Pierre-Dominique Pautet

Atmospheric waves are never perfectly monochromatic and show modulations in amplitude, wavelength, and frequency due to transient background conditions. In observations, they often appear as complex superpositions of multiple interacting wave components. To disentangle this superposition into individual interpretable wave packets and to study their instantaneous spectral properties in an automatic fashion, we introduce a new method: the Wavefield Analysis and Segmentation unraveling an interpretable set of wave packets (WAVEGUISE). Based on the continuous wavelet transform, WAVEGUISE combines watershed segmentation from image processing and density based spatial clustering algorithms to partition the wavelet domain into coherent regions of shared temporal, spatial, and spectral characteristics. We demonstrate the method using three observational data sets of increasing dimensionality: a) a 25-day pressure time series (1-D) from a long-duration super-pressure balloon obtained during the Strateole-2 campaign, b) a 1,400 × 500 km Q-line radiance swath (2-D) from the Atmospheric Waves Experiment (AWE), and c) a OH (3,1) band intensity video sequence (3-D) from the Advanced Mesospheric Temperature Mapper. Applied to these data sets, WAVEGUISE acts as a magnifying glass. The segmentation into a set of dominant and subdominant interpretable wave packets reveals a significantly enhanced level of detail enabling to precisely study dynamical processes such as e.g. wave-wave interaction and to improve wave statistics.

Atmospheric waves are never perfectly monochromatic and show modulations in amplitude, wavelength, and frequency due to transient background conditions. In observations, they often appear as complex superpositions of multiple interacting wave components. To disentangle this superposition into individual interpretable wave packets and to study their instantaneous spectral properties in an automatic fashion, we introduce a new method: the Wavefield Analysis and Segmentation unraveling an interpretable set of wave packets (WAVEGUISE). Based on the continuous wavelet transform, WAVEGUISE combines watershed segmentation from image processing and density based spatial clustering algorithms to partition the wavelet domain into coherent regions of shared temporal, spatial, and spectral characteristics. We demonstrate the method using three observational data sets of increasing dimensionality: a) a 25-day pressure time series (1-D) from a long-duration super-pressure balloon obtained during the Strateole-2 campaign, b) a 1,400 × 500 km Q-line radiance swath (2-D) from the Atmospheric Waves Experiment (AWE), and c) a OH (3,1) band intensity video sequence (3-D) from the Advanced Mesospheric Temperature Mapper. Applied to these data sets, WAVEGUISE acts as a magnifying glass. The segmentation into a set of dominant and subdominant interpretable wave packets reveals a significantly enhanced level of detail enabling to precisely study dynamical processes such as e.g. wave-wave interaction and to improve wave statistics.

High frequency gravity waves (GWs) promote mixing in the mesosphere and lower thermosphere (MLT) via energy transport and momentum deposition. Kelvin‐Helmholtz instabilities (KHI) accompany strong shear throughout the atmosphere and influence GW breaking and mixing processes. Recent observations of GW‐KHI interactions in the MLT revealed the formation of KHI tube and knot dynamics (T&K) that were reproduced in simulations of unperturbed shear layers. However, simulation studies to‐date have not explored the dominant parameters of GW‐KHI interactions and their mixing implications. We present observations and modeling results of KHI advecting through a stationary GW in the MLT. On 18 July 2018, the advanced mesospheric temperature mapper (AMTM) at Tierra del Fuego captured stationary 30–50 km GWs and orthogonal 8–10 km KHI advecting through the wave field. We conduct numerical simulations to assess the influence of shear layer depth and Richardson number (Ri) on GW‐KHI interactions observed in the OH layer. Simulations reveal the intensification and rapid evolution of KHI triggered by shear layer advection through successive GW phases. Shear layers initialized with yield KHI localized to one GW wavelength, whereas KHI from layers with form faster and extend across multiple GW wavelengths. For , T&K control the transition to turbulence for deeper shear layers, whereas shallower layers result in localized KHI billow core breakdowns at earlier times. We assess the impact of these different KHI scales and shear intensities to determine how KHI‐GW interactions promote T&K in different atmospheric environments.

