatures of the analyses have a mean precision of 0.4–1.4 K and a warm bias of 0.4–2.1 K at about 17–18.5 km altitude and 60–85° S. Zonal and meridional winds have a mean precision of 0.9–2.3 m/s and a bias below &pm;0.5 m/s in the same region. Standard deviations related to small-scale fluctuations such as gravity waves are reproduced at levels of 15–60 % for temperature and 30–60 % for the horizontal winds. We also used the balloon observations to validate trajectory calculations, where vertical motions of simulated trajectories were nudged to pressure measurements of the balloons to take into account changes in the overall mass configuration of the balloon-gondola system. We found relative horizontal transport deviations of 4.5–12 % and error growth rates of 60–170 km/day for 15-day trajectories. Dispersion simulations revealed some difficulties with the representation of subgrid-scale wind fluctuations in our Lagrangian transport model, as the spread of air parcels simulated with different analyses was not consistent. Although case studies suggest that the accuracy of trajectory calculations is influenced by meteorological complexity, diffusion generally does not contribute significantly to transport deviations in our analysis. Overall, validation results are satisfactory and compare well to earlier studies using superpressure balloon observations. In most cases, best performance was achieved by the ECMWF operational analysis, having the best spatiotemporal resolution, followed by ERA-Interim, MERRA, and finally NCEP/NCAR, having the lowest spatiotemporal resolution. Future work applying Eulerian or Lagrangian models to study the chemistry and dynamics of the polar vortex may use our validation results as additional guideline for error analyses.

The relationship between gravity wave momentum fluxes and local wind speed is investigated for oceanic regions at high southern latitudes during austral spring. The motivation is to better describe the gravity wave field by identifying a simple relationship between gravity waves and the large-scale flow. The tools used to describe the gravity waves are probability density functions of the gravity wave momentum fluxes. Three independent datasets covering high latitudes in the Southern Hemisphere springtime are analyzed: simulations with a mesoscale model, analyses from the European Centre for Medium-Range Weather Forecasts, and observations from superpressure balloons of the Concordiasi campaign in 2010. A remarkably robust relation is found, with stronger momentum fluxes much more likely in regions of strong winds. The tails of the probability density functions are well described as lognormal. The median momentum flux increases linearly with background wind speed: for winds stronger than 50 m s−1, the median gravity wave momentum fluxes are about 4 times larger than for winds weaker than 10 m s−1. From model output, this relation is found to be relevant from the tropopause to the midstratosphere at least. The flux dependence on wind speed shows a somewhat steeper slope at higher altitude. Several different processes contribute to this relation, involving both the distribution of sources and the effects of propagation and filtering. It is argued that the location of tropospheric sources is the main contributor in the upper troposphere and lowermost stratosphere and that lateral propagation into regions of strong winds becomes increasingly important above.

[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 (

An original and innovative sampling system called AirCore was presented by NOAA in 2010 (Karion et al., 2010). It consists of a long (> 100 m) and narrow (< 1 cm) stainless steel tube that can retain a profile of atmospheric air. The captured air sample has then to be analyzed with a gas analyzer for trace mole fraction. In this study, we introduce a new AirCore aiming at improved resolution along the vertical with the objectives to: (i) better capture the vertical distribution of CO2 and CH4, (ii) provide a tool to compare AirCores and validate the estimated vertical resolution achieved by AirCores. This AirCore-HR (high resolution) consists of a 300 m tube, combining 200 m of 1/8 in. (3.175 mm) tube and a 100 m of 1/4 in. (6.35 mm) tube. This new configuration allows to achieve a vertical resolution of 300 m up to 15 km and better than 500 m up to 22 km (if analysis of the retained sample is performed within 3 hours). The AirCore-HR was flown for the first time during the annual StratoScience campaign from CNES in August 2014 from Timmins (Ontario, Canada). High-resolution vertical profiles of CO2 and CH4 up to 25 km were successfully retrieved. These profiles revealed well defined transport structures in the troposphere (also seen in CAMS-ECMWF high resolution forecasts of CO2 and CH4 profiles) and captured the decrease of CO2 and CH4 in the stratosphere. The multi-instruments gondola from the flight carried two other low-resolution AirCore-GUF that allowed to perform direct comparisons and study the underlying processing method used to convert the sample of air to greenhouse gases vertical profiles. In particular, degrading the AirCore-HR derived profiles to the low resolution of AirCore-GUF yields an excellent match between both sets of CH4 profiles, and shows a good consistency between vertical structures of CO2 and CH4. These results fully validate the theoretical vertical resolution achievable by AirCores. Finally, the uncertainties associated with the measurements are assessed, yielding an average uncertainty below 3 ppb for CH4 and 0.25 ppm for CO2 with the major source of uncertainty coming from the potential loss of air sample on the ground and the choice of the starting and ending point of the collected air sample inside the tube. In an ideal case where the sample would be fully retained, it would be possible to know precisely the pressure at which air was sampled last and thus to improve the overall uncertainty to about 0.1 ppm for CO2 and 2 ppb for CH4.

International audience; Gravity waves (GWs) play an important role in transferring energy and momentum from the troposphere to the middle atmosphere. However, shorter-scale GWs are generally not explicitly resolved in general circulation models but need to be parameterized instead. Super pressure balloons provide direct access to measure GW characteristics as a function of wave intrinsic frequency that are needed for these parameterizations. The 30 s sampling rate of the GPS receivers carried on the balloons deployed in the 2010 Concordiasi campaign in the Antarctic is much higher compared to the previous campaigns and can cover the full range of the GW spectrum. Two among 19 balloons are also equipped with the dual-frequency GPS receivers initially developed for GPS radio occultation research in addition to the single-frequency receivers, which are expected to provide better positions for GW parameter estimations. Improvements of the positions are significant, from 3-10 m horizontal and 5 m vertical to 0.1 and 0.2 m, respectively, which makes it possible to resolve the Eulerian pressure independently of altitude for the intrinsic phase speed estimation. The lower position accuracy in the previous analysis of campaign results from the single-frequency GPS receiver was primarily due to a problem with the onboard clock that is not present in the new results. The impacts of the position improvements on the final GW parameters are highlighted, with larger difference in momentum flux for the shorter-scale GWs than for the longer scale GWs and significant difference in the distribution of the intrinsic phase speed.

