Propagation Direction Of Gravity Waves Research Articles

Oscillations of GW Activities in the MLT Region over Mid-Low-Latitude Area, Kunming Station (25.6° N, 103.8° E)

Gravity wave (GW) activities play a prominent role in the complex coupling process of wave–wave and wave–background circulation around mid-low-latitude and equatorial areas. The wavelengths are wide, from about 10 m to 100 km, with a period from minutes to hours. However, the oscillations of GW activities are apparently different between the period bands of 0.1 to 1 h (HF) and 1 to 5 h (LF). To further understand the characteristics of GW activities, the neutral winds during 2008–2009 with a resolution of 3 min obtained from a medium-frequency (MF) radar in Kunming (25.6° N, 103.8° E) were analyzed. Using two numerical filters, the HF and LF GWs were estimated. Interestingly, the power spectral density grows larger as the frequency increases. It linearly falls with decreasing frequency when the period is less than 2 h. The seasonal variations in both HF and LF GWs are strongly demonstrated in August–September, November, and February–March with maximum meridional variances of 1100 m2 s−2 and 500 m2 s−2 and maximum zonal variances of 800 m2 s−2 and 350 m2 s−2 in, respectively. The turbulent velocity was also calculated and shows similar oscillations with GW activities. Furthermore, the GW propagation direction exhibits strong seasonal variations, which may be dependent on the location of the motivating source and background wind.

Modeling Studies of Gravity Wave Dynamics in Highly Structured Environments: Reflection, Trapping, Instability, Momentum Transport, Secondary Gravity Waves, and Induced Flow Responses.

A compressible numerical model is applied for three‐dimensional (3‐D) gravity wave (GW) packets undergoing momentum deposition, self‐acceleration (SA), breaking, and secondary GW (SGW) generation in the presence of highly‐structured environments enabling thermal and/or Doppler ducts, such as a mesospheric inversion layer (MIL), tidal wind (TW), or combination of MIL and TW. Simulations reveal that ducts can strongly modulate GW dynamics. Responses modeled here include reflection, trapping, suppressed transmission, strong local instabilities, reduced SGW generations, higher altitude SGW responses, and induced large‐scale flows. Instabilities that arise in ducts experience strong dissipation after they emerge, while trapped smaller‐amplitude and smaller‐scale GWs can survive in ducts to much later times. Additionally, GW breaking and its associated dynamics enhance the local wind along the GW propagation direction in the ducts, and yield layering in the wind field. However, these dynamics do not yield significant heat transport in the ducts. The failure of GW breaking to induce stratified layers in the temperature field suggests that such heat transport might not be as strong as previously assumed or inferred from observations and theoretical assessments. The present numerical simulations confirm previous finding that MIL generation may not be caused by the breaking of a transient high‐frequency GW packet alone.

Comparison of gravity wave propagation directions observed by mesospheric airglow imaging at three different latitudes using the M-transform

Abstract. We developed user-friendly software based on Matsuda et al.'s (2014) 3D-FFT method (Matsuda-transform, M-transform) for airglow imaging data analysis as a function of Interactive Data Language (IDL). Users can customize the range of wave parameters to process when executing the program. The input for this function is a 3-D array of a time series of a 2-D airglow image in geographical coordinates. We applied this new function to mesospheric airglow imaging data with slightly different observation parameters obtained for the period of April–May at three different latitudes: Syowa Station, the Antarctic (69∘ S, 40∘ E); Shigaraki, Japan (35∘ N, 136∘ E); and Tomohon, Indonesia (1∘ N, 122∘ E). The day-to-day variation of the phase velocity spectrum at the Syowa Station is smaller and the propagation direction is mainly westward. In Shigaraki, the day-to-day variation of the horizontal propagation direction is larger than that at the Syowa Station; the variation in Tomohon is even larger. In Tomohon, the variation of the nightly power spectrum magnitude is remarkable, which indicates the intermittency of atmospheric gravity waves (AGWs). The average nightly spectrum obtained from April–May shows that the dominant propagation is westward with a phase speed <50 m s−1 at the Syowa Station and east-southeastward with a phase speed of up to ∼80 m s−1 in Shigaraki. The day-to-day variation in Tomohon is too strong to discuss average characteristics; however, a phase speed of up to ∼100 m s−1 and faster is observed. The corresponding background wind profiles derived from MERRA-2 indicate that wind filtering plays a significant role in filtering out waves that propagate eastward at the Syowa Station. On the other hand, the background wind is not strong enough to filter out relatively high-speed AGWs in Shigaraki and Tomohon and the dominant propagation direction is likely related to the distribution and characteristics of the source region, at least in April and May.

