Gravity Wave Fluctuations Research Articles

Climatology of global gravity wave activity and dissipation revealed by SABER/TIMED temperature observations

Gravity wave activity and dissipation in the height range from the low stratosphere to the low thermosphere (25–115 km) covering latitudes between 50°S and 50°N are statistically studied by using 9-year (January 22, 2002–December 31, 2010) SABER/TIMED temperature data. We propose a method to extract realistic gravity wave fluctuations from the temperature profiles and treat square temperature fluctuations as GW activity. Overall, the gravity wave activity generally increases with height. Near the equator (0°–10°), the gravity wave activity shows a quasi-biennial variation in the stratosphere (below 40 km) while from 20° to 30°, it exhibits an annual variation below 40 km; in low latitudes (0°–30°) between the upper stratosphere and the low thermosphere (40–115 km), the gravity wave activity shows a semi-annual variation. In middle latitudes (40°–50°), the gravity wave activity has a clear annual variation below 85 km. In addition, we observe a four-monthly variation with peaks occurring usually in April, August, December in the northern hemisphere and in February, June, October in the southern hemisphere, respectively, above 85 km in middle latitudes, which has been seldom reported in gravity wave activity. In order to study the dissipation of gravity wave propagation, we calculate the gravity wave dissipation ratio, which is defined as the ratio of the gravity wave growth scale height to the atmosphere density scale height. The height variation of the dissipation ratio indicates that strong gravity wave dissipation mainly concentrates in the three height regions: the stratosphere (30–60 km), the mesopause (around 85 km) and the low thermosphere (above 100 km). Besides, gravity wave energy enhancement can be also observed in the background atmosphere.

A method for specifying atmospheric gravity wavefields for long‐range infrasound propagation calculations

AbstractTwo important challenges in infrasound propagation physics are (1) to explain frequently observed infrasound signals in the classical near‐field shadow zones and (2) to accurately predict observed waveform amplitude and signal duration. For these problems, the role that small‐scale internal atmospheric gravity wave fluctuations play has recently been realized. This paper provides a methodology for representing small‐scale internal gravity wave fluctuations which is suitable for infrasound propagation calculations. Adapted from the numerical weather prediction and climate modeling communities, the resulting stochastic gravity wave noise field model is three‐dimensional, time dependent, and self‐consistent with the atmospheric background state. To illustrate the methodology the resultant gravity wavefields are applied to ray‐trace simulations of observed infrasound travel times for a dense seismic network in the Western United States which recorded infrasound signals from a large surface explosion.

Analysis on the Global Morphology of Middle Atmospheric Gravity Waves

AbstractUsing temperature profiles obtained by the SABER/TIMED experiment from January 2002 to December 2009, we have extracted mesoscale temperature perturbations with vertical wavelengths ranging from 2 to 10 km. Global distribution of middle atmospheric gravity wave activity is revealed by observing the temperature perturbations. Comparison shows that gravity wave fluctuations in summer and winter are stronger than those in spring and autumn, below 70 km the disturbance intensity in summer is weaker with respect to winter, and contrary above 70 km. The larger gravity waves appear in winter hemisphere and in the tropics between 25°N and 25°S. Meanwhile, the perturbation peak points in the tropics move northward with increasing altitude. Furthermore, relatively larger values of gravity wave activity are present at the edge of the Antarctic polar vortex. They also appear to vary with longitudes at equatorial latitudes, which may be resulted from wind filtering, topography, planetary wave modulations and other factors. Moreover, gravity wave disturbance intensity changes with height. It indicates a decrease at 25~30 km and an increase above 42 km. Comparing the average distribution of gravity waves of eight years at different heights, we can see that the strength of gravity waves is related to topography obviously at lower altitudes, but the relationship is not significant at higher altitudes. It indicates that the formation of gravity waves is closely correlated with topography, but in the process of propagation the distribution significantly changes with altitudes.

