Gravity Waves Research Articles

Statistical Characteristics of Wavelike Banded Convection Associated With Ducted Gravity Waves Over Southern China

AbstractWavelike banded convection occurs frequently and persistently in southern China, significantly impacting local weather. This study presents the first multi‐year investigation of wavelike banded convection associated with ducted gravity waves in southern China from 2013 to 2021 using radar mosaic maps. These convective bands are highly dependent on the wave ducting environment, which typically exhibits a three‐layer stratification structure. Out of 112 identified cases, two types were further classified based on the presence of a wave ducting vertical structure in the soundings: WaveDuct and nonWaveDuct type. Unlike nonWaveDuct type, the WaveDuct type predominantly happens on winter or spring nights. The phase speeds of WaveDuct convective bands also show a highly positive correlation with the depth of the ducting layer, consistent with wave ducting theory. This study provides a climatological understanding of the frequencies, movements, and environment conditions of wavelike banded convection related to ducted gravity waves over southern China.

X‐Raying Neutral Density Disturbances in the Mesosphere and Lower Thermosphere Induced by the 2022 Hunga‐Tonga Volcano Eruption‐Explosion

AbstractWe present X‐ray observations of the upper atmospheric density disturbance caused by the explosive eruption of the Hunga Tonga‐Hunga Ha'apai (HTHH) volcano on 15 January 2022. From 14 January to 16 January, the Chinese X‐ray astronomy satellite, Insight‐HXMT, was observing the supernova remnant Cassiopeia A. The X‐ray data obtained during Earth's atmospheric occultations allowed us to measure neutral densities in the altitude range of 90–150 km. The density profiles above 110 km altitude obtained before the major eruption are in reasonable agreement with expectations by both GAIA and NRLMSIS 2.0 models. In contrast, after the HTHH eruption, a severe density depletion was found up to 1,000 km away from the epicenter, and a relatively weak depletion extending up to km for over 8 hr after the eruption. In addition, density profiles showed wavy structures with a typical length scale of either 20 km (vertical) or 1,000 km (horizontal). This may be caused by Lamb waves or gravity waves triggered by the volcanic eruption.

On the kinetics of internal gravity waves beyond the hydrostatic regime

We present a new derivation of the kinetic equation for weak, non-hydrostatic internal gravity wave turbulence. The equation is equivalent to the one obtained by Caillol & Zeitlin (Dyn. Atmos. Oceans, vol. 32, issue 2, 2000, pp. 81–112), but it takes a canonical form. We show that it conserves the energy without involving the resonance condition in frequency, and look for the isotropic part of the steady, scale-invariant solutions. We provide a parametrization of the resonant manifold of non-hydrostatic internal gravity wave triadic interactions. This allows us to simplify the collision integral, and to evaluate the transfer coefficients of all triadic interactions. In the hydrostatic limit, our equation is equivalent to the Hamiltonian description of Lvov & Tabak (Phys. Rev. Lett., vol. 87, issue 16, 2001, 168501).

Initiation of prefrontal convection within a Northeast China cold vortex: The roles of elevated front and dry air intrusion

AbstractConvective initiation ahead of a surface cold front within the Northeast China cold vortex of June 1, 2021, is investigated using observations and a convection‐permitting simulation. The initiation of the convection is elevated without identifiable surface mesoscale forcing. Air feeding the initial convective cells originates from at least 1 km above the ground where parcel‐based convective available potential energy is sufficiently large. An elevated front with large equivalent potential temperature gradient and flow convergence across is primarily responsible for the initiation of elevated convection and later organization into a deep convective line. The elevated front is similar to the upper cold front within extratropic cyclones studied elsewhere, but its altitude is lower in this case. Backward‐trajectory analysis of air parcels on the cold side of the elevated front shows that dry air intrusion reinforces the elevated front by increasing the equivalent potential temperature gradient and moment convergence across the front. The convergence forcing combined with access to convective unstable air at the leading edge of the westerly flows east of the elevated front causes the initiation of convection. The development and spreading of a surface cold pool help push the convective line eastward away from the surface cold front located to the west. Orographic gravity waves provide additional forcing so that convection cells are initiated first at the upward branches of the gravity waves, but the overall organization of initial cells into a solid convective line is controlled mainly by mesoscale structures associated with the elevated front.

