Simultaneous optical observations of long-period gravity waves during AIDA '89

Ground-based optical instrumentation supported the AIDA '89 wind measurement comparisons by describing the gravity waves affecting the 80–100 km altitude region during clear dark hours over Puerto Rico. This study tabulates the characteristics of gravity waves with fractional column emission rate amplitudes up to 30% and with periods greater than 45 min as seen in the O₂ airglow layer by MORTI, a sensor of O₂ rotational temperature and column emission rate in twelve look directions. Data from seven other sensors operating at Guanica and the Arecibo Observatory are then compared with the MORTI data to check the consistency of the entire data set with the wave parameters, primarily velocities, deduced from MORTI. Nine nights of visually distinct crests and troughs were found, one of which was dominated by an evanescent wave and the rest by internal waves. The nights of 5/6 April and 4/5 May 1989 were selected for multi-sensor comparisons. The comparisons showed substantial agreement between the MORTI characterizations and the observations by others, and most differences were attributed to complexities introduced by higher frequency components with shorter coherence distances. Nightly summaries of the O₂ rotational temperature and column emission rate are also given.     

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.     

Observations of ion layer motions during the AIDA campaign

The AIDA-89 campaign has yielded the most comprehensive set of low-latitude incoherent scatter radar power profiles and derived electron concentration results ever made. These results have been used to study the time-height trajectories of 80–150 km ion layers and serve to gauge both the periodicity and variability of ion layer structure throughout the campaign. Features of the AIDA ion layer trajectories point to a dynamics ‘zoo’ of processes ranging from multiday-period waves, tides and acoustic-gravity waves (AGWs) to geomagnetic storm effects and evidence of coupled neutral sodium and ion layer/plasma processes. The semidiurnal and diurnal tides are evidenced in the almost always present layers, the Tidal Ion Layers (TILs), which are identified by their regular and periodic trajectories that also display regions of variable mixing or confluence of the various tides. The TILs are contrasted with the truly sporadic layers that include sporadic E and sporadic intermediate layers. The sporadic layers may be formed due to interaction of the tidal wind system with AGWs. The formation process may involve horizontal as well as vertical ion convergence mechanisms and/or various non-linear effects. Limits to the study derive from volume undersampling due to use of the single radar beam.     

Daily tidal,planetary wave,and gravity wave amplitudes over the Canadian prairies

Fifteen months of daily mean winds, semi-diurnal tides, and gravity waves observed from the Canadian Prairies MF radar network were examined to find correlations in their amplitude fluctuations. In all cases, the correlations of the wave/winds between the three sites were greater than 95% significant. Generally, while the correlations between the various wave/winds types were much weaker, there was evidence of gravity wave/tidal and tidal/mean wind interactions.     

A numerical model for gravity wave dissipation in the thermosphere

Two simplified models of internal gravity wave dissipation due to viscosity, thermal conduction and ion-drag, in a multilayered, isothermal thermosphere are developed. Each of these models uses the WKB approximation, ray theory and the time-averaged equation of energy conservation, but whereas one of the models (A) employs all of the gravity wave equations appropriate to a dissipative atmosphere, the other (B) does not. Results derived from these models for one particular wave are compared to each other and also to some previously published results of Klostermeyer, which employed a full-wave, model. A breakdown of the WKB approximation in the lower, non-isothermal thermosphere leads to models A and B underestimating the total dissipation there. In the middle thermosphere model A estimates the dissipation reasonably well, while model B grossly overestimates the dissipation. In the upper thermosphere model A underestimates the total upward energy flux, probably as a result of the neglect of coupling into the dissipative waves at these levels, while no energy remains in model B. Results from model A show that when dissipation due to viscosity and thermal conduction are included correctly and simultaneously, the dissipation due to viscosity can exceed that due to thermal conduction by a factor of three. It is argued that ray theory may either overestimate or underestimate the energy flux reaching the upper boundary of a thermospheric model depending on both its height and the particular thermospheric model used.     

