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.     

Atmospheric tides—a review

Studies on atmospheric tides are reviewed with an emphasis on theory and recent important observations. Numerical simulation seems to be promising to elucidate the variability of observed tides. Radar observation will be powerful to obtain the precise vertical structure of tides in the middle atmosphere as well as the thermosphere. Various unsolved problems are pointed out.     

Middle atmosphere tides

Recent progress in the study of middle atmosphere tides is reviewed. Specific areas where recent progress has occurred include: development of more realistic thermal excitation and numerical simulation models; the role of tides in accelerating and heating the mean flow at the base of the thermosphere; observational efforts which delineate average seasonal, latitudinal and vertical structures of tides, and shorterterm variations of tides about these values; theoretical and observational studies concerning the importance of non-migrating tidal components; the effects of tides on minor constituent concentrations in the upper mesosphere and lower thermosphere. The review concludes with a summary of key problems to be addressed in the future.     

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.     

Tides in the 80–100 km height region

The Institute of Experimental Meteorology (U.S.S.R.) has carried out long-term continuous wind velocity measurements by the meteor radar method for the 80–100 km height region. From this experimental data the seasonal and latitudinal variations of atmospheric tides, as well as the spatial and temporal scales of tidal variability, have been determined.Atmospheric variations with a period of about half-a-day are investigated on the basis of a numerical model. A dependence between the mean wind structure and the semi-diurnal oscillation in the lower thermosphere is established. The influence of stratospheric warmings on semi-diurnal oscillations is also discussed. Numerical experiments show that the mean wind variations cannot explain the observed seasonal variations of a semi-diurnal tide.     

The effects of non-linearity on thermal tides in a 3-D numerical model

Absorption of solar radiation is taken as the cause of atmospheric tides, which are simulated by a global, 3-D primitive equation model for the altitude region 0 km–120 km. To investigate non-linear effects, two model versions are used, one with the complete non-linear equation set, the other with the linear equation set. Tide simulations are then performed under the same conditions (background atmosphere and radiation for solstice) with both model versions and directly compared. The diurnal tide can be regarded as a linear phenomenon, whereas the semi-diurnal tide is modified in lower latitudes above 65 km by non-linear effects. Due to the interaction of the tidal components with the background wind, a strong westward zonal flow is generated in the non-linear model above 70 km.     

Recent progress in tidal modelling

Recent progress in identifying the physical mechanisms governing the propagation of atmospheric tides and in performing numerical simulations of the phenomenon, are reviewed. Specific attention is given to gravity-wave/tidal interactions, non-linear interactions between tides, numerical simulations of the diurnal tide, and month by month simulations of the semidiurnal tide.     

Tides in the middle atmosphere

Some recent progress in the study of tides in the middle atmosphere are reviewed, with special emphasis placed on radar observations at high latitudes, as well as data analysis methods used in the calculation of tidal structures. Observations carried out outside the meteor zone with MST radars and satellites are also presented. Theoretical and numerical advances on the diurnal tide are extensively discussed. Finally, some outstanding problems, which we hope will be solved in the near future are raised: the existence of hemispheric asymmetries in tidal structure; the role played by non-migrating modes at meteor heights and short time scales variations of tides.     

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.     

Monthly simulations of the solar semidiurnal tide in the mesosphere and lower thermosphere

Monthly simulations of the solar semidiurnal tide in the 80–100 km height regime are presented. These calculations benefit from the recent heating rates provided by Groves G. V. (1982a,b) (J. atmos. terr. Phys. 44, 111; 44, 281), the zonally-averaged wind, temperature and pressure fields developed for the new COSPAR international reference atmosphere [Labitzke K., Barnett J. J. and Edwards B. (1985) Handbook for MAP 16, 318], and eddy diffusivities determined from gravity wave saturation climatologies and used by Garcia R. R. and Solomon S. (1985) (J. geophys. Res. 90, 3850) to simulate oxygen photochemistry and transport in the mesosphere and lower thermosphere. Some of the main characteristics of the observed semidiurnal tide at middle and high latitudes are reproduced in our simulations: larger amplitudes in winter months than in summer months, and the bi-modal behavior of the phase with summer-like and winter-like months separated by a quick transition around the two equinoxes. The phase transition is also more rapid in the spring, consistent with observations. The wavelengths are also longer in summer than in winter, at least below 95 km (whereas in July and August the simulations exhibit some discrepancies above this altitude), similar to the observational data. Semidiurnal amplitudes are generally smaller and the phases more seasonally symmetric at middle and low latitudes, as compared with the tidal structures above about 50° latitude. In addition, hemispheric differences in the mean zonal wind result in marked asymmetries in tidal behavior between the Arctic and Antarctic regions, and suggest that a comparative study of tide, gravity wave and mean flow interactions in the Arctic and Antarctic mesosphere and lower thermosphere would be fruitful.     