Linear Spectral Numerical Model for Internal Gravity Wave Propagation

Abstract A three-dimensional linear spectral numerical model is proposed to simulate the propagation of internal gravity wave fluctuations in a stably stratified atmosphere. The model is developed to get first-order estimations of gravity wave fluctuations produced by identified sources. It is based on the solutions of the linearized fundamental fluid equations and uses the fully compressible dispersion relation for inertia–gravity waves. The spectral implementation excludes situations involving spatial variations of buoyancy frequency or background wind. However, density stratification variations are taken into account in the calculation of fluctuation amplitudes. In addition to gravity wave packet free propagation, the model handles both impulsive and continuous sources. It can account for spatial and temporal variations of the sources, encompassing a broad range of physical situations. The method is validated with a monochromatic pressure monopole, which is known to generate St. Andrew’s cross–shaped waves. It is then applied to the case of the ozone layer cooling during a total solar eclipse. The asymptotic response to a Gaussian thermal forcing traveling at constant velocity and the transient response to the 4 December 2002 eclipse show good agreement with previous numerical simulations. Further applications for the model are discussed.

Midlatitude mesosphere/lower thermosphere meridional winds and temperatures measured with meteor radar

A meteor radar to measure hourly mean horizontal winds in the 80–100 km height range and daily temperatures near 90 km is operating at Collm (51.3°N, 13°E) since summer 2004. Here we present changes of the meridional wind jet in comparison with temperature fluctuations from August 2004 to May 2006 at time scales of 11–30 days. Whereas in summer no direct correlation between wind and temperature is visible, the meridional circulation at 85 km in winter is positively correlated and nearly in phase with temperatures, so that northward/southward winds are generally accompanied by higher/lower temperatures. This behaviour is in accordance with the dynamical forcing of mesopause region temperatures, which is modulated by planetary waves, gravity wave fluctuations and stratospheric warmings. Numerical modelling of MLT planetary waves show that temperature oscillations at 90 km and winds at 85 km due to those waves have a phase difference of several days, so that planetary waves probably are not the reason for the temperature/wind coupling.

Vertical wave number spectra of wind fluctuations in the troposphere and lower stratosphere over Gadanki, a tropical station

Spectral analyses of horizontal and vertical winds measured by Gadanki (13.5°N, 79.2°E) MST radar in the troposphere and lower stratosphere have been presented. Average slopes of the wave number spectra obtained in the power-law region are −2.5 for horizontal winds and −2.2 for vertical winds. Spectral indices are found to vary in different altitude zones with steepest slopes in the tropopause region for both horizontal and vertical winds. A seasonal variation of the slope has also been observed with higher value in summer. The ratio of the spectral magnitudes between the stratosphere and troposphere is found to be quite variable with an average value of 4.2 and 5.5 for horizontal and vertical winds, respectively. The power-law region seems to be dominated by gravity wave fluctuations and high correlations are observed between horizontal and vertical winds and between stratospheric and tropospheric energy densities. It is suggested that the saturated gravity wave model may be modified to match the observed spectral features. Spectral growth is found to be inhibited on some days which cannot be explained by gravity wave theory. It is felt that mesoscale spectra can be influenced by mechanism like 2D-turbulence.

Possibility of observable amount of gravitational waves from inflation.

The curvaton and inhomogeneous reheating scenarios for the generation of the cosmological curvature perturbation on large scales represent an alternative to the standard slow-roll scenario. The basic assumption of these mechanisms is that the initial curvature perturbation due to the inflaton field is negligible. This is usually attained by lowering the energy scale of inflation, thereby concluding that the amount of gravitational waves produced during inflation is highly suppressed. We show that the curvaton and inhomogeneous reheating scenarios are compatible with a level of gravity-wave fluctuations which may well be observed in future satellite experiments.

Diurnal Variation of Gravity Wave Activity at Midlatitudes in the Ionospheric F Region

We investigate the short-term fluctuations in the period range from 15 to 180 minutes in the electron density variations of the F region ionosphere. Electron density profiles obtained at the ionospheric stations of Pruhonice (49.9° N, 14.5° E) and Ebro (40.8° N, 0.5° E) at five minute time sampling have been used for this analysis. The diurnal changes of the activity of the acoustic gravity wave fluctuations (AGW) show a clear enhancement during and several hours after sunrise. The periods of such AGW's are about 60 to 75 minutes and these waves propagates vertically through the ionosphere from a source located at an altitude of 180-220 km. The most likely source for these events seems to be passage of the Solar terminator.

