Secondary Gravity Waves Research Articles

Airglow-imager based observation of possible influences of subtropical mesospheric gravity waves on F-region ionosphere over Jammu & Kashmir, India

As a joint research collaboration between the National Atmospheric Research Laboratory (NARL), and the University of Kashmir (KU), NARL installed an all-sky airglow CCD imager (with centre wavelengths of 630 nm, 557.7 nm [2 nm band widths] and 840 nm [150 nm wide band with blocking notch at 866 nm to avoid the contamination of molecular oxygen emissions]) in the University campus in Srinagar (75°E, 34°N, geographic), Jammu and Kashmir, India (western Himalayan region). To understand the upper atmospheric dynamics and ionospheric electrodynamics and their associated physical coupling mechanisms, the imager observes airglow emissions of OH molecules (~ 85 km height; 840 nm) and atomic oxygen occurring at the heights of ~ 97 km (557.7 nm) and ~ 250 km (630 nm). Airglow observations in Kashmir commenced in the night of August 11, 2017 and the present work reports on the characteristics of first-time observation of Medium Scale Travelling Ionospheric Disturbances (MSTIDs with horizontal wavelengths of ~ 100–300 km) over Kashmir region during 20:30—22:30 IST (Indian standard time) on August 15, 2017 (India independence day). Initially, the phase front of MSTIDs was aligned along the north-west and south-east direction and moved at ~ 57 m/s towards the south-west direction and finally the westward direction by aligning along the meridian before they disappeared. Along with SAMI-3 ionospheric model simulations, simultaneous multiwavelength airglow observations indicate that secondary gravity waves generated due to dissipation of upward propagating mesospheric gravity waves in the heights of ~ 85–95 km would have contributed to the generation of MSTIDs in the F region ionospheric plasma through electrodynamical coupling between the E and F region (Perkins instability) ionosphere.

Concentric Secondary Gravity Waves in the Thermosphere and Ionosphere Over the Continental United States on March 25–26, 2015 From Deep Convection

AbstractWe examine the total electron content (TEC) from GPS receivers over the United States on March 25–26, 2015. We observe partial to nearly fully concentric rings of traveling ionospheric disturbances (TIDs) with centers close to deep convection. Many of these TIDs have observed horizontal phase speeds cH > 300 m/s, suggesting they are induced by gravity waves (GWs) created in the thermosphere. We investigate the largest‐amplitude concentric TIDs at 23:00 UT on March 25 and 01:20 UT on March 26. We find that cH and the GW period τr increase linearly with radius and the horizontal wavelength, λH, increases quadratically with radius. This is expected if the GWs are excited by point sources. For these GWs, cH = 150–530 m/s, τr ∼ 8–40 min, and λH ∼ 100–500 km. Using reverse ray‐tracing, no GW with cH > 200 m/s propagates below z = 100 km, 73% of the GWs in the first case cannot propagate below z ∼ 100 km, all of the GWs in the second case cannot propagate below z ∼ 100 km, and the inferred thermospheric point sources are ∼2–4° from deep convection. Because the underlying GWs are most likely excited by a point source and most must be created in the thermosphere, we find that these concentric TIDs are most likely induced by GWs generated in the thermosphere, including those with cH = 150–200 m/s. Their close proximity to deep convection and the TEC map asymmetries suggest these TIDs are likely induced by secondary GWs from local horizontal body forces created by the dissipation of primary GWs from deep convection.

Solar activity dependence of medium-scale traveling ionospheric disturbances using GPS receivers in Japan

