Climatology of gravity waves in the middle atmosphere

An attempt is made at the statistical analysis of small-scale disturbances in the stratosphere and mesosphere with the aid of meteorological rocket observations at many stations from 77°N to 8°S for several years.By applying a high-pass filter to daily rocket data in the height range 20–65 km, wind and temperature fluctuations with characteristic vertical scales close to or less than 10 km are obtained, which are considered to be due to internal gravity waves. Results are expressed in terms of parameters which tend to emphasize smallscale vertical fluctuations and which should provide qualitative measures of gravity wave activity.It is found that the gravity wave activity shows a notable annual cycle in higher latitudes with the maximum in wintertime, while it shows a semiannual cycle in lower latitudes with the maxima around equinoxes. It is also found from the standard deviation around the monthly mean that the temporal variability of gravity waves is very large.     

Some further results on long term mesospheric and lower thermospheric wind observations by the Arecibo UHF radar

A second series of long term mesospheric and lower thermospheric wind observations was conducted at Arecibo (18.4°N, 66.8°W) between 6 and 20 March 1981 using the UHF Doppler radar, following the first observations in August 1980 (Hirota et al., 1983). Zonal and meridional wind velocities were measured during the morning (8–10 LT) and afternoon (13–15 LT) periods. The mean wind profile averaged over the entire observational period shows the predominance of the diurnal tide. The fluctuating wind vector rotates clockwise relative to height with a characteristic vertical scale of about 10 km. The phase difference inferred by a cross correlation analysis between morning and afternoon profiles indicates that the dominant period is about 20–30 h. This oscillation is discussed in relation to internal inertia-gravity waves observed by the same radar in the lower stratosphere. On the other hand, wind fluctuation with a vertical scale larger than 20 km shows a substantial day-to-day variation with a period of 5–8 days. This long period oscillation shows a good correlation with the global scale geopotential height anomalies at 1 mb (46–48 km) observed by the Tiros-N satellite at 20°N. Our evidence suggests that westward travelling planetary-scale waves with zonal wavenumber one may propagate up to the lower thermosphere.     

Dominant vertical scales of gravity waves in the middle atmosphere observed with the MU radar and rocketsondes

We have simultaneously observed wind motions in the altitude range of 5–90 km by means of the MU radar, rocketsondes and radiosondes. Dominant vertical scales of wind fluctuations due to gravity waves were 2–5 km in the lower stratosphere, about 5–15 km in the upper stratosphere and longer than 15 km in the mesosphere. The increase in the vertical scale with altitude is interpreted in terms of the saturation of upward propagating gravity waves. In the stratosphere, the observed vertical wavenumber spectra showed smaller amplitudes and more gradual slopes than the model values. Furthermore, the wind velocity variance in the stratosphere increases exponentially with an e-folding height of about 9 km, implying that the gravity waves were not fully saturated. On the other hand, the spectra in the upper stratosphere and mesosphere agreed fairly well with the model spectra. The variance in the mesosphere seems to cease increase of the wave amplitudes and agrees reasonably well with the model value.     

The analysis of hydroxyl rotational temperatures to characterize moving thermal structures near the mesopause

Hydroxyl (OH) rotational temperatures near 85 km altitude have been monitored at Calgary, Alberta, Canada (51°N, 114°W) since 1981 with the objective of determining velocities, wavelengths and periods associated with moving temperature structures. A technique is described whereby the velocity of moving patterns in two dimensional data sets can be accurately determined and used as a parameter for a global smoothing algorithm. Velocities of the structures in the meridional direction were found to be directed poleward. Corresponding Doppler bulk wind velocities measured near the 95 km height region were directed equatorward indicating the presence of filtering of internal gravity waves by the background wind. Two coherent wave structures were often observed simultaneously during a night. The smaller of the two structures had true wavelengths less than 15–30 km and may be related to billow clouds often reported in noctilucent cloud observations. The second wave has a period on the order of an hour and meridional wavelengths ranging from 100 to 2000 km.     

