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Based on the long‐term observations of the Wuhan and Beijing Mesosphere‐Stratosphere‐Troposphere (MST) radars, the statistic characteristics of the mesospheric vertical winds at midlatitudes are investigated for the first time. The vertical wind data come from the Wuhan MST radar (29.51°N, 114.13°E) during 2012–2017 and from the Beijing MST radar (39.75°N, 116.96°E) during 2012–2014. The subdaily, seasonal, and annual variations of the mesospheric vertical winds at Wuhan and Beijing are presented. No obvious subdaily and seasonal variations are found and the annual variations of the mean vertical winds are downward in most of time. The mean vertical winds at Wuhan become smaller in summer and turn to upward at some heights, while those of Beijing only become smaller in summer. The difference of summer vertical winds over Wuhan and Beijing is closely related to the gravity wave activity. The Wuhan MST radar is located to the east side of the Qinghai‐Tibet plateau and also in the East Asian monsoon region, thus the mesospheric winds over Wuhan may experience more influence of gravity waves compared with those over Beijing. The possible bias sources in the vertical wind measurement of the two MST radars are carefully analyzed and the presented wind velocities are proved to be credible. The measured velocity magnitude is similar to other Very High Frequency radar observations, but much larger than that of the WACCM simulation. The Stokes drift, multicellular circulation structures, the wave influence on the radar echoes are raised as the potential sources for this discrepancy. Key Points: Wuhan and Beijing MST radars are used to study the statistical characteristics of the midlatitude mesospheric vertical windsEstimated mesospheric vertical winds at Wuhan and Beijing are generally downward all the year, and much larger than the model dataMuch more gravity wave activities are responsible for the summer difference of the mesospheric vertical winds [ABSTRACT FROM AUTHOR]

Nighttime airglow images observed at the low‐latitude site of São João do Cariri (7.4°S, 36.5°W) showed the presence of a medium‐scale atmospheric gravity wave (AGW) associated with the 21 August 2017 total solar eclipse. The AGW had a horizontal wavelength of ∼1,618 km, observed period of ∼152 min, and propagation direction of ∼200° clockwise from the north. The spectral characteristics of this wave are in good agreement with theoretical predictions for waves generated by eclipses. Additionally, the wave was reverse ray‐traced, and the results show its path crossing the Moon's shadow of the total solar eclipse in the tropical North Atlantic ocean at stratospheric altitudes. Investigation about potential driving sources for this wave indicates the total solar eclipse as the most likely candidate. The optical measurements were part of an observational campaign carried out to detect the impact of the August 21 eclipse in the atmosphere at low latitudes. Plain Language Summary: The Moon's shadow during a total solar eclipse introduces horizontal temperature gradients in the atmosphere and screens the ozone layer from solar heating. The shadow also travels supersonically, producing instabilities that can generate the so‐called atmospheric gravity wave (AGW). AGWs associated with eclipses are expected to have periodic oscillations with periods ranging from just a few minutes to hours. Additionally, these AGWs can have horizontal wavelengths as large as thousands of kilometers. It is also possible to estimate the propagation path of the AGWs into the atmosphere by solving a system of equations that govern their propagation. This methodology is similar to that of tracing a ray of light that propagates in a varying environment. In the present work, an AGW in the northeast of Brazil was observed with spectral characteristics that indicate association with the 21 August 2017 total solar eclipse. In addition, the ray path matched the Moon's shadow in the stratosphere corroborating with the observational inferences. The AGW was observed by optical instruments during the nighttime, more than 3 h after the end of the eclipse and over 2,000 km away from the Moon's shadow. Key Points: A multi‐instrumented observational campaign was carried out in Brazil to study the effects of the 21 August 2017 solar eclipseAtmospheric gravity waves were observed in the airglow over the Northeastern BrazilAnalyses including reverse ray tracing suggested the eclipse as the likely source for one observed medium‐scale gravity wave [ABSTRACT FROM AUTHOR]