A study of stratospheric GW fluctuations and sporadic E at midlatitudes with focus on possible orographic effect of Andes

Longitudinal dependences of stratospheric gravity wave (GW) fluctuations and lower ionospheric irregularities (sporadic E) at midlatitudes are studied by means of radio occultation data of the Global Positioning System/Meteorology Experiment (GPS/MET) satellite mission. The zonal average of temperature variance of GW fluctuations with vertical scales less than 7 km at northern midlatitudes is observed to be similar to that at southern midlatitudes, but there is a significant interhemispheric difference in the longitudinal dependence of GW fluctuations. The GPS/MET data at northern midlatitudes show a rapid change of the gravity wave distribution from 25 to 35 km height, resulting in a broad maximum of temperature variance located over the Atlantic and Eurasia. We only find in the wave distribution at h = 25 km some weak traces of possible orographic effects. On the other hand, the distribution of GW fluctuations at southern midlatitudes has a strong and sharp maximum over Andes, which is obviously due to orographic wave generation by the interaction of surface wind with the Andean mountain ridge. This observation of the new GPS radio occultation technique is in agreement with previous measurements of spaceborne microwave and infrared limb sounders. The amplitude of the average wave field increases with height over Andes, while the amplitude maximum moves westward, against the prevailing wind. The temperature fluctuations have an apparent, dominant vertical wavelength of around 6 km. In situ measurements by a balloon‐borne rawinsonde at Ushuaia, Argentina (54.7°S, 68.1°W) are compared to a simultaneous GPS/MET temperature profile. The balloon observations of temperature and horizontal wind are interpreted by a large amplitude mountain wave propagating to the upper stratosphere. Wave characteristics and atmospheric background conditions are investigated in detail for this mountain wave observation. Finally, the GPS/MET experiment indicates enhanced sporadic E in the lower ionosphere over Southern Andes. We assume that these plasma irregularities are generated by enhanced, upward wave flux due to the possible orographic effect of Andes.

VHF radar observations of gravity waves at a low latitude

Abstract. Wind observations made at Gadanki (13.5°N) by using Indian MST Radar for few days in September, October, December 1995 and January, 1996 have been analyzed to study gravity wave activity in the troposphere and lower stratosphere. Horizontal wind variances have been computed for gravity waves of period (2-6) h from the power spectral density (PSD) spectrum. Exponential curves of the form eZ/H have been fitted by least squares technique to these variance values to obtain height variations of the irregular winds upto the height of about 15 km, where Z is the height in kilometers. The value of H, the scale height, as determined from curve fitting is found to be less than the theoretical value of scale height of neutral atmosphere in this region, implying that the waves are gaining energy during their passage in the troposphere. In other words, it indicates that the sources of gravity waves are present in the troposphere. The energy densities of gravity wave fluctuations have been computed. Polynomial fits to the observed values show that wave energy density increases in the troposphere, its source region, and then decreases in the lower stratosphere.Key words. Meteorology and atmospheric dynamics (middle atmosphere dynamics; turbulence; waves and tides)

VHF radar observations of gravity waves at a low latitude

Wind observations made at Gadanki (13.5°N) by using Indian MST Radar for few days in September, October, December 1995 and January, 1996 have been analyzed to study gravity wave activity in the troposphere and lower stratosphere. Horizontal wind variances have been computed for gravity waves of period (2-6) h from the power spectral density (PSD) spectrum. Exponential curves of the form eZ/H have been fitted by least squares technique to these variance values to obtain height variations of the irregular winds upto the height of about 15 km, where Z is the height in kilometers. The value of H, the scale height, as determined from curve fitting is found to be less than the theoretical value of scale height of neutral atmosphere in this region, implying that the waves are gaining energy during their passage in the troposphere. In other words, it indicates that the sources of gravity waves are present in the troposphere. The energy densities of gravity wave fluctuations have been computed. Polynomial fits to the observed values show that wave energy density increases in the troposphere, its source region, and then decreases in the lower stratosphere.Key words. Meteorology and atmospheric dynamics (middle atmosphere dynamics; turbulence; waves and tides)

On the reality of upper mesospheric/lower thermospheric turbulent “eddies”