Fractal analysis of gravity wave spectra in the middle atmosphere

Wind fluctuations in the middle atmosphere behave like colored noise processes. They have a continuum of scales without dominant features and a power spectrum density (PSD) that often decays with frequency ƒ as ƒ^−β. Spectral index β is generally obtained through least-square fit to PSD estimated by Fourier methods. Graphs of colored noise have fractal plane-filling properties depending on β. An efficient method for finding β using the fractal dimension (D), based on analysis of 1/ƒ noise in galactic X-ray luminosities by McHardy I. and Czerny B., (1987, Nature325, 696), is described. An empirical relation is found between D and β and its validity is confirmed in limiting cases. Then D is obtained from power-law dependence of a length metric L(μ) on scale μ. The method is applied to middle-atmospheric velocity data from the Poker Flat radar in Alaska. Variations of D follow those in β, from an earlier analysis by Bemraet al., (1986, Handbook for MAP20, 216), but show an offset of 0.1–0.2 even after corrections for outliers, gaps, and additive noise. Usefulness of this method for screening data as an aid to spectral analysis is examined.     

Mesospheric wind studies during AIDA Act '89: morphology and comparison of various techniques

The Arecibo Initiative in Dynamics of the Atmosphere (AIDA) '89 was a multi-instrument campaign designed to compare various mesospheric wind measurement techniques. Our emphasis here is the comparison of the incoherent scatter radar (ISR) measurements with those of a 3.175 MHz radar operating a s an imaging Doppler interferometer (1131). We have performed further analyses in order to justify the interpretation of the long term IDI measurements in terms of prevailing winds and tides. Initial comparison of 14 profiles by Hines et al., 1993, J. atmos. terr. Phys. 55, 241–288, showed good agreement between the ISR and IDI measurements up to about 80 km, with fair to poor agreement above that altitude. We have compiled statistics from 208 profiles which show that the prevailing wind and diurnal and semidiurnal tides deduced from the IDI data provide a background wind about which both the IDI and ISR winds are normally distributed over the height range from 70 to 97 km. The 3.175 MHz radar data have also been processed using an interferometry (INT) technique [Van Baelen and Richmond 1991, Radio Sts. 26, 1209–1218] and two spaced antenna (SA) techniques [Meek, 1980, J. atmos. terr. Phys. 42, 837–839; Briggs. 1984, MAP Handbook, Vol. 13, pp. 166–186] to determine the three dimensional wind vector. These are then compared with the IDI results. Tidal amplitudes and phases were calculated using the generalized analysis of Groves, 1959, S. atmos. terr. Phys. 16, 344–356, historically used on meteor wind radar data. Results show a predominance of the diurnal S₁¹ tidal mode in the altitude range 70–110 km, reaching a maximum amplitude 45 ms⁻¹ at 95 km, with semidiurnal amplitudes being about 10–15 ms⁻¹ throughout the height range considered. There is evidence of the two day wave in data from 86–120 km, with amplitudes on the order of 20 ms⁻¹.     

Numerical simulations of the saturated gravity wave spectra in the atmosphere

A two dimensional numerical model is used to compute the saturation of small scale gravity waves in the region near the critical level. The vertical wave number spectrum of horizontal velocity fluctuations in the unstable region (USR) where shear instability develops is found to be governed by wave-shear interaction and follows a theoretical saturation spectrum ~ω_b²/2m³. Wave-shear interaction is also found to be responsible for the observed fact that the variance of vertical velocity fluctuations is significantly lower than the level predicted by linear gravity wave theory. On the other hand, the corresponding spectrum in the stable region (SR) following a much shallower spectrum ~m⁻² is found to result from the combined effects of wave-wave interactions and eddy diffusion. The key step in our simulation is the separate parameterization of horizontal and vertical eddy diffusion coefficients instead of a constant molecular viscosity coefficient.     