Gravity waves from O2 nightglow during the AIDA '89 campaign III: effects of gravity wave saturation

The O₂ atmospheric (0–1) night airglow emitted within the gravity wave saturation region at ∼90–100 km can serve as a means of studying the wave activity. In this analysis, the atmospheric motions were described by a mean spectral model and an algorithm was developed to infer the wave kinetic energy density and momentum flux from variations in O₂ (0–1) airglow emission rate and rotational temperature. The method was applied to eight nights of data collected by MORTI, a mesopause oxygen rotational temperature imager, during the AIDA campaign of 1989 in Puerto Rico (18°N, 67°W). The observed r.m.s. fractional fluctuations of airglow emission rate and rotational temperature were of the order of ∼0.07–0.15 and ∼0.02–0.04, respectively, and the characteristic vertical wavelengths were estimated at ∼10 2 -20 km. The inferred r.m.s. horizontal velocities and velocity variances were found to be ∼12–25 m/s and ∼150–600 m²/s², with the majority of the horizontal velocity and its variance associated with low-frequency, large-scale wave motions. The estimated momentum fluxes, mainly contributed by high-frequency, small-scale waves, were ∼2–10 m²/s². These results are in good agreement with those obtained from other measurements using different observational methods at low and mid-latitudes.     

Gravity-wave motions in the mesosphere and lower thermosphere observed at Mawson,Antarctica

We present the results of MF radar observations of mean winds and waves in the height range 78–108 km at Mawson (67°S, 63°E), Antarctica. The measurements were made in the period from 1984 to 1990. Climatologies of the prevailing zonal and meridional circulations made with a 12-day time resolution show that the mean circulation remained relatively stable over the 6 yr of observation. Climatologies of gravity-wave motions in the 1–24 h period range were also generated. These reveal that the r.m.s. amplitudes of horizontal wave motions near the mesopause (~90 km) are about 30 m s⁻¹, and that there is some anisotropy in the motions, especially at heights below 90 km. Meridional amplitudes are larger than zonal amplitudes, which suggests a preference for wave propagation in the north-south direction. Comparisons with MST radar wind observations made near the summer solstice at Poker Flat, Alaska (65°N) and at Andøya, Norway (69°N) show similarities with the Mawson observations, but the wave amplitudes and mean motions are larger in magnitude at the northern sites. This suggests hemispheric differences in wave activity that require further study.     

Observations of gravity waves from multispectral mesospheric nightglow emissions observed at 23°S

Simultaneous measurements of the 015 57.7 nm, O₂ atmospheric (0,1) band, NaD and OH (9,4) band emissions obtained during the period October November 1989 at Cachoeira Paulista (23°S, 45°W), Brazil, have been analysed to study gravity waves in the mesospheric region at a low-latitude station in the southern hemisphere. It was found that, when these emissions showed large temporal intensity variations, there were also short period quasi-coherent temporal variations superposed on them, suggesting a possible passage of internal gravity waves in the emission layers. Cross-correlation analysis indicates that the time lag between the different emissions is smaller for short period variations compared with the long period variations. The wave parameters, namely a vertical wavelength of 12 km, a horizontal derived wavelength of 200 km with a period of 80 min, estimated from one of the observed short-period coherent oscillations, are typical of the internal gravity waves at the airglow emission height.     

A study of equatorial waves in the Indian zone

An equatorial wave campaign was conducted at Trivandrum (8.5°N, 77°E), Minicoy (8.3°N, 73°E) and Port Blair (11.7°N, 92.7°E) during June-July 1988. The campaign provided balloon-measured daily wind profiles at all the three stations for 48 days in the 0–30 km altitude range and rocket-measured daily wind profiles at Trivandrum for 42 days in the 31–60 km altitude range. Using these daily wind data a study was made on different equatorial wave modes present in this region. The study revealed evidence of Kelvin waves with period 12–16 days and vertical wavelength ∼ 10 km in the lower stratosphere, with period 6–9.6 days and vertical wavelength of ∼ 10–15 km in the stratospheric-lower mesospheric region and MRG waves with periods 4–4.4 days and vertical wavelength of 10 km in the upper troposphere and lower stratosphere.     