Linking source region and ocean wave parameters with the observed primary microseismic noise

In previous studies, the contribution of Love waves to the primary microseismic noise field was found to be comparable to those of Rayleigh waves. However, so far only few studies analysed both wave types present in this microseismic noise band, which is known to be generated in shallow water and the theoretical understanding has mainly evolved for Rayleigh waves only. Here, we study the relevance of different source region parameters on the observed primary microseismic noise levels of Love and Rayleigh waves simultaneously. By means of beamforming and correlation of seismic noise amplitudes with ocean wave heights in the period band between 12 and 15 s, we analysed how source areas of both wave types compare with each other around Europe. The generation effectivity in different source regions was compared to ocean wave heights, peak ocean gravity wave propagation direction and bathymetry. Observed Love wave noise amplitudes correlate comparably well with near coastal ocean wave parameters as Rayleigh waves. Some coastal regions serve as especially effective sources for one or the other wave type. These coincide not only with locations of high wave heights but also with complex bathymetry. Further, Rayleigh and Love wave noise amplitudes seem to depend equally on the local ocean wave heights, which is an indication for a coupled variation with swell height during the generation of both wave types. However, the wave-type ratio varies directionally. This observation likely hints towards a spatially varying importance of different source mechanisms or structural influences. Further, the wave-type ratio is modulated depending on peak ocean wave propagation directions which could indicate a variation of different source mechanism strengths but also hints towards an imprint of an effective source radiation pattern. This emphasizes that the inclusion of both wave types may provide more constraints for the understanding of acting generation mechanisms.

Sixteen year variation of horizontal phase velocity and propagation direction of mesospheric and thermospheric waves in airglow images at Shigaraki, Japan

AbstractWe analyzed the horizontal phase velocity of gravity waves and medium‐scale traveling ionospheric disturbances (MSTIDs) by using the three‐dimensional fast Fourier transform method developed by Matsuda et al. (2014) for 557.7 nm (altitude: 90–100 km) and 630.0 nm (altitude: 200–300 km) airglow images obtained at Shigaraki MU Observatory (34.8°N, 136.1°E, dip angle: 49°) over ∼16 years from 16 March 1999 to 20 February 2015. The analysis of 557.7 nm airglow images shows clear seasonal variation of the propagation direction of gravity waves in the mesopause region. In spring, summer, fall, and winter, the peak directions are northeastward, northeastward, northwestward, and southwestward, respectively. The difference in east‐west propagation direction between summer and winter is probably caused by the wind filtering effect due to the zonal mesospheric jet. Comparison with tropospheric reanalysis data shows that the difference in north‐south propagation direction between summer and winter is caused by differences in the latitudinal location of wave sources due to convective activity in the troposphere relative to Shigaraki. The analysis of 630.0 nm airglow images shows that the propagation direction of MSTIDs is mainly southwestward with a minor northeastward component throughout the 16 years. A clear negative correlation is seen between the yearly power spectral density of MSTIDs and F10.7 solar flux. This negative correlation with solar activity may be explained by the linear growth rate of the Perkins instability and secondary wave generation of gravity waves in the thermosphere.

Characteristics of mesospheric gravity waves over the southeastern Tibetan Plateau region

AbstractThe Tibetan Plateau (TP), known as “Third Pole” of the Earth, has important influences on global climates and local weather. An important objective in present study is to investigate how orographic features of the TP affect the geographical distributions of gravity wave (GW) sources. Three‐year OH airglow images (November 2011 to October 2014) from Qujing (25.6°N, 103.7°E) were used to study the characteristics of GWs over the southeastern TP region. Along with the almost concurrent and collocated meteor radar wind measurements and temperature data from SABER/TIMED satellite, the propagation conditions of three types of GWs (freely propagating, ducted, or evanescent) were estimated. Most of GWs exhibited ducted or evanescent characteristics. Almost all GWs propagate southeastward in winter. The GW propagation directions in winter are significantly different from other airglow imager observations at northern middle latitudes. Wind data and convective precipitation fields from the European Centre for Medium‐Range Weather Forecasts reanalysis data are used to study the sources of GWs on the edge of the TP. Using backward ray‐tracing analysis, we find that most of the mesospheric freely propagating GWs are located in or near the large wind shear intensity region (~10 km–~17 km) on the southeastern edge of the TP in spring and winter. The averaged value of momentum flux is 11.6 ± 5.2 m2/s2 in winter and 7.5 ± 3.1 m2/s2 in summer. This work will provide valuable information for the GW parameterization schemes in general circulation models in TP region.