In order to reveal solar activity dependence of the medium-scale traveling ionospheric disturbances (MSTIDs) at midlatitudes, total electron content (TEC) data obtained from a Global Positioning System (GPS) receiver network in Japan during 22 years from 1998 to 2019 were analyzed. We have calculated the detrended TEC by subtracting the 1-h running average from the original TEC data for each satellite and receiver pair, and made two-dimensional TEC maps of the detrended TEC with a spatial resolution of 0.15° × 0.15° in longitude and latitude. We have investigated MSTID activity, defined as delta I/overline{I}, where delta I and overline{I} are standard deviation of the detrended TEC and the average vertical TEC within the area of 133.0°–137.0° E and 33.0°–37.0° N for 1 h, respectively. From each 2-h time series of the detrended TEC data within the same area as the MSTID activity, auto-correlation functions (ACFs) of the detrended TEC were calculated to estimate the horizontal propagation velocity and direction of the MSTIDs. Statistical results of the MSTID activity and propagation direction of MSTIDs were consistent with previous studies and support the idea that daytime MSTIDs could be caused by atmospheric gravity waves, and that nighttime MSTIDs were caused by electro-dynamical forces, such as the Perkins instability. From the current long-term observations, we have found that the nighttime MSTID activity and occurrence rate increased with decreasing solar activity. For the daytime MSTID, the occurrence rate increased with decreasing solar activity, whereas the MSTID activity did not show distinct solar activity dependence. These results suggest that the secondary gravity waves generated by dissipation of the primary gravity waves propagating from below increase under low solar activity conditions. The mean horizontal phase velocity of the MSTIDs during nighttime did not show a distinct solar activity dependence, whereas that during daytime showed an anticorrelation with solar activity. The horizontal phase velocity of the daytime MSTIDs was widely distributed from 40 to 180 m/s under high solar activity conditions, whereas it ranged between 80 and 200 m/s, with a maximum occurrence at 130 m/s under low solar activity conditions, suggesting that gravity waves with low phase velocity could be dissipated by high viscosity in the thermosphere under low solar activity conditions.

Numerical Simulation of Mountain Waves over the Southern Andes. Part I: Mountain Wave and Secondary Wave Character, Evolutions, and Breaking

AbstractThis paper addresses the compressible nonlinear dynamics accompanying increasing mountain wave (MW) forcing over the southern Andes and propagation into the mesosphere and lower thermosphere (MLT) under winter conditions. A stretched grid provides very high resolution of the MW dynamics in a large computational domain. A slow increase of cross-mountain winds enables MWs to initially break in the mesosphere and extend to lower and higher altitudes thereafter. MW structure and breaking is strongly modulated by static mean and semidiurnal tide fields exhibiting a critical level at ~114 km for zonal MW propagation. Varying vertical group velocities for different zonal wavelengthsλxyield initial breaking in the lee of the major Andes peaks forλx~ 50 km, and extending significantly upstream for largerλxapproaching the critical level at later times. The localized extent of the Andes terrain in latitude leads to “ship wave” responses above the individual peaks at earlier times, and a much larger ship-wave response at 100 km and above as the larger-scale MWs achieve large amplitudes. Other responses above regions of MW breaking include large-scale secondary gravity waves and acoustic waves that achieve very large amplitudes extending well into the thermosphere. MW breaking also causes momentum deposition that yields local decelerations initially, which merge and extend horizontally thereafter and persist throughout the event. Companion papers examine the associated momentum fluxes, mean-flow evolution, gravity wave–tidal interactions, and the MW instability dynamics and sources of secondary gravity waves and acoustic waves.

On the Seeding of Periodic Equatorial Plasma Bubbles by Gravity Waves Associated With Tropical Cyclone: A Case Study

AbstractUnderstanding the day‐to‐day variability of equatorial plasma bubble (EPB) irregularities has been a major challenge for many decades that makes the deterministic prediction of EPBs remain elusive. In the present study, we report a case of intense and periodic EPBs observed during 8 and 9 April 2013 by the 47‐MHz Equatorial Atmosphere Radar at Kototabang, Indonesia. The periodic EPBs separated by about 200–250 km were initiated before sunset. The presunset onset and development of these periodic EPBs were discussed in light of the gravity waves (GWs) excited in connection with the deep convection due to the tropical cyclone (TC) Victoria. The outgoing long‐wave radiation measurement by very high‐resolution radiometer (VHRR) onboard Indian meteorological satellite Kalpana‐1 shows the occurrence of deep convective activity during these days. The presence of upward propagating GWs from the deep convective region associated with TC Victoria was confirmed using the GPS radio occultation (GPS‐RO) observations in the troposphere and stratosphere. The GW signatures at ionospheric altitudes were also observed from the Ionosonde observations over magnetic equator, and medium‐scale (~300 km) GWs were observed from the GPS‐TEC (Total Electron Content) data near to the magnetic equator and cyclone center. From the GW parameters observed from GPS‐TEC and GPS‐RO, we surmise that the secondary GWs generated by the dissipation of primary GWs associated with TC Victoria could have served as a seeding source on the generation of periodic EPBs during these two consecutive days.