Permanent monitoring of the upper mesosphere and lower thermosphere wind fields (prevailing and semidiurnal tidal components) obtained from LF D1 measurements in 1991 at the Collm Geophysical Observatory

The wind field of the upper mesosphere and lower thermosphere region (85–105 km) over Central Europe (52°N, 15°E) has been continually and reliably recorded by regular daily D1 radio wind measurements in the LF range (177, 225 and 270 kHz) using commercial radio transmitters. These measurements show the prevailing winds, the tidal wind components and the effects of internal gravity waves, as well as the seasonal and irregular variations of these parameters. The height of the wind measurements is determined by measuring the travel time differences between corresponding modulation bursts in the sky wave and in the ground wave. Using a quasi-online calculation procedure, the results are available immediately. Therefore they are useful for monitoring the upper atmospheric circulation with regard to upper atmosphere meteorology in the future. Vertical profiles of the wind field parameters can be derived with the aid of the combined wind and height measurements. Height-time cross-sections of the monthly mean prevailing winds and semidiurnal wind components have been calculated almost continuously for the last 10 years. The present paper deals with recent results for the year 1991.     

Dynamic regime of the mesopause—lower thermosphere at mid-latitudes of the northern hemisphere by radio meteor observations

The coherent pulse Meteor Automatic Radar System (MARS) based at Kharkov (49°30′N, 36°51′E) was used to measure zonal winds in the altitude range 80–105 km in the period from November 1986 to December 1990. It was found that, for the greater part of the year, the zonal prevailing wind component was in the eastward direction. The change from eastward to westward direction begins in the lower thermosphere in February–March, propagating downwards to the mesosphere, and it remains there until June–July. The structure of semidiurnal tides has general regularities at different sites. Annual variations in the monthly mean values of semidiurnal vertical wavelengths are practically the same, both in the northern and southern hemispheres. Wavelengths are more than 100 km in summer months, whereas they are less than 60 km in winter months.Studies of internal gravity wave (IGW) parameters in the height range of 80–105 km have shown that the internal gravity wave amplitude does not exceed 30 m/s, the vertical wavelength is in the range of 10–30 km, the horizontal wavelengths are 100–800 km and the horizontal phase velocities are in the range 20–160 m/s. The propagation and breaking of upward and downward IGW at heights of 80–100 km have been recorded.     

A simple model of gravity-wave momentum and energy fluxes transferred through the middle atmosphere to the upper atmosphere

Vertical fluxes of momentum and energy through the middle atmosphere are calculated by using a simple semi-empirical model of quasi-monochromatic internal gravity waves with dominant vertical wavenumbers. In this model those dominant gravity waves are assumed to saturate and break at each observational altitude by an effective critical-layer mechanism. The dominant value of the vertical wave-number is expressed by an exponential function of altitude, decreasing upward with a scale height of 34 km. This expression gives the momentum and energy flux densities decreasing upward with scale heights of 12 and 18 km, respectively, and typical values at 100 km altitude are estimated as 4 × 10⁻⁵ Pa and 4 × 10⁻³ W/m². A heat flux induced by wavebreaking turbulence also has an order of magnitude similar to that of the wave energy flux. Variabilities around these values and comparisons with other momentum and heat inputs to the upper atmosphere are only briefly discussed.     

Variations of the gravity wave characteristics with height,season and latitude revealed by comparative observations

This paper reviews some recent observations of gravity wave characteristics in the middle atmosphere, revealed by co-ordinated observations with the MU radar in Shigaraki (35°N, 136°E) and nearby rocketsonde experiments at Uchinoura (31°N, 131°E). We further summarize the results of comparative studies on the latitudinal variations of the gravity wave activity, which were detected by additionally employing data obtained with MF radars at Adelaide (35°S, 139°E) and Saskatoon (52, 107W) and lidar observations at Haute Provence (44, 6E).The seasonal variation of gravity wave activity detected with the MU radar in the lower stratosphere showed a clear annual variation with a maximum in winter, and coincided with that for the jet-stream intensity, indicating a close relation between the excitation of gravity waves and jet-stream activity at middle latitudes. The long-period (2–21 h) gravity waves seemed to be excited near the ground, presumably due to the interaction of flow with topography, and the short-period (5 min 2 h) components had the largest kinetic energy around the peak of jet-stream.We found an increase with height in the vertical scales of dominant gravity waves, which can be explained in terms of a saturation of upward propagating gravity waves. The values of the horizontal wind velocity variance generally increased in the stratosphere and lower mesosphere, but they became fairly constant above about 65 km due to the wave saturation, resulting in the active production of turbulent layers.Although the gravity wave energy showed an annual variation in the lower atmosphere, it exhibited a semiannual variation in the mesosphere, with a large peak in summer and a minor enhancement in winter. Lidar observations reasonably interpolated the seasonal variations in the intermediate height regions.The gravity wave energy in the mesosphere, with periods less than about 2 h, was consistently larger in summer than in winter at all the stations, i.e. at 35N, 44N,52 N and 35 S. However, the values were generally larger at 35 N than at 52 N. which was found from a comparison of l-yr observations at Shigaraki and Saskatoon. Furthermore, a comparison between Shigaraki and Adelaide, located at the conjugate points relative to the equator, revealed that the gravity-wave energy in the mesosphere was found to be fairly similar, when we compared the values in summer/winter in each hemisphere.     