Evidence of high frequency gravity wave forcing on the meridional residual circulation at the mesopause region

Data of high frequency gravity wave propagation direction from globally distributed stations indicate a meridional preference of mesospheric gravity waves to be globally oriented toward the summer pole. This orientation is opposite to the mean residual circulation (from summer to winter pole) at mesospheric altitudes. We discuss here a number of dynamic mechanisms including filtering that may be responsible for the preferential wave orientation, and the effects of the gravity wave forcing imposed on the meridional flow due to dissipative waves. Using nightglow image data recorded in three distinct latitude stations, we have estimated the meridional wave drag (i.e, deceleration) of about -4.6±0.2m/s/day during the summer, and 3.8±0.2m/s/day during the winter, which is significant because the meridional flow has small magnitude. This is a component of dynamic forcing in the mesopause region, not heretofore recognized.

Horizontal velocities and propagation directions of gravity waves in the ionosphere over the Czech Republic

Using a five‐point continuous Doppler sounding system operating at 3.59 MHz, developed at the Institute of Atmospheric Physics AS CR (IAP), we investigate propagation directions and velocities of gravity waves (GWs) at altitudes from ∼150 to ∼250 km over the western part of the Czech Republic. The velocities and directions are computed from the time delays between the observations of corresponding GWs at different reflection points that correspond to various sounding paths. We focused on the GWs that produce an S‐shaped trace in Doppler shift spectrograms and are observed close to sunrise and sunset. We have selected about 100 of such events. Our results show that the analyzed GWs propagate with typical horizontal velocities from ∼100 to ∼200 m/s. The north‐south component of GW velocities depends on the season, it is directed northward in the summer and southward in the winter. At the same time, the north‐south component of the neutral winds calculated by the horizontal wind model HWM07 has the opposite sign. Typical observed periods of the analyzed waves range from ∼10 to ∼30 min.

Variability of gravity wave occurrence frequency and propagation direction in the upper mesosphere observed by the OH imager in Northern Colorado

Based on 5 years of OH imager data between September 2003 and September 2008 over Yucca Ridge Field Station, CO (40.7ºN, 104.9ºW), we presented the variation of gravity wave (GW) occurrence frequency and propagation direction in the upper mesosphere. In summer the GW occurrence frequency was extremely high at above 95% compared to other seasons (around 85%). The GW propagation direction showed a strong northward (poleward) preference in summer and a southward (equatorward) preference in winter. This could be possibly due to ducting of waves in the mesopause thermal structure and wave generation by the strong deep convection located at south side in summer and possible storms located at north side in winter. Westward traveling waves were rare, but eastward were frequent. In addition to seasonal variability, significant interannual variability was also observed.

Using PFISR measurements and gravity wave dissipative theory to determine the neutral, background thermospheric winds

Understanding the propagation and dissipation of an atmospheric gravity wave (GW) in the thermosphere requires an accurate dissipative GW dispersion relation, the GW's horizontal wavelength and period, and the background neutral winds and temperatures. Conversely, if the GW's horizontal wavelength, period, and vertically‐varying vertical wavelengths are known instead along with the background temperatures, then the background, horizontal neutral winds along the GW propagation direction can be calculated using GW dissipative theory. Recent daytime observations using the Advanced Modular Incoherent Scatter Radar (AMISR) located in Poker Flat, Alaska, the Poker Flat Incoherent Scatter Radar (PFISR), have obtained these latter parameters. Using PFISR data for a GW on December 13, 2006, we calculate the average, background, horizontal neutral winds at z ∼ 160–240 km.

Tidal signatures in mesospheric turbulence

Abstract. We search for the presence of tidal signatures in high latitude mesospheric turbulence as parameterized by turbulent energy dissipation rate estimated using a medium frequency radar, quantifying our findings with the aid of correlation analyses. A diurnal periodicity is not particularly evident during the winter and spring months but is a striking feature of the summer mesopause. While semidiurnal variation is present to some degree all year round, it is particularly pronounced in winter. We find that the maximum in the summer 24-h variation corresponds to that of the westward phase of the diurnal tide, and that the maximum in the winter 12 h variation corresponds to that of the southward phase of the semidiurnal tide. This information is used to infer the horizontal propagation direction of gravity waves: during the summer the eastward direction is consistent with closure of the summer vortex, while in winter the inferred directions require more complex arguments.