First Direct Observational Evidence for Secondary Gravity Waves Generated by Mountain Waves Over the Andes

AbstractA mountain wave with a significant brightness temperature amplitude and ~500 km horizontal wavelength was observed over the Andes on 24–25 July 2017 in Atmospheric Infrared Sounder (AIRS)/Aqua satellite data. In the Modern‐Era Retrospective Analysis for Research and Applications, version 2 (MERRA‐2), reanalysis data, the intense eastward wind flowed over the Andes. Visible/Infrared Imaging Radiometer Suite (VIIRS)/Suomi‐NPP (National Polar‐orbiting Partnership) did not detect the mountain waves; however, it observed concentric ring‐like waves in the nightglow emissions at ~87 km with ~100 km wavelengths on the same night over and leeward of the Southern Andes. A ray tracing analysis showed that the mountain waves propagated to the east of the Andes, where concentric ring‐like waves appeared above a region of mountain wave breaking. Therefore, the concentric ring‐like waves were likely secondary waves generated by momentum deposition that accompanied mountain wave breaking. These results provide the first direct evidence for secondary gravity waves generated by momentum deposition.

Secondary Gravity Waves Generated by Breaking Mountain Waves Over Europe

AbstractA strong mountain wave, observed over Central Europe on 12 January 2016, is simulated in 2D under two fixed background wind conditions representing opposite tidal phases. The aim of the simulation is to investigate the breaking of the mountain wave and subsequent generation of nonprimary waves in the upper atmosphere. The model results show that the mountain wave first breaks as it approaches a mesospheric critical level creating turbulence on horizontal scales of 8–30 km. These turbulence scales couple directly to horizontal secondary waves scales, but those scales are prevented from reaching the thermosphere by the tidal winds, which act like a filter. Initial secondary waves that can reach the thermosphere range from 60 to 120 km in horizontal scale and are influenced by the scales of the horizontal and vertical forcing associated with wave breaking at mountain wave zonal phase width, and horizontal wavelength scales. Large‐scale nonprimary waves dominate over the whole duration of the simulation with horizontal scales of 107–300 km and periods of 11–22 minutes. The thermosphere winds heavily influence the time‐averaged spatial distribution of wave forcing in the thermosphere, which peaks at 150 km altitude and occurs both westward and eastward of the source in the 2 UT background simulation and primarily eastward of the source in the 7 UT background simulation. The forcing amplitude is 2 that of the primary mountain wave breaking and dissipation. This suggests that nonprimary waves play a significant role in gravity waves dynamics and improved understanding of the thermospheric winds is crucial to understanding their forcing distribution.

Self-Acceleration and Instability of Gravity Wave Packets: 3. Three-Dimensional Packet Propagation, Secondary Gravity Waves, Momentum Transport, and Transient Mean Forcing in Tidal Winds.

Dong et al. (2020, https://doi.org/10.1029/2019JD030691) employed a new compressible model to examine gravity wave (GW) self‐acceleration dynamics, instabilities, secondary gravity wave (SGW) generation, and mean forcing for GW packets localized in two dimensions (2D). This paper extends the exploration of self‐acceleration dynamics to a GW packet localized in three dimensions (3D) propagating into tidal winds in the mesosphere and thermosphere. As in the 2D packet responses, 3D GW self‐acceleration dynamics are found to be significant and include 3D GW phase distortions, stalled GW vertical propagation, local instabilities, and SGW and acoustic wave generation. Additional 3D responses described here include refraction by tidal winds, localized 3D instabilities, asymmetric SGW propagation, reduced SGW and acoustic wave responses at higher altitudes relative to 2D responses, and forcing of transient, large‐scale, 3D mean responses that may have implications for chemical and microphysical processes operating on longer time scales.