Contributions of gravity waves to the momentum,heat and turbulent energy budget of the upper mesosphere and lower thermosphere

The role of gravity waves for the momentum and heat budget of the atmosphere between approximately 70 and 110 km height is considered. Parameterization schemes for vertical gravity wave diffusivity, generalized Rayleigh friction, viscous force, heat conduction and kinetic energy dissipation are reviewed. Eddy diffusion parameterization and its relation to the gravity wave approach is also discussed and it is shown that principal similarities exist in both concepts, especially when irregular (stochastic) contributions to the perturbations are modeled. Special attention is paid to the dissipation of perturbation kinetic energy and its contribution to the heat budget of the mesopause region. It is concluded that the amount of energy which can be attributed to the part of the gravity wave spectrum contributing to generalized Rayleigh friction above the mesopause is of the order of 10% of the total perturbation energy.     

Simultaneous nightglow and Na lidar observations at Arecibo during the AIDA-89 campaign

Simultaneous nightglow and Na lidar observations made at Arecibo during the AIDA-89 campaign are reported from between 0000 and 0530 LT (Local Time) on 6 April 1989 and on 9 April 1989. On 9 April the observations are consistent with the presence of a large amplitude 8.5 km vertical wavelength, 4 h period wave propagating through the 85–91 km region. A lower amplitude 80 min period wave is also observed. The results imply that the O₂ atmospheric band intensity peaked near 91 km while the OH Meinel (6,2) band intensity peaked near 85 km. The OH Meinel temperature and intensity are 160° out of phase which can be explained by low eddy diffusion and high ozone densities near the mesopause. The integrated Na abundance from 87 to 89 (90–92) km correlates well with the OH Meinel (O₂ atmospheric) band intensity. On 5–6 April the OH Meinel and O₂ atmospheric band intensities are not well correlated. The OH Meinel intensity is correlated with the integrated Na abundance from 86 to 88 km. Both the Na and OH measurements reveal the presence of an approximately 1 h period wave. The OH temperature data appear to be consistent with the OH Meinel band originating near 85 km. The O₂ atmospheric band data show the presence of a 2 h period wave. The integrated Na abundance data suggest that the O₂ atmospheric band peaks between 90 and 94 km. A large sporadic Na event which occurs near 6 UT appears related to the presence of a gravity wave near 95 km. In all of the observed waves there is good agreement between the wave parameters derived separately by the optical airglow and Na lidar techniques.     

Vertical structure of AGW associated ionospheric fluctuations in the E- and lower F-region observed with EISCAT—a case study

The vertical structure of AGW (atmospheric gravity wave) associated fluctuations of ionospheric plasma parameters for the 100–240 km altitude range in the daytime of 7 September 1988 has been investigated by making use of the data provided by the Tromsø measurements in the EISCAT CP1 observation mode.The wave power profile vs height has been studied by integrating the power spectral density in each altitude. The essential feature of the power variation can be explained in terms of the energy conservation of AGWs propagating in a dissipative thermosphere. Intrinsic propagation parameters of the dominant AGW have been successfully estimated with a method based on the retrieval of the Doppler effect due to the horizontal prevailing wind. From the fluctuation structure analysis in a time-altitude frame, a downcoming AGW has been clearly identified. This downcoming wave might have been reflected from a wind shear at the altitude around 200 km, which is inferred from the meridional prevailing wind profile.     