The radar volume reflectivity and turbulent kinetic energy dissipation rate in the Antarctic mesosphere were estimated from the polar mesosphere winter echoes (PMWE) recorded using a vertical beam of the PANSY radar, a Mesosphere‐Stratosphere‐Troposphere radar at Syowa Station (69°S, 40°E), over a period of 4 years. The observed radar volume reflectivity exhibits a lognormal distribution in the range of 2 × 10−18 to 5 × 10−15 m−1 for a height region of 55–82 km. The turbulent energy dissipation rate estimated from the spectral widths of the PMWE ranges from 10−4 to 100 m2 s−3. From monthly histograms of the turbulent energy dissipation rate for a fixed solar zenith angle (SZA) and height, it was found that the summer‐to‐winter transition of the turbulent energy dissipation rate occurs in March, while the winter‐to‐summer transition occurs in September. This seasonal variation agrees well with that of gravity wave activity, suggesting that the turbulence in the mesosphere is likely caused by gravity wave breaking. Key Points: The turbulent kinetic energy dissipation rates in the Antarctic mesosphere from PMWE spectral widths are estimatedThe dissipation rates at heights of 55–82 km are in the range of 10–4 to 100 m2 s–3The summer‐to‐winter (winter‐to‐summer) transition of the turbulence parameters occurs in April (September) [ABSTRACT FROM AUTHOR]

Based on the round‐trip intelligent sounding system, the structure function and spectrum analysis method were carried out to analyze the scale interactions between the small‐scale gravity wave and turbulence in the middle stratosphere. The result showed that the generation of turbulence was closely related to the Kelvin‐Helmholtz (KH) billows. An upscale energy cascade occurred on the spatial scale of KH billows, while a downscale energy cascade occurred on a larger spatial scale (gravity wave scale), which resulted in the breaking of the gravity wave and generation of turbulence. From the multilevel signal measurement, the obvious interactions between turbulence and gravity can be found. The two‐dimensional turbulence that the inverse energy cascade in 2‐D exists simultaneously with a direct enstrophy cascade to the small scales was found, which is considered to be the first‐hand observation result of relevant height. Key Points: The two‐dimensional turbulence is found with bidirectional energy transmissionKH billows are considered to be an important factor in the excitation of turbulenceConvergence and divergence on different scales cause instability within the wave [ABSTRACT FROM AUTHOR]

The tropical cyclone (TC)‐induced concentric gravity waves (CGWs) are capable of propagating upward from convective sources in the troposphere to the upper atmosphere and creating concentric traveling ionospheric disturbances (CTIDs). To examine the CGWs propagation, we implement tropical cyclone‐induced CGWs into the lower boundary of global ionosphere‐thermosphere model with local‐grid refinement (GITM‐R) and simulate the influence of CGWs on the ionosphere and thermosphere. GITM‐R is a three‐dimensional non‐hydrostatic general circulation model for the upper atmosphere with the local‐grid refinement module to enhance the resolution at the location of interest. In this study, CGWs induced by the typhoon Meranti in 2016 have been simulated. Information of the TC shape and moving trails is obtained from the TC best‐track dataset, and the gravity wave patterns are specified at the lower boundary of GITM‐R (100 km altitude). The horizontal wavelength and phase velocity of wave perturbation at the lower boundary are specified to be consistent with the TEC observations. The simulation reveals a clear evolution of CTIDs, which shows reasonable agreement with the GPS‐TEC observations. This is the first time the typhoon‐driven TEC perturbation has been simulated in a general circulation model. To further examine the dependence of the CTIDs on the wavelength and frequency of the gravity wave perturbation at the lower boundary, different waveforms have been tested as well. The magnitude of CTIDs has a negative correlation with the period but a positive correlation with the wavelength when the horizontal phase velocities are sufficiently fast against the critical‐level absorption. Key Points: Tropical cyclone‐induced gravity waves have been simulated using a general circulation model (GCM) for the upper atmosphereThe model has been used is the 3‐D global ionosphere‐thermosphere model with local‐grid refinement (GITM‐R)The simulation results reveal a clear evolution of traveling ionospheric disturbances (TIDs) and show a reasonable agreement with observations [ABSTRACT FROM AUTHOR]