Self‐Acceleration and Instability of Gravity Wave Packets: 2. Two‐Dimensional Packet Propagation, Instability Dynamics, and Transient Flow Responses

AbstractResults of two‐dimensional and narrow three‐dimensional (2‐D and 2.5‐D) simulations of a gravity wave (GW) packet localized in altitude and along its propagation direction employing a new, versatile compressible model are described. The simulations explore self‐acceleration and instability dynamics in an idealized atmosphere at rest under mean solar conditions in a domain extending to an altitude of 260 km and 1,800 km horizontally without artificial dissipation. High resolution in the central 2.5‐D domain enables the description of 3‐D instability dynamics accounting for breaking, dissipation, and momentum deposition within the GW packet. 2‐D results describe responses to localized self‐acceleration effects, including generation of secondary GWs (SGWs) at larger scales able to propagate to much higher altitudes. 2.5‐D results exhibit instability forms consistent with previous 3‐D simulations of instability dynamics and cause SGW generation and propagation at smaller spatial scales to weaken significantly compared to the 2‐D results. SGW responses at larger scales are driven primarily by GW/mean flow interactions arising at early stages of the self‐acceleration dynamics prior to strong GW instabilities and dissipation. As a result, they exhibit similar responses in both the 2‐D and 2.5‐D simulations and readily propagate to high altitudes at large distances from the initial GW packet. A companion paper examines these dynamics for an initial GW packet localized in three dimensions and evolving in a representative 3‐D tidal wind field.

Physical Effects of the Lipetsk Meteoroid: 3

Comprehensive modeling studies of the processes induced in all geospheres by the passage and explosion of the meteoroid near the city of Lipetsk (Russia) on June 21, 2018, have been conducted. Magnetic, electric, electromagnetic, ionospheric, and seismic effects, as well as the effects of acoustic-gravity waves have been estimated. The magnetic effect of turbulence has been shown to be insignificant. The magnetic effect of the ionospheric currents and the current in the wake of the meteoroid could be substantial (~1 nT). Under the action of an external electric field, a transient current pulse with the strength of current up to 104 A could occur. The electrostatic effect could be accompanied by the accumulation of an electric charge of 1 mC producing the electric field intensity of 0.01–1 MV/m. The flow of the electric current in the wake of the meteoroid could result in the generation of an electromagnetic pulse in the 40–80 kHz band with the electric field intensity of 1–10 V/m. The electromagnetic effect of infrasound has been determined to be significant (1–10 V/m and 1–10 nT). The absorption of the shock wave at ionospheric dynamo region altitudes (100–150 km) could generate secondary atmospheric gravity waves with the 0.1–1 relative amplitude. The passage of the meteoroid acted to produce a plasma wake and noticeable disturbance not only in the lower but also in the upper atmosphere in the range of no less than 1000 km. The possibility of occurrence of the electrophonic effect, the generation of the ion and magnetic sound by infrasound, and the generation of gradient-drift and drift-dissipative instabilities are discussed. A conclusion is drawn that magnetic, electric, and electromagnetic effects dealt with in this paper significantly fill in the gaps in the theory of physical effects produced by meteoroids in the Earth–atmosphere–ionosphere–magnetosphere system. The magnitudes of magnetic, electric, electromagnetic, ionospheric, and acoustic effects were significant. The magnitude of the earthquake caused by the meteoroid explosion did not exceeded 1.7. The mean rate of the fall of celestial bodies similar to the Lipetsk meteoroid is equal to 0.68 yr–1.

Numerical Modeling of the Generation of Tertiary Gravity Waves in the Mesosphere and Thermosphere During Strong Mountain Wave Events Over the Southern Andes

AbstractWe investigate the effects on the mesosphere and thermosphere from a strong mountain wave (MW) event over the wintertime Southern Andes using a gravity wave (GW)‐resolving global circulation model. During this event, MWs break and attenuate at z∼50–80 km, thereby creating local body forces that generate large‐scale secondary GWs having concentric ring structure with horizontal wavelengths λH=500–2,000 km, horizontal phase speeds cH=70–100 m/s, and periods τr∼3–10 hr. These secondary GWs dissipate in the upper mesosphere and thermosphere, thereby creating local body forces. These forces have horizontal sizes of 180–800 km, depending on the constructive/destructive interference between wave packets and the overall sizes of the wave packets. The largest body force is at z=80–130 km, has an amplitude of ∼2,400 m/s/day, and is located ∼1,000 km east of the Southern Andes. This force excites medium‐ and large‐scale “tertiary GWs” having concentric ring structure, with λH increasing with radius from the centers of the rings. Near the Southern Andes, these tertiary GWs have cH=120–160 m/s, λH=350–2,000 km, and τr∼4–9 hr. Some of the larger‐λH tertiary GWs propagate to the west coast of Africa with very large phase speeds of cH≃420 m/s. The GW potential energy density increases exponentially at z∼95–115 km, decreases at z∼115–125 km where most of the secondary GWs dissipate, and increases again at z>125 km from the tertiary GWs. Thus, strong MW events result in the generation of medium‐ to large‐scale fast tertiary GWs in the mesosphere and thermosphere via this multistep vertical coupling mechanism.

