Dynamic coupling between the lower and upper atmosphere by tides and gravity waves

Results of a General Circulation Model simulation of the dynamics of the middle atmosphere are shown focusing our attention to the tidal wave mean flow interaction and propagation of migrating diurnal and semidiurnal tides in the model. It is shown that migrating tidal waves are well simulated and the amplitude growth with height is effectively suppressed by the convective adjustment in the model. It is also shown that the dissipating solar diurnal tide plays an important role in inducing mean zonal winds in the low latitude region of the lower thermosphere. The behavior of non-migrating diurnal tides is also analyzed to show that non-migrating diurnal tides have significant amplitudes in the lower thermosphere. It is suggested that the non-migrating diurnal tide, which propagates against background mean zonal winds, has the possibility to propagate into the middle to high latitude region due to the Doppler effect.     

The impact of gravity waves on nitric oxide in the lower thermosphere

Observations of nitric oxide (NO) by the Solar Mesosphere Explorer (SME) during equinox indicate a lower-thermosphere equatorial minimum which is at variance with theoretical predictions. To address this discrepancy a zonally averaged model of the thermosphere and upper mesosphere is used to evaluate the influence of a latitude variation in turbulence. Five numerical simulations were performed with different latitude structures of eddy diffusion (K_T), ranging from uniform in latitude, peaks at low, mid-, or high-latitude, to a hemispherically asymmetric distribution. A local increase in eddy diffusion causes the lower thermosphere to cool and induces a latitude pressure gradient that drives horizontal and vertical winds. The circulation, turbulent transport and temperature dependent chemistry act to change the distribution of species. Comparison of the model predictions of NO with SME data, and simulated wind and temperature structure with empirical climatology, indicates a preference for a midlatitude peak in K_T.     

Non-linear interaction between the diurnal and semidiurnal tides: terdiurnal and diurnal secondary waves

Many years of measurements obtained using French meteor radars at Garchy (Lat. 47°N) and Montpazier (Lat. 44°N) are used to show the existence of an 8 h oscillation. Some examples of the structure of this wave are displayed and compared with measurements performed at Saskatoon (Lat. 52°N) and Budrio (Lat. 45°N). This wave can be interpreted as the solar driven terdiurnal tide, or as the result of the non-linear interaction between the diurnal and semidiurnal tides. Both hypotheses are tested with numerical models. Incidentally, the possible existence of a 24 h wave resulting from this interaction is also studied.     

Inter-annual variability of tides in the mesosphere and lower thermosphere

The inter-annual variation in diurnal and semi-diurnal atmospheric tides between 85 and 95 km has been studied for various years between 1978 and 1988. Observations comprised wind measurements from the medium frequency SA mode wind radars at Adelaide (35°S), Christchurch (44°S) and Saskatoon (52°N) and the meteor wind radar at Durham (43°N). Although the observations include the interval between solar maximum and solar minimum, there is in general no correlation between tidal amplitudes and solar activity. In contrast with earlier studies there does appear to be a positive correlation between solar activity and the amplitude of the semi-diurnal tide, but only during the southern summer and simultaneous northern winter.     

The semidiurnal tides at the midlatitude lower thermosphere over East Siberia

Results from the study of semidiurnal tides in the horizontal wind field at 85–95 km over East Siberia are presented. The seasonal variation of tidal amplitudes and the effects of stratospheric warmings are discussed.     

A quartic dispersion equation for internal gravity waves in the thermosphere

A new quartic dispersion equation in the square of the complex vertical wave number is derived by employing the ‘shallow atmosphere’ approximation and an ion drag approximation. These approximations allow the coefficients of the quartic equation to be given in terms of the corresponding cubic equation, which neglects the Coriolis force and the zonal ion drag component, but modified to take into account these neglected effects. Coupling between the extraordinary viscosity wave mode and the other three wave modes is highlighted and numerical solutions are compared for this quartic equation, an exact eighth order equation and the cubic equation. For the first time the validity of using the ‘shallow atmosphere’ approximation to describe internal gravity wave motions is demonstrated.     

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.     