The variations of the horizontal phase velocity of an internal gravity wave, generated by wave "blocking" or "reflection" due to an inhomogeneous wind field, have been predicted theoretically and numerically investigated but had yet to be captured experimentally. In this paper, through a collaborative observation campaign using a sodium (Na) Temperature/Wind lidar and a collocated Advanced Mesospheric Temperature Mapper (AMTM) at Utah State University (USU), we report the first potential evidence of such a unique gravity wave process. The study shows that a small‐scale wave, captured by the AMTM, with initial observed horizontal phase velocity of 37 ± 5 m/s toward the northwest direction, experienced a large and increasing headwind as it was propagating in the AMTM field of view. This resulted in significant deceleration along its initial traveling direction, and it became quasi‐stationary before it was "reflected" to the opposite direction at later time. The USU Na lidar measured the horizontal wind and temperature during the event, when the wave was found traveling within a temperature inversion layer and experiencing an increasing headwind relative to the wave. The wind agrees well with the expected value for wave blocking suggested by the wave tracing theory, implying the existence of a large horizontal wind gradient that night near the OH layer altitudes. The study indicates the critical role of horizontal winds and their horizontal gradients in determining propagation in vertical and horizontal directions. Key Points: We observe blocking and reflection of a small‐scale gravity waveThe wave blocking event occurred when the wave was experiencing strong and increasing headwindThe wave was propagating in an upper mesosphere temperature inversion layer formed within the OH layer [ABSTRACT FROM AUTHOR]

We have studied atmospheric gravity waves (AGWs) and nighttime medium‐scale traveling ionospheric disturbances (MSTIDs) by applying three‐dimensional spectral analysis technique to 557.7‐ and 630.0‐nm airglow images at Shigaraki (SGK) (35°N, 136°E, 1999–2017) and Rikubetsu (RIK) (44°N, 144°E, 1999–2017), Japan, Athabasca (ATH), Canada, (55°N, 247°E, 2005–2017), and Magadan (MGD), Russia (60°N, 151°E, 2008–2017), focusing on their horizontal wavenumber spectra. For the AGWs in 557.7‐nm images, the power spectra in summer are stronger than in other seasons, probably due to stronger tropospheric convection. The highest energy content of the waves are mostly at wavelengths between 20 and 300 km at MGD, ATH, and RIK, while it is above 200 km at SGK. The largest power spectral density is obtained at RIK at wavelengths of 30–100 km and then ATH. The slopes of the horizontal wavenumber spectra varies from −2.77 to −3.22. From the MSTIDs in 630.0‐nm images, the power spectra in summer at RIK and SGK are stronger than those in other seasons regardless of solar activity. The power spectra in solar quiet time are stronger than those in solar active time at all four stations. These features can be explained by the Perkins instability with coupling between sporadic E and F layers. The spectral slope decreases with increasing latitudes. Weak positive correlations were obtained between the daily wave power of AGWs in 557.7‐nm images and MSTIDs in 630.0‐nm images, suggesting that the MSTIDs in the thermosphere may be partially generated by the AGWs from the mesopause region. Plain Language Summary: In this paper we study atmospheric gravity waves (AGWs) and nighttime medium‐scale traveling ionospheric disturbances (MSTIDs) observed at four stations in Japan, Canada, and Russia, in 557.7‐ and 630.0‐nm airglow images over more than 10 years. The 557.7‐nm airglow has an emission layer at altitudes of 90–100 km (mesopause region). The waves seen in the 557.7‐nm airglow images mainly indicates AGWs. The 630.0‐nm airglow has an emission layer at altitudes of 200–300 km (bottomside ionosphere). The waves seen in the 630.0‐nm airglow images mainly indicates MSTIDs in the ionosphere. The AGWs in the mesopause region are the main driver of global atmospheric circulation in the middle atmosphere. The MSTIDs in the bottomside ionosphere are one of the causes of the satellite positioning error. We show typical energy content, propagation direction, and wavelengths of these waves at these two altitudes and discuss possible reason of the observed characteristics. These results contribute to our understanding of generation and propagation of AGWs and MSTIDs in the upper atmosphere. Key Points: We conducted the statistical analysis of the horizontal wavenumber spectra of waves in airglow images in Japan, Canada, and RussiaDifferences of horizontal wavenumber spectra at different locations, seasons, and solar activities are quantitatively obtainedWeak positive correlation of the power between atmospheric gravity waves and medium‐scale traveling ionospheric disturbances was observed [ABSTRACT FROM AUTHOR]