Temperature in the upper atmosphere is one of the critical parameters that is closely associated with the in situ chemistry and neutral dynamics, especially the atmospheric waves with various scales, such as tide and gravity waves. To study the small‐scale wave activities in the mesopause region, the Atmospheric Waves Experiment (AWE) was launched and attached onto the International Space Station (ISS) in November 2023. Since then, it has been providing high resolution horizontal global temperature mapping of the mesopause region by deducing the layer‐averaged rotational temperature of the hydroxyl (OH) layer, centered near 87 km altitude. In this study, we are evaluating the AWE measured temperatures with those obtained by the TIMED/SABER instrument, through building several customized weighting functions for the SABER temperature profiles. The preferred algorithm utilizes the simultaneous SABER observation of the OH layer to establish the customized weighting function of the SABER temperatures. Overall, with 13,122 overlapping samples, the mean temperature difference is less than 1 K with a standard deviation of ∼5 K, demonstrating consistent high accuracy of AWE temperature measurements. In addition, using the ground‐based Na LiDAR and an Advanced Mesospheric Temperature Mapper (AMTM), operating at Utah State University (USU) and Bear Lake Observatory (BLO), respectively, the AWE measured temperatures are further validated during the AWE overpasses.

The mesosphere and lower thermosphere (MLT) dynamics play a crucial role in the vertical coupling of the whole atmosphere and are a key component of space weather processes. However, accurately representing the MLT in whole atmosphere models remains challenging due to sparse observational coverage and limited assimilation of high‐altitude wind data. In this study, we evaluate the feasibility and impact of assimilating MLT wind observations from a ground‐based meteor radar network into the Whole Atmosphere Community Climate Model (WACCM) using the Ensemble Adjustment Kalman Filter (EAKF) within the Data Assimilation Research Testbed (DART). Two Observing System Simulation Experiments (OSSEs) were conducted for the 2018/2019 Northern Hemisphere winter. In the first OSSE, synthetic observations of lower atmospheric state variables and SABER and MLS neutral temperatures are assimilated. In the second OSSE, additional synthetic ground‐based MLT wind measurements were assimilated to assess their added value. Results show that assimilating meteor radar winds leads to localized improvements in neutral wind fields near the observation sites. Notably, the assimilation of ground‐based MLT winds results in substantial and widespread reductions in neutral temperature error, highlighting strong wind–temperature correlations within the model ensemble. However, wind corrections remain largely confined in both space and time, emphasizing the need for adjustments to the data assimilation methodology to extend their impact. These findings demonstrate the potential of ground‐based MLT wind assimilation to enhance upper atmospheric state estimation and emphasize the importance of optimizing localization, inflation, and multivariate update strategies.

The NASA Atmospheric Waves Experiment (AWE) employs an OH imager that began measurements aboard the International Space Station (ISS) on 22 November 2023. The motivation for AWE was to quantify gravity wave (GW) responses to diverse sources and their influences extending into the mesosphere and higher altitudes. AWE measures temperatures at 2‐km cross‐track resolution in a 600‐km wide imaging swath at ∼87 km during nighttime. AWE swaths reveal a remarkable range of nighttime GW and instability responses from small to very large spatial scales over diverse terrain, deep convection, and oceans where no sources are apparent. Mountain wave responses over small islands and large‐scale orography exhibit horizontal wavelengths of λ _h ∼ 10–300 km or longer. Convective GWs likewise reveal λ _h ∼ 10–300 km or larger over land and ocean sources. AWE also provides clear evidence of secondary GWs having λ _h ∼ 50–500 km in response to strong MW sources. Additionally, the ISS orbit enables repeat observations over all sites at an ∼4‐day cadence and 2–3 successive swaths at an ∼1.5 hr cadence over all sites near the ISS orbit inclination of 51.6°. AWE swaths enable assessments of along‐track, cross‐track, and 2‐dimensional wavenumber spectra of GW temperatures, T′ ² , and velocity covariances, < u _h ' w ′>, for u _h ′ = ( u ′, v ′) spanning GW horizontal wavelengths, λ _h ∼ 30–1,000 km or longer. These spectral evaluations reveal the major contributions to GW pseudomomentum fluxes, ρ < u _h ' w ′>, to occur at GW λ _h ∼ 30–300 km.