The structure of atmospheric turbulence and its relationship to the refractive index discontinuities responsible for the scattering of radio waves are discussed. Emphasis is placed more on the nature of the intermittent scatterers observed at upper mesospheric/lower thermospheric altitudes than on volume scattering. The preferred model is of transient patches composed of vortex strings. An overview of the methods used to estimate the rate of dissipation of turbulent energy using MF radar data which emphasizes the advantages of imaging Doppler interferometry is presented, together with a review of pertinent laboratory investigations of coherent structures. An attempt is made to remove the contamination by gravity waves from imaging Doppler interferometry fluctuating velocity data. Emphasis is given to the fact that all radar measurements of turbulent intensity to date are upper estimates, not only because of the presence of gravity wave fluctuations contaminating the data, but also because the turbulence itself is not stationary and homogeneous but intermittent in both space and time. The intermittent nature of the turbulence, in which the refractive index discontinuities responsible for the radar backscatter are associated with decaying, homogeneous turbulent coherent structures, is in contrast to the theory of volume scattering, which assumes the radar pulse volume to be partially filled with stationary, homogeneous turbulence.

Radio Scintillations in Venus's Atmosphere: Application of a Theory of Gravity Wave Generation

Radio scintillations in Pioneer Venus radio Occultation data are simulated assuming that the index of refraction fluctuations in Venus's atmosphere responsible for the scintillations are directly caused by gravity wave fluctuations. The gravity waves are created by a global convection layer between 50- and 55-km attitude in Venus's atmosphere and propagate vertically. The authors compare the simulated scintillations with data from Pioneer Venus. These gravity waves can explain the spectral shape and amplitude of the radio scintilations. The shape at high frequencies is controlled by wave breaking, which yields a saturated spectrum. The amplitude is subject to parameters such as the intensity of the convection, the angle between the zonal winds and the beam path, and the zonal wind profile at polar latitudes. To match the observed amplitude of the scintillations, the velocity variations of the energy-bearing eddies in the convection must be at least 2 m s−1. This value is consistent with the Venus balloon results of Sagdeev et al. and is in the middle of the range considered by Leroy and Ingersoll in their study of convectively generated gravity waves. The later study, combined with the lower bound on velocity from the present study, then yields lower bounds on the vertical fluxes of momentum and energy in the Venus atmosphere.

Simulation of lidar measurements of gravity waves in the mesosphere

Lidar measurements of mesospheric gravity wave characteristics are simulated using a simple numerical model. Simple theory is used to superimpose gravity wave fluctuations upon typical background densities and sodium abundances in the height range 80–100 km, which we then assume are perfectly observed by a Rayleigh and Na lidar, respectively. These lidar “data” are then analyzed for their gravity wave content using the standard reduction and analysis methods which are routinely applied to these data sets. Our goal is to assess whether this analysis of lidar data faithfully recovers the underlying wave field. In both quasi‐monochromatic and spectral wave studies, the height range over which Na number density can be measured and the shape of the background Na profile impose a limit on the gravity wave information which can be extracted from Na lidar data. The limitations due to background layer shape do not exist for Rayleigh lidar measurements. The simulations reveal that quasi‐monochromatic gravity waves with vertical wavelengths larger than approximately 10 km may not be reliably retrieved from Na lidar data. It is also shown that there are limitations on the accurate extraction of spectral parameters from Na lidar data due to the limited vertical extent of the mesospheric Na layer.

Middle and upper atmosphere radar observations of ionospheric density gradients produced by gravity wave packets

By observing the ionospheric F region simultaneously in multiple beams with the middle and upper atmosphere (MU) radar, we have been able to track the passage of gravity waves and measure the horizontal electron density gradients that they produce. Our previous work on this topic, on one day of observations, verified that the gradient structure and fluctuations were consistent with a gravity wave source. The present work (1) looks at several days of observation to confirm the routine existence of these gradient structures; (2) identifies individual wave packets; (3) finds evidence that long trains of gravity wave fluctuations are composed of several juxtaposed wave packets of one or two cycles each rather than a single wave of many cycles; (4) finds that during quiet periods strong waves are present only during the daytime, but during disturbed periods they are present day and night; and (5) notes several other characterizations.