Convective bursts occur frequently in tropical cyclones and help their intensification by diabatic heating, but their quantitative importance has not been established. By using the high‐frequency observation of infrared brightness temperature with Himawari‐8, a latest‐generation geostationary meteorological satellite, convective bursts in Typhoon Lan (2017) were studied. Aided with a series of numerical simulations, it was revealed that the anvil edges of many bursts are associated with finite‐amplitude gravity waves consistent with internal bores, creating warm anomalies by subsidence ahead of the edges. As the edges spread, they are thinned, and their propagation speeds are often decreased. In many such instances, gravity waves, now linear, are separated from the edges to propagate away, spreading convective heating. It is proposed that by quantifying these processes with geostationary satellites, diabatic heating by convective bursts can be estimated to diagnose their impacts on tropical‐cyclone intensification. Plain Language Summary: Convective bursts (CBs) are intense long‐lasting cumulonimbus that occur frequently in tropical cyclones (TCs). Many studies suggested that condensation heating associated with CBs is an important factor in TC intensification but to what extent they are important has not been established. The latest‐generation (also known as the third‐generation) geostationary meteorological satellites such as Himawari‐8 provided opportunities to observe TCs at frequencies much higher than before. We studied the anvil clouds (outflow clouds near the tropopause) of CBs in Typhoon Lan (2017) by using infrared images from Himawari‐8. We also conducted numerical simulations to help interpretation. It was found that the anvil of a CB frequently spreads as an internal gravity wave in such a way that the anvil edge is preceded by the subsidence and temperature increase due to an internal bore (finite‐amplitude gravity wave similar to tidal bore). Since the anvil spreads circularly, its edge is thinned gradually. It was found that its expansion speed is often slowed down, and the gravity wave is separated to propagate further ahead, disseminating convective heating. It is proposed that geostationary‐satellite observations can be used to estimate convective heating associated with each CB. It will help establish the impacts of CBs in TC intensification. Key Points: Intensifying tropical cyclones often have convective bursts with circular shaped anvilsThe anvil edge often starts to spread as a finite‐amplitude internal gravity wave and proceeds to emanate a linear gravity waveIt is proposed that geostationary satellites can be used to quantify the impact of convective burst on tropical‐cyclone intensification [ABSTRACT FROM AUTHOR]

The disturbances of the ionosphere caused by thunderstorms or lightning events in the troposphere have an impact on global navigation satellite system (GNSS) signals. Gravity waves (GWs) triggered by thunderstorms are one of the main factors that drive short-period Travelling Ionospheric Disturbances (TIDs). At mid-latitudes, ionospheric GWs can be detected by GNSS signals. However, at low latitudes, the multi-variability of the ionosphere leads to difficulties in identifying GWs induced by thunderstorms through GNSS data. Though disturbances of the ionosphere during low-latitude thunderstorms have been investigated, the explicit GW observation by GNSS and its propagation pattern are still unclear. In this paper, GWs with periods from 6 to 20 min are extracted from band-pass filtered GNSS carrier phase observations without cycle-slips, and 0.2–0.8 Total Electron Content Unit (TECU) magnitude perturbations are observed when the trajectories of ionospheric pierce points fall into the perturbed region. The propagation speed of 102.6–141.3 m/s and the direction of the propagation indicate that the GWs are propagating upward from a certain thunderstorm at lower atmosphere. The composite results of disturbance magnitude, period, and propagation velocity indicate that GWs initiated by thunderstorms and propagated from the troposphere to the ionosphere are observed by GNSS for the first time in the low-latitude region.