Ground-based lidar measurements with the CORAL instrument at the southern tip of South America (53.8S, 67.8E) provide vertical temperature measurements and gravity wave characteristics at a resolution of 15 min throughout the middle atmosphere, up to the OH emission layer. Of this layer, the AWE instruments onboard the International Space Station provides global imaging of OH brightness and temperature at 2 km x 2 km resolution. We identified coincident and common-volume observations during winter 2024 and analyze these for gravity wave and instability dynamics. The geographic region is known for strong orographic wave forcing and deep propagation of gravity waves into the middle atmosphere during winter, including mountain wave breaking, secondary wave generation, and the generation of vortex rings and Kelvin-Helmholtz instabilities in the upper mesosphere. Gravity waves identified by lidar are typically 2-3 hours in period and 10-15 km in vertical wavelength in the upper stratosphere, lower mesosphere. In the upper mesosphere, smaller scales prevail. By wavelet analysis, we find that periods down to 40 min sporadically occur in thin, confined altitude layers. These observations are combined with AWE measurements showing varied local responses including mesospheric bores, small-scale mountain waves and large- and small-scale vortex rings and ring clusters related to the breaking of the gravity waves propagating from below.

We find observational evidence for mountain waves (MWs) and secondary gravity waves (2GWs) in the OH*(3‐1) layer above Tierra del Fuego, Argentina. On the night of 21–22 May 2018, the Advanced Mesospheric Temperature Mapper (AMTM) obtained temperatures at km. During the westerly phase of the semi‐diurnal tide, arc‐shaped quasi‐stationary structures in temperature maps indicated MWs with horizontal wavelengths of 20–40 km. A co‐located temperature measurement from the COmpact Rayleigh Autonomous Lidar (CORAL) confirmed quasi‐stationary structures down to 15 km, also indicating MWs. Temperature profiles revealed a convectively unstable region within a MW phase front at 58–66 km—a sign of MW breaking, momentum deposition, local body forces, and generation of 2GWs. After a wind reversal in the mesosphere/lower thermosphere (MLT) observed by the Southern Argentina Agile Meteor Radar (SAAMER), a gravity wave propagated southeastward with an intrinsic phase speed . Using 1‐D cross‐wavelet analysis, we derived spectral properties of this fast wave and performed ray‐tracing, using the Japanese Whole Atmosphere Reanalysis as background. The fast wave likely originated near 63 km above the Torres del Paine region, known for large‐amplitude MWs. We conclude that tropospheric forcing excites MWs over Tierra del Fuego (observed) and over the Torres del Paine massif (not observed) km northwest. The latter MWs likely generate 2GWs upon breaking in the lower mesosphere, observed over Tierra del Fuego after a turning‐level disappearance. A novel momentum‐flux (MF) retrieval, based on co‐located AMTM, CORAL, and SAAMER measurements, automatically identifies dominant wave features, incorporating measurement and OH*(3‐1) layer variability to infer realistic MF uncertainties.

High frequency Gravity waves (GWs) promote mixing in the mesosphere and lower thermosphere (MLT) via energy transport and momentum deposition. Kelvin-Helmholtz instabilities (KHI) accompany strong shear throughout the atmosphere and influence GW breaking and mixing processes. Recent observations of GW-KHI interactions in the MLT revealed the formation of KHI tube and knot dynamics (T&K) that were reproduced in simulations of unperturbed shear layers. However, simulation studies to-date have not explored the dominant parameters of GW-KHI interactions and their mixing implications. We present observations and modeling results of KHI advecting through a stationary GW in the MLT. On 18 July 2018, the advanced mesospheric temperature mapper (AMTM) at Tierra del Fuego captured stationary 30-50 km GWs and orthogonal, 8-10 km KHI advecting through the wave field. We conduct direct numerical simulations (DNS) to assess the influence of shear layer depth and Richardson number (Ri) on GW-KHI interactions observed in the OH layer. DNS reveal the intensification and rapid evolution of KHI triggered by shear layer advection through successive GW phases. Shear layers initialized with Ri = 0.15 yield KHI localized to one GW wavelength, whereas KHI from layers with Ri = 0.05 form faster and extend across multiple GW wavelengths. For Ri = 0.05, T&K control the transition to turbulence for deeper shear layers, whereas shallower layers result in localized KH billow core breakdowns at earlier times. We assess the impact of these different KHI scales and shear intensities to determine how KHI-GW interactions promote mixing in different atmospheric environments.