Studies on atmospheric gravity wave activity in the troposphere and lower stratosphere over a tropical station at Gadanki

Abstract. MST radars are powerful tools to study the mesosphere, stratosphere and troposphere and have made considerable contributions to the studies of the dynamics of the upper, middle and lower atmosphere. Atmospheric gravity waves play a significant role in controlling middle and upper atmospheric dynamics. To date, frontal systems, convection, wind shear and topography have been thought to be the sources of gravity waves in the troposphere. All these studies pointed out that it is very essential to understand the generation, propagation and climatology of gravity waves. In this regard, several campaigns using Indian MST Radar observations have been carried out to explore the gravity wave activity over Gadanki in the troposphere and the lower stratosphere. The signatures of the gravity waves in the wind fields have been studied in four seasons viz., summer, monsoon, post-monsoon and winter. The large wind fluctuations were more prominent above 10 km during the summer and monsoon seasons. The wave periods are ranging from 10 min-175 min. The power spectral densities of gravity waves are found to be maximum in the stratospheric region. The vertical wavelength and the propagation direction of gravity waves were determined using hodograph analysis. The results show both down ward and upward propagating waves with a maximum vertical wave length of 3.3 km. The gravity wave associated momentum fluxes show that long period gravity waves carry more momentum flux than the short period waves and this is presented.

Relationship between propagation direction of gravity waves in OH and OI airglow images and VHF radar echo occurrence during the SEEK-2 campaign

Abstract. We report simultaneous observations of atmospheric gravity waves (AGW) in OI (557.7nm) and OH airglow images and VHF radar backscatter from field-aligned irregularities (FAI) in the E-region during the SEEK-2 (Sporadic-E Experiment over Kyushu 2) campaign period from 29 July to 9 August 2002. An all-sky imager was operated at Nishino-Omote (30.5 N, 130.1 E), Japan. On 14 nights, 17 AGW events were detected in OI and OH airglow images. AGW propagated mostly toward the northeast or southeast. From comparison with the E-region FAI occurrence, which is detected by a nearby VHF radar (31.57MHz), we found that AGW tended to propagate southeastward during FAI events. This result suggests that the interaction between AGW and E-region plasma plays an important role in generating FAI. Furthermore, polarization electric fields generated directly by AGW may contribute to the FAI generation. Keywords. Atmospheric composition and structure (Airglow and aurora), Ionosphere (Ionospheric irregularities, Mid-latitude ionosphere)

Atmospheric gravity wave propagation direction observed by airglow imaging in the South American sector

Airglow all sky imaging observation has been carried out in three different locations in south America, at Cachoeira Paulista (22.7°S, 45.0°W) in 1999, São João do Cariri (7.5°S, 36.5°W) in 2001 and Boa Vista (2.8°N, 60.7°W) in 2002. Comparing the atmospheric gravity wave characteristics retrieved from the image data for the three different sites and including a previous work at Alcântara (2.3°S, 44.5°W) carried out by Taylor et al. [1997. Journal of Geophysical Research 102 (D22) 26,283–26,299], we found that there is a preferential propagation direction, from the Continent to the Atlantic Ocean. The observed wave propagation directions reveal that a major part of the waves have their direction from Continent toward Ocean. The possible source of the wave generation is discussed.

A Numerical Study on the Interpretation of Data from Open Atmospheric Balloon Soundings

Abstract Wrong information may be extracted from balloon soundings if neither appropriate interpretation and processing nor evaluations of certain inevitable distortions or artifacts on atmospheric measurements are performed. A numerical code that finds solutions to the dynamical and thermal equations describing an open balloon in the atmosphere is used to develop flight simulations under diverse conditions. The results are then employed to point out that a valid determination of values for diverse variables is intrinsically difficult. It is shown that the distance between the balloon and gondola may be chosen to optimize the information to be obtained from observations obtained during ascent and descent, so that even without an accurate balloon-tracking system, it may be possible to reconstruct horizontal wind fluctuations from the measurements. Vertical air oscillations may be only grossly inferred in some cases. The propagation direction of gravity waves detected during a sounding may be inferred and vertical wavelengths may typically be determined with a 10% accuracy. Air velocity measurements performed during flotation may be used to find shears.