Accurate precipitation prediction is of great significance for regional flood control and disaster mitigation. This study introduced a prediction model based on the least square support vector machine (LSSVM) optimized by the genetic algorithm (GA). The model was used to estimate the precipitation of each meteorological station over the source region of the Yellow River (SRYE) in China for 12 months. The Ensemble empirical mode decomposition (EEMD) method was used to select meteorological factors and realize precipitation prediction, without dependence on historical data as a training set. The prediction results were compared with each other, according to the determination coefficient (R2), mean absolute errors (MAE), and root mean square error (RMSE). The results show that sea surface temperature (SST) in the Niño 1 + 2 region exerts the largest influence on accuracy of the prediction model for precipitation in the SRYE (RSST2= 0.856, RMSESST= 19.648, MAESST= 14.363). It is followed by the potential energy of gravity waves (Ep) and temperature (T) that have similar effects on precipitation prediction. The prediction accuracy is sensitive to altitude influences and accurate prediction results are easily obtained at high altitudes. This model provides a new and reliable research method for precipitation prediction in regions without historical data.

Numerous recent studies report the Coseismic Tropospheric Disturbances (CTDs) during large earthquakes. Their presence suggests the importance of atmospheric seismology in a possible earthquake forecasting scenario. The origin mechanism and associated energetics of CTDs are not well understood though the observations associate them with the atmospheric waves. We present the numerical modeling of coupled dynamics of Gravity waves (GWs) and convective instability (CI) in the dry troposphere that produces the CTDs, in the form of pressure disturbances, of observed magnitudes. The study reveals the altitude and epicentral distribution of CTDs and elaborates the relative role of GWs and CI in the generation and intensification of CTDs. The study finds that mega and strong earthquakes disturb the troposphere to a similar level as the severe meteorological weather.

Abstract The Swarm satellites observed small-amplitude (0.1–5 nT) magnetic fluctuations perpendicular to the geomagnetic field. These so-called magnetic ripples (MRs) have a period of around a few tens of seconds along the satellite orbit in the topside ionosphere at middle and low latitudes. They are spatial structures from small-scale field-aligned currents. We investigated the following three characteristics of the MRs. First, we used Swarm observations to confirm their basic characteristics obtained from the Challenging Minisatellite Payload satellite. That is, the global distribution of the average MR amplitudes has clear geographic, seasonal, and local time dependence that is highly correlated with ionospheric conductivities. Second, we found that the average amplitudes of the MRs derived from the Swarm-B satellite, which flies at a ~50 km higher altitude, are slightly smaller than those of the Swarm-A and Swarm-C. This difference suggests that the location of the origin of MRs is below ~460 km altitude, i.e., not in the magnetosphere. Last, to provide evidence of correlation between the MRs and meteorological phenomena, we performed statistical and event analyses with typhoon track data, which are a source of acoustic and gravity waves. The data from 54 typhoons during the period from November 26, 2013, to July 31, 2016, were used for statistical analysis. The results show that the average amplitudes of the MRs during typhoon activity on the dawn, dusk, and night sides are larger than those during non-typhoon conditions. Event analyses indicate amplitude enhancements of the MRs around typhoons, and the latitude of the enhancement migrated with the typhoon. These analyses indicate that typhoon activity is correlated with MR activity and that cumulus convection activity other than typhoons may also affect MR amplitudes. Graphical abstract .