A simplified approach to internal gravity wave damping by viscous dissipation

Internal gravity wave perturbation velocity subject to constant dynamic viscosity and constant kinematic viscosity are approximately derived based on an energy conservation principle. When the dynamic viscosity is assumed to be a constant, the velocity at any height relative to the velocity at the saturation height, Z_sat, is found to be solely dependent on the number of scale heights measured from Z_sat. Gravity wave energy dissipation due to constant dynamic viscosity primarily occurs from one scale height below to one scale height above the saturation height. When the kinematic viscosity is assumed to be a constant, the perturbation amplitude either increases or decreases monotonically with height depending on whether the energy dissipation rate due, to viscosity, can offset the effect of the decreasing atmospheric density with increasing height. The derivations are made simple by assuming that the non-dissipative dispersion relation is applicable to the dissipative situation. The condition for the assumption to be approximately valid is also given.     

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.     

The non-linear interaction of a gravity wave with the vortical modes

The wave-wave interaction theory has been used successfully in describing one class of weakly non-linear wave phenomena. The application of this theory to the atmosphere shows the possibilities of energy and momentum transfer among three interacting gravity waves, as well as from the gravity wave to the other modes of motion. It has been found that the non-resonant interaction of a gravity wave with two vortical modes can proceed at a reasonably rapid rate. With the gravity wave viewed as the primary wave and the two vortical modes as the secondary waves, the interaction equation can be linearized and solved. The resulting analytic formula gives the growth rate of the interaction. In the absence of the Earth's rotation, the growth is limited to a threshold effect. The theory shows that whenever the horizontal air parcel velocity of a gravity wave exceeds a factor of √2 times the horizontal trace velocity of the wave, energy and momentum transfer from the gravity wave to the vortical modes can proceed. The rotation of the Earth will blur this threshold effect by making the interaction more likely to occur. Thus, through this mechanism, a gravity wave can transfer its energy and momentum to the horizontal velocity field in the vortical mode. In this sense, the small scale vortical motions would serve as the sink of both energy and momentum of a propagating gravity wave. When scales of vortical modes reach sufficiently small values, dissipation through viscosity becomes important. At this scale and smaller, the vortical modes are damped out quickly and its energy spectrum must exhibit a sharp decay.     

Formation of mesospheric VHF echoing layers due to a gravity wave motion

We present mesospheric backscattered VHF echo power and wind velocity data indicating the co-existence of a threefold strongly echoing layer and a wave motion, observed on 20 September 1985 with the MU radar at Shigaraki (34.9°N, 136.1°E), Japan. The echoing layers are clearly connected with the vertical and horizontal wind perturbations due to the wave. The analysis of the wind data have shown that the wave motion is due to an internal inertia-gravity wave with the vertical and horizontal wavelengths of 6 and 400 km, respectively, and period of 5.6 h. Evaluating the atmospheric stability in the wave field with the estimated wave parameters, the echoing layers are shown to be consistent with statically stable regions generated by the wave. It is suggested from our results that Fresnel scattering is a dominant echoing mechanism for a VHF radar beam in the mesosphere, as well as in the lower stratosphere.     

The nonlinear mechanism of gravity wave generation by meteorological motions in the atmosphere

Using asymptotic expansions of the hydrodynamic equations in the Rossby number and the method of multiple time scales, we derive approximate expressions for the inhomogeneous “forcing” terms which describe the continuous generation of inertio-gravity waves by quasi-geostrophic motions. As a result of numerical modelling applied to the evolution of tropospheric meso- and macro-scale wave sources, the values of these forcing terms are estimated. A three-dimensional numerical simulation of wave propagation from a mesometeorological tropospheric eddy into the upper atmosphere was done to estimate the gravity wave response to the sources described. The results of the calculations show that the most part of the wave energy propagates quasi-horizontally carried by two-dimensional inertio-gravity waves. At the same time, a part of the energy is transported into the upper atmosphere by internal-gravity waves which can create regions of wave disturbance in the upper atmosphere at considerable distances from the source site. The amplitudes of these waves increase with increasing intensity and decreasing time scales of the wave sources and can reach the values observed in the upper atmosphere.     

Long term periodicities in lower thermospheric wind variations

An analysis of long term meteor radar wind measurements for three temperature latitude stations shows that significant solar cycle oscillations with periods of 22 years and 11 years can be detected in the data.     