International audience; The impact of high-frequency gravity waves on homogeneous-freezing ice nucleation in cold cirrus clouds is examined using parcel model simulations driven by superpressure balloon measurements of temperature variability experienced by air parcels in the tropical tropopause region. We find that the primary influence of high-frequency waves is to generate rapid cooling events that drive production of numerous ice crystals. Quenching of ice nucleation events by temperature tendency reversal in the highest-frequency waves does occasionally produce low ice concentrations, but the overall impact of high-frequency waves is to increase the occurrence of high ice concentrations. The simulated ice concentrations are considerably higher than indicated by in situ measurements of cirrus in the tropical tropopause region. One-dimensional simulations suggest that although sedimentation reduces mean ice concentrations, a discrepancy of about a factor of 3 with observed ice concentrations remains. Reconciliation of numerical simulations with the observed ice concentrations will require inclusion of physical processes such as heterogeneous nucleation and entrainment.

We use the Weather Research and Forecast (WRF) model to simulate a large-scale tropical tropopause layer (TTL) cirrus in order to understand the formation and life cycle of the cloud. This cirrus event has been previously described through satellite observations by Taylor et al. (2011). Comparisons of the simulated and observed cirrus show a fair agreement and validate the reference simulation regarding cloud extension, location and life time. The validated simulation is used to understand the causes of cloud formation. It is shown that several cirrus clouds successively form in the region due to adiabatic cooling and large-scale uplift rather than from convective anvils. The structure of the uplift is tied to the equatorial response (equatorial wave excitation) to a potential vorticity intrusion from the midlatitudes. Sensitivity tests are then performed to assess the relative importance of the choice of the microphysics parameterization and of the initial and boundary conditions. The initial dynamical conditions (wind and temperature) essentially control the horizontal location and area of the cloud. However, the choice of the microphysics scheme influences the ice water content and the cloud vertical position. Last, the fair agreement with the observations allows to estimate the cloud impact in the TTL in the simulations. The cirrus clouds have a small but not negligible impact on the radiative budget of the local TTL. However, for this particular case, the cloud radiative heating does not significantly influence the simulated dynamics. This result is due to (1) the lifetime of air parcels in the cloud system, which is too short to significantly influence the dynamics, and (2) the fact that induced vertical motions would be comparable to or smaller than the typical mesoscale motions present. Finally, the simulation also provides an estimate of the vertical redistribution of water by the cloud and the results emphasize the importance in our case of both rehydration and dehydration in the vicinity of the cirrus.

International audience; In 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, the momentum-flux pdfs tend to behave like lognormal distributions. Monte Carlo simulations are undertaken to examine the role played by critical levels in influencing the shape of momentum-flux pdfs. In particular, the study finds that the lognormal shape may result from the propagation of a wave spectrum into a varying background wind field that generates the occurrence of frequent critical levels. © 2012 American Meteorological Society.

International audience; Stratospheric balloons from the Vorcore campaign have provided a unique description of the gravity-wave field in the lower stratosphere above Antarctica and the Southern Ocean, during the austral spring of 2005. Mesoscale simulations are carried out to analyze further the gravity-wave field. First, the realism of the simulated waves is assessed by comparison to the observations. A satisfactory overall agreement is found, but different behaviour is noted for orographic waves (overestimation in the simulations relative to the observations) and non-orographic waves (underestimation). Second, the gravity-wave field is analyzed in more detail than was possible from observations alone. It is necessary to distinguish and quantify orographic and non-orographic waves separately. Orographic waves are larger and more intermittent, yet affect only a limited geographical region. Hence, although orographic sources stand out as 'hot spots' for gravity waves, their contribution to momentum fluxes entering the stratosphere is comparable to or smaller than the contribution of non-orographic sources. A diagnostic for intermittency, the Gini coefficient, is proposed. It clearly marks the distinction between orographic and non-orographic sources (intermittencies of ~ 0.8 and ~ 0.5 respectively). Sensitivity to resolution is quite small regarding the spatial structure of the gravity-wave field, but is significant for the amplitudes. The momentum flux values increase by ~ 2 when the horizontal resolution is doubled, and possible biases of both simulations and observations are discussed. Nonetheless, the good agreement between observations and simulations and the complementary information on the biases of each dataset promises that in the future these different estimates of gravity-wave momentum fluxes may converge. © 2012 Royal Meteorological Society.

The observations from the Vorcore campaign (September 2005– January 2006) provide a unique dataset for the study of gravity waves in the lower stratosphere because of their wide coverage (150 000 observations from the South Pole to about 40°S) and of the quasi-Lagrangian nature of the superpressure balloons used. Numerical simulations with a mesoscale model are undertaken in order to investigate further the observed gravity waves, their sources and the induced momentum fluxes. The need for a high spatial resolution makes it necessary to find a compromise between the size of the model domain and the length of the simulations. We describe preliminary simulations used to determine the best configuration of the model for this purpose, and show that the simulations compare reasonably well with the observations. Model results complement the observations and provide new insights into orographic and non-orographic wave sources. Copyright © 2010 Royal Meteorological Society