Enhanced extra mixing in low-mass stars approaching the RGB tip and the problem of Li-rich red-clump stars

Abstract A few percent of red giants are enriched in Lithium with A(Li) > 1.5. Their evolutionary status has remained uncertain because these Li-rich giants can be placed both on the red-giant branch (RGB) near the bump luminosity and in the red clump (RC) region. However, thanks to asteroseismology, it has been found that most of them are actually RC stars. Starting at the bump luminosity, RGB progenitors of the RC stars experience extra mixing in the radiative zone separating the H-burning shell from the convective envelope followed by a series of convective He-shell flashes at the RGB tip, known as the He-core flash. The He-core flash was proposed to cause fast extra mixing in the stars at the RGB tip that is needed for the Cameron-Fowler mechanism to produce Li. We propose that the RGB stars are getting enriched in Li by the RGB extra mixing that is getting enhanced and begins to produce Li, instead of destroying it, when the stars are approaching the RGB tip. After a discussion of several mechanisms of the RGB extra mixing, including the joint operation of rotation-driven meridional circulation and turbulent diffusion, the Azimuthal Magneto-Rotational Instability (AMRI), thermohaline convection, buoyancy of magnetic flux tubes, and internal gravity waves, and based on results of (magneto-) hydrodynamics simulations and asteroseismology observations, we are inclined to conclude that it is the mechanism of the AMRI or magnetically-enhanced thermohaline convection, that is most likely to support our hypothesis.

Tidal Spin-up of Subdwarf B Stars

Hot subdwarf B (sdB) stars are stripped helium-burning stars that are often found in close binaries, where they experience strong tidal interactions. The dissipation of tidally excited gravity waves alters their rotational evolution throughout the sdB lifetime. While many sdB binaries have well-measured rotational and orbital frequencies, there have been few theoretical efforts to accurately calculate the tidal torque produced by gravity waves. In this work, we directly calculate the tidal excitation of internal gravity waves in realistic sdB stellar models and integrate the coupled spin–orbit evolution of sdB binaries. We find that for canonical sdB (M sdB = 0.47 M ⊙) binaries, the transitional orbital period below which they could reach tidal synchronization in the sdB lifetime is ∼0.2 day, with weak dependence on the companion masses. For low-mass sdBs (M sdB = 0.37 M ⊙) formed from more massive progenitor stars, the transitional orbital period becomes ∼0.15 day. These values are very similar to the tidal synchronization boundary (∼0.2 day) evident from observations. We discuss the dependence of tidal torques on stellar radii, and we make predictions for the rapidly rotating white dwarfs formed from synchronized sdB binaries.

Rainfall Enhancement Downwind of Hills Due to Stationary Waves on the Melting Level and the Extreme Rainfall of December 2015 in the Lake District of Northwest England

This paper investigates how stationary gravity waves generated by flow over orography enhance rainfall, with particular attention to the role of induced waves in the melting level. The findings reveal a new mechanism by which gravity wave flow focuses precipitation, amplifying rainfall intensity downwind of hills. This mechanism, which depends on the differential velocities of rain and snow, offers fresh insights into how orographic effects can intensify rainfall. A two-dimensional diagnostic model based on linear gravity wave theory is used to investigate the record-breaking rainfall of December 2015 in the Lake District of northwest England. The pattern of ascent is shown to have a qualitatively good fit to that of the Met Office’s operational high-resolution UKV model averaged over 24 h, suggesting that orographically excited stationary waves were the principal cause of the rain. Precipitation trajectories imply that a persistent downstream elevated wave caused by the Isle of Man supported a spray of seeding ice particles directed towards the Lake District, and that these grew whilst suspended in strong upslope flow before being focused by the undulating melting-level into intense shafts of rain.