Effects of solar tides on the zonal mean circulation in the lower thermosphere: solstice condition

Effects of momentum deposition due to solar diurnal and semi-diurnal tidal waves on the zonal mean circulation in the mesosphere and lower thermosphere for a solstice condition are discussed. In the present model, the system of zonally averaged equations and the system of perturbation equations are integrated simultaneously, so that the propagation of tidal waves is affected not only by the basic mean fields but also by the induced zonal mean fields due to the momentum deposition. Results for two different vertical eddy diffusion profiles are presented. It is shown that the solar tides make a significant contribution to the generation of the mean zonal winds in the upper mesosphere and the lower thermosphere. Below 120 km the main contribution is due to propagating diurnal tides, while above 120 km it is due to semidiurnal tides.     

Radar studies of gravity waves and tides in the middle atmosphere: a review

This review deals with recent radar studies of gravity waves and tides in the middle atmosphere, roughly over regions of 10–30 and 60–90 km. The techniques are briefly discussed and their limitations are pointed out. In the troposphere-stratosphere region, buoyancy oscillations, gravity-wave critical-layer interactions, and gravity waves excited by cumulus convection have been observed. Pronounced short-period (10–20 min) waves have frequently been detected in the mesosphere, and in some cases these have been identified as evanescent and trapped gravity wave modes. Diurnal and semidiurnal tides have been observed in the stratosphere and mesosphere at low and mid latitudes, but the corresponding tidal modes are not unambiguously resolved. The need for obtaining more comprehensive data bases with the existing radar systems is emphasized for further tidal and wave studies in the middle atmosphere.     

Simulation of the global structure of stationary planetary waves in the mesosphere and lower thermosphere

When simulating the global structure of stationary planetary waves (SPW) the problem of obtaining the numerical solution in the equatorial region appears. It results from the presence of apparent singularities in the operator of the SPW latitudinal structure when the Coriolis parameter is small. The new method based on SPW latitudinal operator inversion is proposed. This method permits the difficulties arising from the simulation of stationary large scale disturbances at low latitudes to be avoided. The global structure of SPW with zonal wave number m = 1 at the mesosphere and lower thermosphere heights has been calculated for the background zonal wind distribution representing a climatic picture of the solstice conditions. In the region of the mean zonal westerlies the SPW penetration across the equator is obtained. The SPW at low latitudes are shown to appear most significantly in the zonal component of the wind velocity. The influence of planetary wave motions on the distribution of longlived species in the ionospheric D-region and at the heights of lower thermosphere are discussed.     

High-latitude tidal behavior in the mesosphere and lower thermosphere

The high-latitude structure of the mean winds and tides is described in this paper using climatologies prepared from radar data during the Atmospheric Tides Middle Atmosphere Program. The monthly evolution of the amplitude and phase of the tides is discussed. Comparison between the southern and northern hemispheres indicate that the diurnal tide is stronger in the southern hemisphere and that the antisymmetric diurnal tidal modes are dominant. The semidiurnal tide is larger than the diurnal tide. The vertical wavelength structure is significantly different between the southern and northern hemisphere. Comparisons with recent tidal models show several discrepancies.     

Variability of winds and temperatures in the lower thermosphere

High-resolution daytime incoherent scatter radar measurements of plasma temperatures and drifts in the ionospheric E-region above Millstone Hill (42.6°N, 71.5°W) have been used to derive horizontal neutral winds and temperatures in the lower thermosphere (105–130 km) during five multi-day campaigns in 1987–1991. The underlying semi-diurnal tidal component has been determined from the observations, with characteristic average amplitudes of 50 ± 15 m/s and 30 ± 10 K. Phase propagation with altitude follows the expected structure of semi-diurnal tidal modes, but reveals complex coupling of tidal modes, particularly above 115 km. Day-to-day variability in the winds and temperatures is large, and the deviations from the semi-diurnal harmonic can exceed 40 m/s and 50 K. No strong correlations have so far been found with geophysical parameters to explain the observed variability.     

Infrared radiative cooling in the mesosphere and lower thermosphere

This paper discusses the current status of calculating infrared cooling by CO₂ in the mesosphere and lower thermosphere. It is desirable to have fast but accurate procedures for use in dynamic models. The most difficult region is from 70 to 90 km, where cooling rates are strongly influenced or, in the case of the summer mesopause region, dominated by the absorption of radiation emitted by underlying layers, with the hot bands and isotopic bands playing a significant role. A three-energy-level model is derived for the excited population levels of a CO₂ molecule. Vibrational-vibrational coupling between isotopes is also included as significant. Results from model calculations for cooling rates and NLTE source functions are presented. Global average infrared cooling rates appear to be in reasonable balance with solar heating rates, considering the uncertainties in calculating both these terms. Radiative cooling rates by CO₂ above 100 km are strongly dependent on atomic oxygen concentrations and on the rate of energy exchange between atomic oxygen and CO₂. Likewise, NO cooling, which is important above 120 km, is proportional to atomic oxygen concentrations. Since CO₂, NO and O concentrations can all vary with motions, these dependencies suggest interesting feedbacks to atmospheric dynamics.     