Characteristics of the Quiet‐Time Hot Spot Gravity Waves Observed by GOCE Over the Southern Andes on 5 July 2010

AbstractWe analyze quiet‐time data from the Gravity Field and Ocean Circulation Explorer satellite as it overpassed the Southern Andes at z≃275 km on 5 July 2010 at 23 UT. We extract the 20 largest traveling atmospheric disturbances from the density perturbations and cross‐track winds using Fourier analysis. Using gravity wave (GW) dissipative theory that includes realistic molecular viscosity, we search parameter space to determine which hot spot traveling atmospheric disturbances are GWs. This results in the identification of 17 GWs having horizontal wavelengths λH = 170–1,850 km, intrinsic periods τIr = 11–54 min, intrinsic horizontal phase speeds cIH = 245–630 m/s, and density perturbations 0.03–7%. We unambiguously determine the propagation direction for 11 of these GWs and find that most had large meridional components to their propagation directions. Using reverse ray tracing, we find that 10 of these GWs must have been created in the mesosphere or thermosphere. We show that mountain waves (MWs) were observed in the stratosphere earlier that day and that these MWs saturated at z∼ 70–75 km from convective instability. We suggest that these 10 Gravity Field and Ocean Circulation Explorer hot spot GWs are likely tertiary (or higher‐order) GWs created from the dissipation of secondary GWs excited by the local body forces created from MW breaking. We suggest that the other GW is likely a secondary or tertiary (or higher‐order) GW. This study strongly suggests that the hot spot GWs over the Southern Andes in the quiet‐time middle winter thermosphere cannot be successfully modeled by conventional global circulation models where GWs are parameterized and launched in the troposphere or stratosphere.

A statistical analysis of medium-scale traveling ionospheric disturbances during 2014\u20132017 using the Hong Kong CORS network

We report a statistical study of medium-scale traveling ionospheric disturbances (MSTIDs) during 2014–2017 in the low-latitude China region using the Hong Kong Continuously Operating Reference Stations network with a baseline length of 10–15 km. Polynomial fitting and the multichannel maximum entropy method are utilized to derive MSTID parameters from global positioning system total electron content data. We find that MSTIDs can be sorted into three types in this region. One type is daytime MSTIDs, which are mainly distributed during 1000–1700 local time (LT) in spring, autumn and winter. The second type is nighttime MSTIDs, which mainly occur from 2200 to 0300 LT in spring and summer. The third type is morning MSTIDs, mainly occurring from 0500 to 0700 LT in spring and autumn. No clear difference in the MSTID parameters is distinguishable between the cyclone period and non-cyclone period, which may be due to the distance restriction for cyclone identification and different propagation distances of the primary gravity waves (GWs) and secondary GWs.

Orographic Primary and Secondary Gravity Waves in the Middle Atmosphere From 16‐Year SABER Observations

AbstractThe seasonal and height dependencies of the orographic primary and larger‐scale secondary gravity waves (GWs) have been studied using the temperature profiles measured by Sounding of the Atmosphere using Broadband Emission Radiometry (SABER) from 2002 to 2017. At ~40°S and during Southern Hemisphere winter, there is a strong GW peak over the Andes mountains that extend to z ~ 55 km. Using wind and topographic data, we show that orographic GWs break above the peak height of the stratospheric jet. At z ~ 55–65 km, GW breaking and momentum deposition create body forces that generate larger‐scale secondary GWs; we show that these latter GWs form a wide peak above 65 km with a westward tilt. At middle latitudes during summer in the respective hemisphere, orographic GW breaking also generates larger‐scale secondary GWs that propagate to higher altitudes. Both orographic primary and larger‐scale secondary GWs are likely responsible for most of the non‐equatorial peaks of the persistent global distribution of GWs in SABER.