Small-scale wind fluctuations within melting layers of winter storms: results from WINTRE-MIX

Abstract Investigations into the melting layer (ML) of winter storms have revealed small-scale fluctuations in the horizontal wind that could significantly affect surface precipitation type and the evolution of the ML. Despite previous evidence of such fluctuations, essential questions remain concerning their characteristics and the forces driving them. Therefore, this study characterizes small-scale horizontal wind fluctuations (<1 km in length with perturbation magnitudes < 3 m s−1) and their environments within the ML of winter storms. This analysis uses data from a scanning X-band Doppler radar collected during the Winter Precipitation Type Research Multi-Scale Experiment (WINTRE-MIX), conducted during February and March 2022. We present three case studies where small-scale horizontal wind fluctuations are identified using along-radial and along-azimuthal radial velocity perturbations. These cases cover the range of environmental conditions observed during WINTRE-MIX, including: i) a descending ML with change in surface p-type from snow to rain, ii) a steady ML with a surface p-type transition from freezing rain to rain due to surface cold air erosion, and iii) a steady ML with a surface p-type transition from freezing rain to ice pellets due to surface cold air advection. Forcing mechanisms for small-scale wind fluctuations during each case are attributed to static instability, vertically trapped gravity waves, and/or shear instability inferred from rawinsonde data, HRRR analysis, and radar data. Our findings suggest that static instability, gravity waves, and shear instability drive the ML’s small-scale wind fluctuations and may influence surface precipitation-type transitions.

Typhoon Early Warning and Monitoring Based on the Comprehensive Characteristics of Oceanic and Ionospheric Echoes from HFSWR: The Case of Typhoon Muifa

As devastating natural disasters, typhoons pose a tremendous threat to human society, making effective typhoon early warning and monitoring crucial. To address this challenge, High Frequency Surface Wave Radar (HFSWR), which can observe oceanic parameters such as typhoon wind fields in real time and even capture the dynamic changes in the ionosphere, has become an effective tool for typhoon monitoring. This paper investigates the interaction mechanisms about Typhoon-Acoustic Gravity Waves (AGWs)-Ionosphere, as well as Typhoon-Ocean Waves for HFSWR, and simulates these interaction processes within HFSWR. Then a typhoon early warning and monitoring scheme for HFSWR has been proposed: In the first stage, the S-shaped ionospheric disturbances observed by HFSWR are utilized as precursor signals for early typhoon warnings. In addition, the second stage involves analyzing changes in first-order oceanic echo spectral peak ratio to pinpoint when the typhoon eye enters the radar detection range, thus initiating the typhoon monitoring phase. Subsequently, the measured data from HFSWR collected during Typhoon “Muifa” (2212) in conjunction with the proposed scheme are evaluated in detail. The results indicate that AGWs generated by typhoons can propagate into non-typhoon areas within the detection range, causing S- shaped ionospheric disturbances and providing approximately 6 h of early warning. At around 8:05 (UTC+8), an increasing trend in the first-order spectral peak ratio was noted, indicating the entry of the typhoon eye into the detection range, which closely aligns with the official typhoon path and marks the transition to the monitoring phase. The proposed scheme is expected to enhance the capability for typhoon early warning and real-time monitoring in specific sea areas and mitigate the risks associated with typhoon-related disasters.

Momentum, energy and vorticity balances in deep-water surface gravity waves

The particle trajectories in irrotational, incompressible and inviscid deep-water surface gravity waves are open, leading to a net drift in the direction of wave propagation commonly referred to as the Stokes drift, which is responsible for catalysing surface wave-induced mixing in the ocean and transporting marine debris. A balance between phase-averaged momentum density, kinetic energy density and vorticity for irrotational, monochromatic and spatially periodic two-dimensional water waves is derived by working directly within the Lagrangian reference frame, which tracks particle trajectories as a function of their labels and time. This balance should be expected as all three of these quantities are conserved following particles in this system. Vorticity in particular is always conserved along particles in two-dimensional inviscid flow, and as such even in its absence it is the value of the vorticity that fundamentally sets the drift, which in the Lagrangian frame is identified as the phase-averaged momentum density of the system. A relationship between the drift and the geometric mean water level of particles is found at the surface, which highlights connections between the geometry and dynamics. Finally, an example of an initially quiescent fluid driven by a wavelike pressure disturbance is considered, showing how the net momentum and energy from the surface pressure disturbance transfer to the wave field, and recognizing the source of the mean Lagrangian drift as the net momentum required to generate an irrotational surface wave by any conservative force.