Gravity wave activity in the upper mesosphere over Urbana,Illinois: lidar observations and analysis of gravity wave propagation models

We analyze 375 h of Na Wind/Temperature lidar measurements of the mesopause region (≈ 80–105 km) Na density and temperature profiles on 57 nights distributed over 2 yr at Urbana, Illinois. These observations yield a high-resolution seasonal data set of gravity wave activity in the upper mesosphere. From this data, we present measurements of the Brunt-Väisälä period, the relative atmospheric density perturbations and their spectra, and the parameters of 143 quasi-monochromatic gravity waves. The direct measurement of the Brunt-Väisälä period allows accurate calculation of the horizontal velocity perturbations and vertical displacement perturbations from the density measurements. The horizontal velocity and vertical displacement vertical wave number spectrum magnitudes and indices show considerable seasonal and nightly variability. The gravity wave amplitudes, wavelengths, and observed periods exhibit systematic relationships similar to those found in previous studies, and are consistent with the MU radar measurements of intrinsic gravity wave parameters. Here, we present a detailed analysis of the observations in terms of Diffusive-Filtering Theory models of gravity wave propagation. The magnitudes of the vertical wave number spectrum, the form of the joint vertical wave number and frequency spectrum, and the systematic relationships between the monochromatic gravity wave parameters are consistent with the Diffusive-Filtering model. We compare these results with a variety of radar, lidar, and airglow observations from other sites. This observational study suggests that the complex nonlinear interactions of the gravity wave field may be modeled successfully as a diffusive damping process, where the effective diffusivity is a function of the total wave variance.     

Comparison of lower thermospheric winds measured by a Polarizing Michelson Interferometer and a Fabry-Perot spectrometer during the AIDA campaign

The Polarizing Atmospheric Michelson Interferometer, PAMI, a new version of the Wide Angle Michelson Interferometer, was used to measure winds in the lower thermosphere during the AIDA campaign. In the polarizing instrument, the optical path difference is changed simply by rotating a polarizing filter external to the interferometer. This allows a very simple scanning mechanism as described by Birdet al. [(1992) J. Phys.]Results of measurement of the 557.7 nm emission were obtained from the AIDA observation campaign in Puerto Rico (with PAMI located at 17°57′0″N, 66°52′42″W). Co-ordinated observations of atmospheric motions were made by PAMI along with other optical and radio measurements during April and May 1989. By comparing with the Arecibo Fabry-Perot instrument, the first wind comparisons of a FabryPerot spectrometer and a Michelson interferometer are presented. The agreement is very good in most cases, but there are times when there is a constant wind offset for several hours and a few occasions of major disagreement. It is concluded that the constant offset results from the 50 km difference in the locations of the two stations; the major disagreement may result from contamination, for the MI, of winds in the F-region during ionospheric disturbances.     

Measurements of mesospheric gravity wave momentum fluxes and mean flow accelerations at Adelaide,Australia

Forty-one days of measurements of the upward flux of zonal momentum associated with internal atmospheric gravity waves propagating in the upper mesosphere and lower thermosphere, made in thirteen 2–5 day periods, in each season, for the years 1981 and 1982 are presented, and the zonal mean flow acceleration is calculated for each period. For five periods of observation the upward fluxes of both zonal and meridional momentum are presented and for these, the total mean flow acceleration is calculated. When averaged over periods of 2–5 days, the magnitude of the upward flux of zonal momentum is typically less than about 3 m² s⁻¹, with the largest values tending to occur in the summer and winter months, suggesting a semi-annual variation with minima at the equinoxes, although large fluctuations in magnitude and sign are possible. About 70% of the upward flux of horizontal momentum appears to be due to motions with periods less than 1 h and their contribution to the mean flow acceleration is comparable. The zonal mean flow acceleration is often in the correct sense, and of sufficient magnitude, to decelerate the zonal wind component and to balance the Coriolis torque due to the mean meridional wind, when experimental uncertainties are taken into account. When averaged over periods of around 3 days, zonal mean flow accelerations with magnitudes of up to 190 m s⁻¹ day⁻¹ were calculated, but more typical values are between 50 and 80 m s⁻¹ day⁻¹. Magnitudes of the meridional and zonal mean flow accelerations were found to be similar, so that the total mean flow acceleration is not aligned with the zonal direction in general.