Observational evidence of a saturated gravity wave spectrum in the mesosphere

Five vertical profiles of scalar horizontal winds have been measured at high resolution (25m) in the range from 80–95 km during the last salvo of the MAC/SINE campaign in the summer 1987 at Andenes, Northern Norway (69.3°N). Our purpose in this study is to examine the consistency of the motion spectrum with the saturated spectrum of gravity waves proposed by Smith S. A., Fritts D. C. and Van Zandt T.E., (1987, J. atmos. Sci. 44, 1404). An analysis of vertical wavenumber spectra of the five horizontal wind profiles is presented and it is found that (a) the average slope of the five vertical wavenumber spectra is −3.0 ± 0.2 for wavelengths in the range from 6.4 km to 100 m. The slope is considerably steeper than the vertical wavenumber spectra of the horizontal velocity discussed in the literature, (b) the average vertical wavenumber spectrum shows that there is excellent agreement between the observed spectrum and the saturated spectrum in both slope and amplitude, suggesting that saturation processes do indeed act to control spectral amplitudes at large wavenumbers, and (c) a dominant vertical wavelength of 6.4 km is found in the mesosphere. Taken together, our observations provide further support for the saturated spectrum theory.     

Summertime stratospheric wind measurements above the South Pole

Observations of the mean wind flow and wave motions in the stratosphere at the South Pole are presented. The atmospheric motions are determined from the tracking of a high altitude, zero-pressure balloon launched from Amundsen-Scott Station during the austral summer of 1985–1986. The balloon position was precisely monitored by an optical theodolite for a large portion of the flight so that small scale motions could be resolved. The mean flow above the pole was approximately 3ms⁻¹. Atmospheric motions characteristic of internal gravity waves were observed with an intrinsic period of approximately 4.5 h and vertical and horizontal wavelengths of approximately 2.5km and 125km, respectively. The horizontal perturbation velocity of the observed waves was large compared to the mean horizontal flow velocity. The implication is that wave motions play a dominant role in the transport of stratospheric constituents in regions where the mean winds are light, such as over the South Pole during austral summer.     

Some further results on the lower stratospheric winds and waves over Jicamarca

The VHF radar at Jicamarca, Peru (12.0°S, 76.9°W) was used to probe the tropical lower stratosphere on 3–4 October and 6–8 December 1977. Velocity data obtained during these experiments exhibit features indicative of winds and waves in the stratosphere as follows:

• 1.(1) The amplitude and phase of a diurnal oscillation in the observed horizontal wind compare well with theoretically predicted values of the solar diurnal tide. An observed semidiurnal oscillation differs considerably from theoretical values, although minimal phase variation with height is a fundamental property of both theory and observation. The observed vertical wind oscillations are larger than theoretical values of vertical tidal components, although the data is consistent with recent rocket observations.
• 2.(2) Dominant velocity oscillations with periods near the Brunt-Väisälä period are frequently observed.
• 3.(3) Downward phase progression of the westerly regime of the quasi-biennial wind oscillation is observed during the course of the two observations. A long-period oscillation with a period exceeding two days also appears to be superposed on the quasi-biennial oscillation.
• 4.(4) Systematic differences are found between horizontal winds measured by the radar and those measured by rawinsondes launched from the Lima-Callao airport some 30 km west of Jicamarca.

     

Heights of MF radar scatter (1986/87) and the wind field (55–95 km): Saskatoon,Canada

The Saskatoon MF radar (2.2 MHz) at 52°N, 107°W, has been used to measure the heights of occurrence of radar scatter during four seasons, and twelve months of 1986/87. Mean winds, and gravity waves are also available, by the spaced antenna method and from the same radar echoes. Certain heights, called elsewhere ‘preferred heights’, are identified near 60km, 70km, 75km in summer, and 80–86 km. Several layers have seasonal and diurnal variations. Associations with electron density gradients (rocket data), mean wind shear in summer, and gravity wave amplitude-minima in the equinoxes are effectively demonstrated. Case studies, involving 3 h data sets of radar scatter and wind elaborate the comparison: gravity waves of long period (τ > 6 h) are shown to modulate the scattering process.     

Daytime mesospheric temperatures over the low-latitude station Thumba derived from rocket-borne solar Lyman-α absorption measurements

Molecular oxygen concentrations obtained from the rocket measurements of the absorption profile of solar Lyman-α radiation, in the upper atmosphere over Thumba (8°33′N, 76°52′E), have been used to derive gas temperatures in the 70–90 km altitude region. A mean reference temperature profile for the daytime mesosphere over Thumba obtained from these measurements shows (1) temperatures lower than those given by the CIRA model and (2) a mesopause around 83 km with a temperature of 175 K.