Role of Cosmological Models in Organic Evolution in Space

Using the dynamic Friedman-Robertson-Walker Cosmological Model of the universe, the possibility of organic evolution in Space in the early universe is explored. According to this model, the universe is expected to perform simple harmonic motion between expansion and contraction. At the end of contraction, the universe assumes the smallest possible finite size with no boundary, while the entire mass of the universe gathers together as gravity particles, or gravity waves, at the center of gravity of universe. As soon as the density of the gravity particles is extremely high and the temperature reaches the flame temperature, a sudden violent explosion hurls the entire material into space uniformly in all directions when the expansion starts. It is in this period of expansion where the precursors of DNA/RNA, such as water, oxides of phosphorus, sugars, heterocyclic bases, compounds of nitrogen, etc. are formed.

A Study of Stochastic Low-frequency Variability for Galactic O-type Stars

In order to explore how the ubiquitous stochastic low-frequency (SLF) variability of O-type stars is related to various stellar characteristics, we compiled a sample of 150 O-type stars observed via ground-based spectroscopic surveys, alongside photometric data obtained from the Transiting Exoplanet Survey Satellite (TESS). We analyzed 298 light curves obtained from TESS Sectors 1–65 for the stars in our sample. Leveraging the spectroscopic parameters, we used Bonnsai to determine masses, radii, fractional main-sequence ages, and mass-loss rates for stars of our sample. Subsequently, we identified possible correlations between the fitted parameters of SLF variability and stellar properties. Our analysis unveiled four significant correlations between the amplitude and stellar parameters, including mass, radius, fractional main-sequence ages, and mass-loss rate. For stars with ≳30 M ⊙, we observed a decrease in characteristic frequency and steepness with increasing radius. Finally, we compared various physical processes that may account for the SLF variability with our results. The observed SLF variability may arise from the combined effects of the iron convection zone (FeCZ) and internal gravity waves (IGWs), with IGWs potentially more dominant in the early stages of stellar evolution, and the contribution of FeCZ becoming more significant as stars evolve. Meanwhile, our results indicate that the SLF variability of O-type stars bears certain signatures of the line-driven wind instability and granulation.

Improved Orographic Gravity Wave Drag Parameterization Accounting for the Nonhydrostatic Effect in the Weather Research and Forecasting Model: Tests for Short-Range Forecast of Northeast China Cold Vortices

Abstract The parameterization of orographic gravity wave drag (OGWD) is essential for accurate numerical weather prediction in regions of complex terrain. Current OGWD schemes assume hydrostatic OGWs but the parameterized OGWs in fine-resolution models with narrow subgrid-scale orography can be significantly affected by the nonhydrostatic effects (NHE). In our recent work, the OGWD scheme in the Model for Prediction Across Scales (MPAS) was revised by accounting for the NHE on the surface wave momentum flux of upward-propagating OGWs. Herein, the revised OGWD scheme is implemented in the Weather Research and Forecasting (WRF) model to evaluate its performance in short-range weather forecast. Two sets of 36-hr WRF simulations are conducted for nine Northeast China cold vortices (NECVs) that occurred in the warm season of 2011 using the original and revised OGWD schemes. Results show that the WRF model tends to underestimate the intensity of the NECVs, producing too high geopotential height. When accounting for the NHE in the OGWD scheme, the NECV intensity biases are significantly reduced. Analyses reveal that the NHE act to weaken the lower-tropospheric OGWD by decreasing the surface wave momentum flux, which strengthens the NECV in the lower troposphere. Consequently, the strengthened low-level cyclonic circulation increases the post-trough cold advection to the southwest of the NECV which in turn enhances the NECV in the mid-upper troposphere with reduced geopotential height. The NHE are found to increase as the model horizontal resolution increases, suggesting greater importance of the NHE in the OGWD parameterization of high-resolution numerical models.