The Excitation of Secondary Gravity Waves From Local Body Forces: Theory and Observation

AbstractWe examine the characteristics of secondary gravity waves (GWs) excited by a localized (in space) and intermittent (in time) body force in the atmosphere. This force is a horizontal acceleration of the background flow created when primary GWs dissipate and deposit their momentum on spatial and temporal scales of the wave packet. A broad spectrum of secondary GWs is excited with horizontal scales much larger than that of the primary GW. The polarization relations cause the temperature spectrum of the secondary GWs generally to peak at larger intrinsic periods τIr and horizontal wavelengths λH than the vertical velocity spectrum. We find that the one‐dimensional spectra (with regard to frequency or wave number) follow lognormal distributions. We show that secondary GWs can be identified by a horizontally displaced observer as “fishbone” or “>” structures in z − t plots whereby the positive and negative GW phase lines meet at the “knee,” zknee, which is the altitude of the force center. We present two wintertime cases of lidar temperature measurements at McMurdo, Antarctica (166.69°E, 77.84°S) whereby fishbone structures are seen with zknee=43 and 52 km. We determine the GW parameters and density‐weighted amplitudes for each. We show that these parameters are similar below and above zknee. We verify that the GWs with upward (downward) phase progression are downward (upward) propagating via use of model background winds. We conclude that these GWs are likely secondary GWs having ground‐based periods τr=6–10 hr and vertical wavelengths λz=6–14 km, and that they likely propagate primarily southward.

Numerical Modeling of the Excitation, Propagation, and Dissipation of Primary and Secondary Gravity Waves during Wintertime at McMurdo Station in the Antarctic

AbstractWe analyze the results of the gravity wave (GW)‐resolving, high‐resolution Kühlungsborn Mechanistic general Circulation Model in July at McMurdo Station (166.69°E and 77.84°S), where strong downslope eastward winds create strong mountain wave (MW) events. These MWs have horizontal wavelengths of λH≃230 km, propagate to z∼ 40–60 km, and can have upward phases in time if the eastward wind accelerates in time. Additionally, inertia‐GWs (IGWs) with λH∼ 500–800 km and ground‐based periods of τr∼ 5–6 hr are generated in the troposphere from unbalanced, large‐scale flow. The density‐scaled GW amplitudes are ∼10 times smaller at z∼ 80–100 km than at z < 50 km because of severe wave dissipation. “Fishbone” structures are seen at z∼ 30–60 km with upward (downward) phases in time below (above) the “knee” at zknee. We horizontally filter the perturbations to isolate the GWs in a fishbone structure for a particular MW event. We find that these GWs have strikingly similar parameters below and above zknee=46 km, with ground‐based horizontal phase speeds of cH∼ 40–60 m/s, τr∼ 9–10 hr, λH∼ 1,600–2,050 km, vertical wavelengths of λz∼ 18–25 km, and azimuths of Υ= 145° –151° east of north. We show that these are secondary GWs excited by a body force at zknee created by MW dissipation approximately 400 km northwest of McMurdo 2.5 hr earlier and that the secondary GW scales and propagation directions are consistent with this force. Importantly, we show that most of the GWs at z > 70 km are secondary GWs not primary GWs from the troposphere.

Multiyear Observations of Gravity Wave Momentum Fluxes in the Midlatitude Mesosphere and Lower Thermosphere Region by Meteor Radar

AbstractMultiyear high‐frequency gravity wave (GW) momentum fluxes and variances in the mesosphere and lower thermosphere region are revealed using four meteor radars along 120°E longitude at Northern Hemisphere midlatitudes for the first time, which are located at Mohe (53.5°N, 122.3°E), Beijing (40.3°N, 116.2°E), Mengcheng (33.3°N, 116.5°E), and Wuhan (30.5°N, 114.2°E), respectively. The seasonal and latitudinal variations of GW momentum fluxes in the midlatitude are discussed. The directions of the monthly mean zonal momentum fluxes are mostly against the background mean zonal winds, which agree well with the selective filtering mechanism. The seasonal variations of meridional momentum fluxes have similar trends over all four stations. The latitudinal variations in the seasonal variation of GW momentum fluxes are mainly due to the latitudinal variations of background winds and GW sources. The unexpected eastward momentum fluxes in winter over Beijing are likely caused by the secondary GWs propagating eastward from the source region over the Tibetan Plateau (25°–40°N, 70–100°E). The GW variances show a V‐shaped structure indicating annual and semiannual variations over four stations in zonal component. A quasi‐4‐month oscillation was observed over Mohe, Mengcheng, and Wuhan in meridional component. The background winds play decisive roles in these GW variance structures.