Gravity waves impacting ice shelves illicit a suite of responses that can affect ice shelf integrity. Broadband seismometers deployed on the Ross Ice Shelf, complemented by a near‐icefront seafloor hydrophone, establish the association of strong icequake activity with ocean gravity wave amplitudes (AG) below 0.04 Hz. The Ross Ice Shelf‐front seismic vertical displacement amplitudes (ASV) are well correlated with AG, allowing estimating the frequency‐dependent transfer function from gravity wave amplitude to icefront vertical displacement amplitude (TGSV(f)). TGSV(f) is 0.6–0.7 at 0.001–0.01 Hz but decreases rapidly at higher frequencies. Seismicity of strong icequakes exhibits spatial and seasonal associations with different gravity wave frequency bands, with the strongest icequakes observed at the icefront primarily during the austral summer when sea ice is minimal and swell impacts are strongest. Key Points: Rift and icefront icequakes were identifiedIcefront icequakes are most frequent during the austral summer when swell forcing is greatest in the absence of sea iceAn empirical ocean‐to‐ice shelf displacement transfer function is obtained [ABSTRACT FROM AUTHOR]

Instances of multiple excitation of large‐scale gravity waves in the auroral region by solar wind Alfvénic fluctuations are rare. In this paper we report an unusual instance, in which sequential large‐scale gravity waves launched from northern and southern auroral regions are clearly linked to intermittent southward magnetic fields of Alfvénic fluctuations embedded in three successive stream interaction regions. The continuous gravity wave activity persists for ∼7 days, the longest duration currently tracked. The Alfvénic fluctuations are mainly dominated by Alfvén and slow waves, which might play a role in the solar wind‐magnetosphere interaction, leading to impulsive auroral electrojet activity and in turn exciting the sequential gravity waves. Key Points: Instances of multiple excitation of large‐scale gravity waves by Alfvénic fluctuations are rareSequential traveling atmospheric disturbances (TADs) observed over ∼7 daysAlfvénic fluctuations might play a role in driving impulsive auroral electrojet activity, which excites gravity waves [ABSTRACT FROM AUTHOR]

We report the first extraterrestrial observation of traveling ionospheric disturbances (TIDs) in the ionosphere of Mars (below ≈160 km) by NASA's Mars Atmosphere and Volatile EvolutioN (MAVEN) spacecraft. TIDs are ionospheric waves manifesting as an oscillation in density, velocity, and composition of ionospheric plasma. The TIDs were encountered in the Martian dynamo region, where ions are collisionally bound to the neutral thermosphere but electrons remain frozen in to the local magnetic field. The underlying physical mechanism for the periodic ionospheric waves appears to be a bulk motion of the entire ionosphere driven by atmospheric gravity waves in the surrounding thermosphere which have propagated upward from the lower atmosphere. Magnetic perturbations were observed in association with the TIDs, which we attribute to electromagnetic induction from the bulk motion of the ionosphere. We thus demonstrate that atmospheric disturbances near the surface of Mars can propagate upward through the entire thickness of the atmosphere and drive ionospheric perturbations at spacecraft altitudes. Key Points: We report the first extraterrestrial observation of traveling ionospheric disturbances at MarsMartian TIDs result from atmospheric gravity waves which have propagated upward from the lower atmosphereTIDs generate magnetic effects not seen at Earth and may be a key driver of turbulent mixing above the homopause [ABSTRACT FROM AUTHOR]

Stationary features indicative of topographic gravity waves were identified at the cloud top of Venus with the 283‐nm channel of the Ultraviolet Imager (UVI) onboard Akatsuki, and their geographical and local time dependences were studied. At this wavelength the absorption by SO2 dominates. To extract stationary structures with respect to the surface, we averaged multiple images to smooth out moving features and applied high‐pass filtering to emphasize small structures. We found that stationary features appear exclusively above highlands and that they tend to appear between noon and evening. The stationary features seem to be synchronized with those observed in the cloud top temperature maps taken by the Longwave Infrared Camera (LIR). It was shown using a gravity wave model that the scale height of SO2 should be smaller than that of the cloud around the cloud top to reproduce the observed phase relationship between the stationary features seen in the Ultraviolet Imager and Longwave Infrared Camera images. Plain Language Summary: Topographic gravity waves, which are atmospheric waves created by winds flowing over mountains, are found to be ubiquitous in the Venus atmosphere. These waves are expected to play crucial roles in the momentum balance of the global‐scale atmospheric circulation. The spatial and temporal variabilities of such waves were studied using 283‐nm images taken by an ultraviolet camera onboard the Venus orbiter Akatsuki. To extract cloud structures related to topographic gravity waves that are stationary with respect to the surface, multiple images were averaged to smooth out moving features. Topographic waves were found to appear above highlands from noon to evening. The vertical distribution of SO2, which is the main absorber at this wavelength, around the cloud top was constrained from the observed wave features based on the modeling of wave‐induced transport. Key Points: Stationary features were observed at the cloud top of Venus in the ultraviolet and attributed to topographic gravity wavesThe features appear above highlands and between noon and the eveningThe scale height of SO2 was constrained based on modeling of the observed features [ABSTRACT FROM AUTHOR]