Atmospheric Gravity Wave Detection in Low-Light Images: A Transfer Learning Approach

Atmospheric gravity waves, as a key fluctuation in the atmosphere, have a significant impact on climate change and weather processes. Traditional observation methods rely on manually identifying and analyzing gravity wave stripe features from satellite images, resulting in a limited number of gravity wave events for parameter analysis and excitation mechanism studies, which restricts further related research. In this study, we focus on the gravity wave events in the South China Sea region and utilize a one-year low-light satellite dataset processed with wavelet transform noise reduction and light pixel replacement. Furthermore, transfer learning is employed to adapt the Inception V3 model to the classification task of a small-sample dataset, performing the automatic identification of gravity waves in low-light images. By employing sliding window cutting and data enhancement techniques, we further expand the dataset and enhance the generalization ability of the model. We compare the results of transfer learning detection based on the Inception V3 model with the YOLO v10 model, showing that the results of the Inception V3 model are greatly superior to those of the YOLO v10 model. The accuracy on the test dataset is 88.2%.

A Statistical Study of Polar Mesospheric Cloud Fronts in the Northern Hemisphere

AbstractComplex spatial structures in polar mesospheric cloud (PMC) images provide visual clues to the dynamics that occur in the summer mesosphere. In this study, we document one such structure, a PMC front, by analyzing PMC images in the northern hemisphere from the Cloud Imaging and Particle Size (CIPS) instrument onboard the aeronomy of ice in the mesosphere (AIM) satellite. A PMC front is defined as a sharp boundary that separates cloudy and mostly clear regions, and where the clouds at the front boundary are brighter than the clouds in the cloudy region. We explore the environment that supports the formation of PMC fronts using near‐coincident temperature and water vapor observations from the Sounding of the Atmosphere using Broadband Emission Radiometry (SABER) satellite instrument. A comparison of PMC front locations to near‐coincident temperature profiles reveals the presence of inversion layers at PMC altitudes. The adiabatic and superadiabatic topside lapse rates of these temperature inversions indicate that some of the identified inversion layers may have been formed by gravity wave (GW) dissipation. The structure of the squared buoyancy frequency profiles indicates a stable layer or thermal duct that can be associated with large‐amplitude mesospheric inversion layers (MILs) that extend large distances. These inversion layers may be conducive to horizontal wave propagation. We hypothesize that ducted GWs may be a formation mechanism of PMC fronts.

Initiation of a Supercell by Convectively Generated Gravity Waves in the Simulated Kaiyuan Tornadic Event of 3 July 2019

Abstract An EF4-rated supercell tornado occurred on 3 July 2019 in Kaiyuan, China, causing heavy casualties. A three-level nested-grid high-resolution numerical simulation is used to investigate the initiation of the tornadic supercell. Automatic weather station (AWS) data, FY-4A visible satellite data, and Doppler radar data are used to verify the model simulation. The most important aspects of the simulated pre-supercell mesoscale convective system (MCS) and the initiation of the supercell agree with observations. Detailed investigation of the model results reveals that the initial cells form first above a convective boundary layer (CBL) on the dry side of a surface dryline. Above the CBL is a moist layer in terms of relative humidity and the layer is stable. Convectively generated gravity waves (GWs) emanating from the MCS and propagating southward along the stable layer above the CBL provide localized forcing for the actual triggering of initial cells at specific locations. The associated perturbation potential temperature and vertical velocity patterns confirm that the GWs trigger a series of cloud bands. The additional lifting by the updraft of a horizontal convective roll in the CBL underneath the GW updraft works together to promote faster growth of the initial cell that later becomes the supercell. Examination of the Scorer parameter profiles shows favorable conditions for vertical trapping of GWs along the wave guide in the stable layer, preventing the radiation of wave energy to the upper levels.

Offshore wind turbine noise propagation and the "ground effect" of the sea: a comparison of Effective Flow Resistivity (EFR) of still and moving water surfaces

With the proliferation of Offshore Wind Turbine (OWT) development, the potential annoyance problems caused onshore from long range propagation of noise over the sea surface generated by the turbines must be investigated thoroughly. Measurement of the Ground Effect in relation to the sea surface is onerous and under-researched. In this suite of experiments, a method similar to that described in ANSI S1.18-1999 "Template method for Ground Impedance" is used to compare the differences in effective flow resistivity (EFR) between water surfaces that have 1) a surface that is still, 2) gravity waves that move in the direction of the noise propagation 3) gravity waves that flow opposite the direction of noise propagation. These results are further compared to standard surfaces such as soil and concrete. The frequency response of the surface under these different scaled-down surface gravity wave conditions may provide some insight into the behaviour of lower frequencies over the large scale surface wave conditions at sea.