Statistical characteristics of gravity waves observed by an all‐sky imager at Darwin, Australia

An all‐sky airglow imager with a cooled charge‐coupled device camera in place at Darwin (12.4°S, 131.0°E), Australia, since October 2001 has been used to obtain two‐dimensional gravity wave images in the mesopause region. Using airglow images of the OI (557.7 nm, emission altitude ∼96 km) and OH band (720–910 nm, ∼86 km) emissions obtained for October 2001 to August 2002, we investigated the wavelengths, phase velocities, and propagation directions of gravity waves. Wave occurrence in OH images (60–90%) is higher than in OI images (30–70%) for all seasons. The waves have wavelengths of less than 90 km (peak: 30–50 km) and phase velocities of less than 90 m/s (peak: 30–60 m/s). Most of the waves propagate in the meridional direction, and the directionality strongly depends on the season. In winter, waves propagate both poleward and equatorward, while in summer almost all waves propagate poleward. An examination of airglow‐imaging statistics at Adelaide (35°S, 138°E), Australia, obtained by Walterscheid et al. [1999], leads us to conclude that this clear directionality is caused by the location of the wave sources and by the wave ducting processes; that is, poleward waves in summer come from an equatorial convective source through a thermal duct structure. The effect of wind filtering on the waves is also discussed for zonal wave propagation.

Mesospheric gravity waves over a tropical convective region observed by OH airglow imaging in Indonesia

From OH airglow imaging observation carried out over one year in Indonesia, 74 events of gravity waves in the MLT (Mesosphere and Lower Thermosphere) region were extracted. Observed period, horizontal wavelength and observed horizontal phase speeds of gravity waves were typically 5–13 min, 13–45 km and 37–75 m/s, respectively. Propagation directions were mostly southward except for the period between December and February, when eastward propagation was preferential. Spatial distributions of tropospheric clouds, estimated with GMS (Geostationary Meteorological Satellite) were consistent with the propagation direction of gravity waves, i.e., these clouds mainly existed in the opposite direction to the propagation direction of waves. This suggests that horizontal propagation characteristics of the short period gravity waves in the low latitude MLT region are mainly affected by the distribution of the wave sources in the troposphere, and the effect of the background mean wind in the middle atmosphere is smaller, as it is weaker (∼20 m/s) in the equatorial region.

Gravity wave propagation directions inferred from satellite observations including smearing effects

We simulate space‐based, sublimb viewing observations of airglow brightness fluctuations caused by atmospheric gravity wave interactions with the O2 atmospheric airglow, and we demonstrate that because of the geometry associated with such observations, the brightness fluctuations observed for the optically thick 0–0 band emission will always appear stronger for waves traveling toward the observer (the satellite). The effect should be most noticeable for waves having relatively small vertical wavelengths (∼10 km) and horizontal wavelengths of 50 km or greater. For waves of short (∼100 km) horizontal wavelength, the brightness fluctuation anisotropy with respect to viewing direction may also be evident in the optically thin 0–1 band emission. We demonstrate that the waves will be observable despite the fact that an instrument requires a certain finite integration time to achieve a desired signal‐to‐noise ratio. Therefore the 180° ambiguity in wave propagation direction associated with space‐based observations may be eliminated for waves of small vertical wavelength that are dissipating in the upper mesosphere and lower thermosphere. It is these same waves that may be expected to be important to the energy and momentum budgets of the mesosphere/lower thermosphere region.

Seasonal variation of gravity waves with various temporal and horizontal scales in the MLT region observed with radar and airglow imaging

Abstract Seasonal variations of activity and horizontal propagation direction of gravity waves in the MLT region at mid-latitude over Shigaraki, Japan (35 N) are investigated for various time and spatial scales using the MU radar observations and recent airglow imaging observations. Semiannual variation with solsticial maxima and equinoctial minima are generally observed for short period (

Resolving ambiguities in gravity wave propagation directions inherent in satellite observations: A simulation study

We simulate space‐based, sub‐limb viewing observations of airglow brightness fluctuations caused by atmospheric gravity wave interactions with the O2 atmospheric airglow, and we demonstrate that, due to the geometry associated with such observations, the brightness fluctuations observed for the optically thick 0–0 band emission will always appear stronger for waves traveling towards the observer (satellite). The effect should be most noticeable for waves having relatively small vertical wavelengths (∼10 km) and horizontal wavelengths of 50 km or greater. For waves of short (∼100 km) horizontal wavelength, the brightness fluctuation anisotropy with respect to viewing direction may also be evident in the optically thin 0–1 band emission. Therefore, the 180° ambiguity in wave propagation direction associated with space‐based observations may be eliminated for waves dissipating in the upper mesosphere and lower thermosphere.