Satellite observations of middle atmosphere–thermosphere vertical coupling by gravity waves

Abstract. Atmospheric gravity waves (GWs) are essential for the dynamics of the middle atmosphere. Recent studies have shown that these waves are also important for the thermosphere/ionosphere (T/I) system. Via vertical coupling, GWs can significantly influence the mean state of the T/I system. However, the penetration of GWs into the T/I system is not fully understood in modeling as well as observations. In the current study, we analyze the correlation between GW momentum fluxes observed in the middle atmosphere (30–90 km) and GW-induced perturbations in the T/I. In the middle atmosphere, GW momentum fluxes are derived from temperature observations of the Sounding of the Atmosphere using Broadband Emission Radiometry (SABER) satellite instrument. In the T/I, GW-induced perturbations are derived from neutral density measured by instruments on the Gravity field and Ocean Circulation Explorer (GOCE) and CHAllenging Minisatellite Payload (CHAMP) satellites. We find generally positive correlations between horizontal distributions at low altitudes (i.e., below 90 km) and horizontal distributions of GW-induced density fluctuations in the T/I (at 200 km and above). Two coupling mechanisms are likely responsible for these positive correlations: (1) fast GWs generated in the troposphere and lower stratosphere can propagate directly to the T/I and (2) primary GWs with their origins in the lower atmosphere dissipate while propagating upwards and generate secondary GWs, which then penetrate up to the T/I and maintain the spatial patterns of GW distributions in the lower atmosphere. The mountain-wave related hotspot over the Andes and Antarctic Peninsula is found clearly in observations of all instruments used in our analysis. Latitude–longitude variations in the summer midlatitudes are also found in observations of all instruments. These variations and strong positive correlations in the summer midlatitudes suggest that GWs with origins related to convection also propagate up to the T/I. Different processes which likely influence the vertical coupling are GW dissipation, possible generation of secondary GWs, and horizontal propagation of GWs. Limitations of the observations as well as of our research approach are discussed. Keywords. Ionosphere (ionosphere–atmosphere interactions)

Secondary Gravity Waves in the Winter Mesosphere: Results From a High‐Resolution Global Circulation Model

AbstractThis study analyzes a new high‐resolution general circulation model with regard to secondary gravity waves in the mesosphere during austral winter. The model resolves gravity waves down to horizontal and vertical wavelengths of 165 and 1.5 km, respectively. The resolved mean wave drag agrees well with that from a conventional model with parameterized gravity waves up to the midmesosphere in winter and up to the upper mesosphere in summer. About half of the zonal‐mean vertical flux of westward momentum in the southern winter stratosphere is due to orographic gravity waves. The high intermittency of the primary orographic gravity waves gives rise to secondary waves that result in a substantial eastward drag in the winter mesopause region. This induces an additional eastward maximum of the mean zonal wind at z ∼ 100 km. Radar and lidar measurements at polar latitudes and results from other high‐resolution global models are consistent with this finding. Hence, secondary gravity waves may play a significant role in the general circulation of the winter mesopause region.

Numerical Study of Traveling Ionospheric Disturbances Generated by an Upward Propagating Gravity Wave

AbstractUsing a global atmosphere‐ionosphere coupled model, the characteristics and excitation source of traveling ionospheric disturbances (TIDs) during geomagnetically quiet periods are studied. This is the first paper concerning the simulation of TIDs generated by upward propagating gravity waves (GWs) that are spontaneously generated in the model. The dominant horizontal wavelengths of the simulated TIDs range from 700 to 1,500 km. The dominant periods and horizontal phase velocities of TIDs are 45–90 min and 250–300 m s−1, respectively. These features are the same as those of GWs in the 250–300 km height region. The phase of the electron density variations due to TIDs descends with increasing time, which is characteristic of the upward propagation of GWs. These electron density variations that are caused due to TIDs are explained by the transport processes of a neutral wind along a geomagnetic field line. These results indicate that the electron density variations respond locally to the passage of neutral wind fluctuations associated with upward propagating GWs. The GWs that excite TIDs are secondary GWs, which are generated in the mesosphere and lower thermosphere via the dissipation/breaking of tropospheric GWs. The magnitudes of TIDs at middle latitudes are larger in winter than in summer. The mechanisms of seasonal and day‐to‐day variations in TIDs that are caused due to GWs are discussed in this study.