Internal gravity waves in the middle atmosphere and their impact on infrasound propagation across the International Monitoring System

Infrasound technology is used to monitor atmospheric events in order to guarantee compliance with the Comprehensive Nuclear-Test-Ban Treaty (CTBT). Over the 60 infrasound stations initially planned, 53 are operational to date as part of the International Monitoring System (IMS). As opposed to the waveform-related technologies that are used by the IMS to monitor the two other media of the geosphere (the underground and the oceans), infrasound waves are travelling in a continuously varying propagation medium. From minutes to hours and from a few hundreds of meters in the vertical to thousands of kilometers in the horizontal, atmospheric perturbations affect detection capabilities of the IMS at various scales. These perturbations need to be taken into account, for infrasound analysis to be able to provide accurate localization and characterization of sources of interest. In particular, internal gravity waves (atmospheric buoyancy waves) pose a serious challenge to atmospheric modelling because of the unresolved involved processes. They can dramatically affect transmission losses across the globe. Through numerical experiments and using high resolution modelling outputs of the atmosphere, backed up by atmospheric measurements, we demonstrate how gravity waves impact infrasound attenuation and drive signal amplification across the IMS.

Ocean Surface Gravity Waves Excited by the 2022 Eruption of Hunga Tonga‐Hunga Ha'apai Volcano

AbstractOn 15 January 2022, a massive underwater eruption occurred at the Hunga Tonga‐Hunga Ha'apai Volcano. The plume reached the mesosphere, and the eruption excited a significant atmospheric Lamb wave, which forced the tsunami. The complicated tsunami waveforms due to ocean‐atmosphere coupling prevented inferring the force history of the excitation. To address this, we analyze ocean surface gravity waves (OSWs) from 15 to 40 mHz, which are decoupled from the Lamb wave due to their slower phase velocities. Modeling these OSWs, we infer that the excitation started at 4:00 UTC with an amplitude of N and lasted for 5 hr, followed by a sub‐event at 8:40 UTC. The observations suggest an initial blowout of seawater above the summit and a subsequent outflow that excited a tsunami below 5 mHz. The 2 hr delayed OSW excitation from 6 to 15 mHz may indicate seawater inflow into the crater.

Global-scale gravity wave analysis methodology for the ESA Earth Explorer 11 candidate CAIRT

Abstract. In the past, satellite climatologies of gravity waves (GWs) have initiated progress in their representation in global models. However, these could not provide the phase speed and direction distributions needed for a better understanding of the interaction between GWs and the large-scale winds directly. The ESA Earth Explorer 11 candidate CAIRT could provide such observations. CAIRT would use a limb-imaging Michelson interferometer resolving a wide spectral range, allowing temperature and trace gas mixing ratio measurements. With the proposed instrument design, a vertical resolution of 1 km, along-track sampling of 50 km, and across-track sampling of 25 km in a 400 km wide swath will be achieved. In particular, this allows for the observation of three-dimensional (3D), GW-resolving temperature fields throughout the middle atmosphere. In this work, we present the methodology for the GW analysis of CAIRT observations using a limited-volume 3D sinusoidal fit (S3D) wave analysis technique. We assess the capability of CAIRT to provide high-quality GW fields by the generation of synthetic satellite observations from high-resolution model data and comparison of the synthetic observations to the original model fields. For the assessment, wavelength spectra, phase speed spectra, horizontal distributions, and zonal means of GW momentum flux (GWMF) are considered. The atmospheric events we use to exemplify the capabilities of CAIRT are the 2006 sudden stratospheric warming (SSW) event, the quasi-biennial oscillation (QBO) in the tropics, and the mesospheric preconditioning phase of the 2019 SSW event. Our findings indicate that CAIRT would provide highly reliable observations not only of global-scale GW distributions and drag patterns but also of specific wave events and their associated wave parameters. Even under worse-than-expected noise levels of the instrument, the resulting GW measurements are highly consistent with the original model data. Furthermore, we demonstrate that the estimated GW parameters can be used for ray tracing, which physically extends the horizontal coverage of the observations beyond the orbit tracks.