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.     

The meteorological jet stream as a source of medium scale gravity waves in the thermosphere: an experimental study

Results are reported of a co-ordinated experimental study of medium scale gravity waves involving use of a multistatic incoherent scatter system together with two networks of polarimeters monitoring transmissions from a geostationary satellite during a six-day period in July 1974. The latter observations give access to the horizontal propagation parameters of the waves while the former technique yields information on the vertical structure of both waves and the ambient atmosphere. A reverse ray tracing analysis, through the atmosphere, for each gravity wave is described and its validity and accuracy discussed in detail. For the majority of the waves the reverse group path can be followed down to tropopause level and comparison with meteorological data has shown that many of the possible source regions of the observed waves appear to lie in proximity to the jet stream, with some evidence to suggest a preference for the polar side of the jet. Other wave sources are found to be close to regions of convectively unstable cold polar air. Wave energetics are discussed together with possible generation mechanisms, the latter in the form of non-linear interaction of shear flow instabilities in the jet stream and penetrative convection.     

Sporadic neutral metal layers in the mesosphere and lower thermosphere

Although the existence of thin ionized layers at heights around 100 km has been known for many years, it is only much more recently that thin neutral metal layers have been observed. Such layers, initially sodium and more recently calcium and iron, have been detected by lidar. The layers, with thicknesses between about 100m and several kilometres, and concentrations between about 10² and 10⁵ cm⁻³, occur most frequently between 90 and 100 km, and are normally superimposed on a background layer about 10 km thick. The occurrence of thin neutral layers appears to be latitude dependent, and is strongly linked to the appearance of Es on ionograms. Several causative mechanisms have been suggested, none of which appears to be capable of providing an altogether satisfactory explanation for the formation of the layers.     

Evaluation of the vertical flux of energy into the thermosphere from medium scale gravity waves generated by the jet stream

Since the paper of Hines (1960), the heating of the thermosphere by gravity waves is well known. Until recently there were no statistical evaluations of their energy. The present paper is a study of this subject. From experimental data of a Faraday rotation experiment and with the help of a measurement theory and a propagation model in a dissipative atmosphere, we evaluated the mean energy flux for the medium scale gravity waves detected during July 1974. We found 0.11 erg cm⁻² s⁻¹ for waves with horizontal phase velocity between 100 and 180 m s⁻¹. We also evaluated the heights of energy deposition which are from 120 to 180 km. This study shows that the heating by gravity waves may be important for the thermospheric equilibrium.     

EISCAT observations of tidal modes in the lower thermosphere

EISCAT has made regular measurements of plasma velocity at heights between 101 and 133 km in the E-region and at 279 km in the F-region as part of the Common Programme CP1. Correcting for the effect of the electric field as determined in the E-region, it is possible to estimate the neutral wind velocity in the E-region for a number of days in the period 1985–1987 when magnetic conditions were relatively quiet. These velocities display diurnal and semi-diurnal tidal oscillations. The diurnal tide varies considerably from day to day in both amplitude and phase. The semi-diurnal tide also varies in amplitude but displays a fairly consistent phase at each height and the variation of phase with height below 110 km indicates a dominant (2,4) mode. Above 120 km the variation of phase with height is slower which suggests that at these heights the (2, 4) mode is attenuated and the (2, 2) mode is more important. The results agree well with previous measurements at high latitude.     

Eddy diffusion models for the mesosphere and lower thermosphere

Two-dimensional eddy diffusion around mesopause heights is modelled by means of a set of parameters relating eddy transports to average fields of meteorological quantities like wind variance and potential temperature. The so-called K-theory approach is used. The parameters chosen in this study allow easy modification of the K-models. This is thought to enable better adjustment and further development of the transport parameterization in accordance with future exploration of the transport phenomena and their physical background in the middle atmosphere. The models in their present state have been used to study a few problems associated with the climatological behaviour of the mesosphere and lower thermosphere: the relative importance of convective and eddy transports, implications of the mean winter anomaly of ionospheric absorption, the problem of strong mean vertical winds near the mesopause and the formation of the NO density minimum around 85 km altitude.