Rapp, Dörnbrack, and Preusse (2018, https://doi.org/10.1029/2018GL079142) uses radio occultation observations and meteorological data from the European Centre for Medium Range Weather Forecasting to quantify the effect of inertial instability on stratospheric temperature variability. A very interesting by‐product of their study is the take‐away message that inertial instability "pancake structures" may be misidentified as being caused by gravity waves. We place this result in the context of inertial instability research. We concur with Rapp et al.'s caution to researchers to first examine the large‐scale meteorological environment to rule out inertial instability when analyzing temperature fluctuations that appear to be gravity wave signals in vertical profile data. Plain Language Summary: Rapp, Dörnbrack, and Preusse (2018) uses satellite observations and meteorological data to quantify the effect of inertial instability on stratospheric temperature variability. A very interesting by‐product of their study is the take‐away message that inertial instability "pancake structures" may be misidentified as being caused by gravity waves. We place this result in the context of inertial instability research. We concur with Rapp et al.'s caution to researchers to first examine the large‐scale meteorological environment to rule out inertial instability when analyzing temperature fluctuations that appear to be gravity wave signals in vertical profile data. Key Points: This commentary on Rapp, Dörnbrack, and Preusse (2018) reinforces the significance of their results and provides historical contextInertial instability "pancake structures" can masquerade as gravity wave signaturesResearchers must examine the large‐scale meteorological situation before attributing temperature perturbations to gravity waves [ABSTRACT FROM AUTHOR]

The present study investigates the global vertical wave number spectra of potential energy density in the stratosphere by utilizing the COSMIC temperature data in the period January 2007 to February 2014. The statistical analysis shows that the spectral amplitude (defined as the maximum spectral energy density) varies from 0.1 × 104 m2 s−2 (cycle/m)−1 to 3.2 × 104 m2 s−2 (cycle/m)−1, generally smaller than the theoretical saturated spectral amplitude, with remarkable temporal and spatial variations. Due to the latitudinal and seasonal variations of the background winds and wave sources, the amplitude exhibits obvious quasi‐biennial, annual and semi‐annual cycles at low latitudes, and the annual cycle at middle and high latitudes. The spectral slope, ranging from −1.6 to −3.0, is systematically less negative than the canonical value of −3, and exhibits a quasi‐biennial cycle at low latitudes, annual cycle at middle and high latitudes, and semi‐annual cycle at boreal high latitudes. Different from the amplitude, the slope shows weaker temporal and latitudinal variations, strongly indicating a more universal feature of the slope, which mainly results from the wave propagation process (such as the wave–wave or wave–background flow interactions) rather than the wave source characteristics. Furthermore, we statistically study the longitudinal variation of the spectral parameters. The spectral amplitude at different latitudes shows an obvious and distinctive longitudinal distribution, which is attributed to the latitude‐dependent jet and front mechanisms and/or orography. However, the slope has no evident longitudinal distribution except over around 120°W, 67°W and 85°E. This may be accounted for by the fact that orography‐excited gravity waves are easily influenced by the background winds due to their low frequencies. Interestingly, the amplitude and slope have negative correlation at most latitudes. Larger spectral amplitudes could mean that the gravity wave spectrum is more strongly saturated, resulting in a spectral slope closer to the theoretical value of −3. Time–latitude variations of the spectral slope S. The dashed lines denote the start of each year. The blank indicates the lack of available data [ABSTRACT FROM AUTHOR]