A nonlinear simulation of the thermal diurnal tide

A simplified middle atmosphere general circulation model is used to investigate the nonlinear behavior of the thermal diurnal tidal waves. In the model, only a westward moving diurnal tide generated by heating with zonal wavenumber 1 is considered. The tidal wave propagation is simulated by a full nonlinear calculation with a convective adjustment scheme and a Richardson number dependent vertical eddy diffusion.The numerical results show that the growth of the diurnal tide due to the density stratification is effectively suppressed and a relatively constant amplitude distribution with height is realized by the convective adjustment in the lower thermosphere. It is also shown that mean zonal winds and mean meridional circulations are induced by the diurnal tidal waves in the region where the tidal waves are breaking by convective instability, in accordance with the wave-mean flow interaction theorem.     

A study of meteor radar winds from two locations in the British Isles

Winds and tides have been measured by a two-station meteor radar system which has increased spatial resolution compared with single station radars used in the past. Narrow radar beams, pointing SW from Sheffield (53.5°N, 1.6°W) and 30°N of W from Shrivenham (51.5°N, 1.6°W), are arranged to converge over the U.K. MST radar site near Aberystwyth, thus defining a unique atmospheric volume in which meteor wind components are simultaneously measured from the two radar sites. The resultant ‘true’, or local, wind vector is compared with the spatially averaged vector obtained with the aid of beams pointing SW and NW from Sheffield only. It is found that the ‘true’ and averaged tidal winds are in good agreement, as expected from their large scale sizes, and that the main advantages of the dual station technique lie in the resolution of a small scale structure such as that related to internal atmospheric gravity waves. By the simultaneous deployment of two-station meteor radar, MST radar and LIDAR, such waves may now be studied through a large vertical section of the atmosphere in a geographically localized area.     

The 2-day wave in the middle atmosphere as simulated in a general circulation model extending from the surface to 100 km

The 2-day wave observed in the mesosphere and lower thermosphere has been reproduced in a general circulation model of the atmosphere run for fixed January conditions. The wave was confined to the summer hemisphere between 50 and 100 km, and was most strongly evident in the meridional velocity where it caused a reversal in the direction of this wind approximately every 24 h. Similar but smaller fluctuations could be detected in the zonal wind and temperature. The synoptic distributions from the model confirm that the 2-day wave is a zonal wave number 3 phenomenon and that it progresses westwards. These distributions have maximum amplitudes occurring at higher latitudes than observed, probably owing to the mean wind intensity in the model summer hemisphere being slightly underestimated. Quite marked interactions occurred between the high latitude and tropical features of the synoptic meridional velocity distribution as the wave progressed. The wave had a very small phase variation with altitude, and, except for a region near 70 km, exhibited hardly any sign of baroclinic activity. The formulation of the model eliminates atmospheric tides or orography as forcing agents responsible for the excitation of the 2-day wave.     

Middle atmosphere and lower thermosphere processes at high latitudes studied with the EISCAT radars

The middle and upper atmosphere and the ionosphere at high latitudes are studied with the EISCAT incoherent scatter radars in northern Scandinavia. We describe here the investigations of the lower thermosphere and the E-region, and the mesosphere and the D-region. In the auroral zone both these altitude regions are influenced by magnetospheric processes, such as charged particle precipitation and electric fields, which are measured with the incoherent scatter technique. Electron density, neutral density, temperature and composition are determined from the EISCAT data. By measuring the ion drifts, electric fields, mean winds, tides and gravity waves are deduced. Sporadic E-layers and their relation to gravity waves, electric fields and sudden sodium layers are also investigated with EISCAT. In the mesosphere coherent scatter occurs from unique ionization irregularities. This scatter causes the polar mesosphere summer echoes (PMSE), which are examined in detail with the EISCAT radars. We describe the dynamics of the PMSE, as well as the combination with aeronomical processes, which could give rise to the irregularities. We finally outline the future direction which is to construct the EISCAT Svalbard Radar for studying the ionosphere and the upper, middle and lower atmosphere in the polar cap region.     

Upper atmosphere semidiurnal tides at Christchurch (44°S) and Scott Base (78°S)

Lunar and solar semidiurnal tides have been determined from winds measurements in the 82 to 100 km height range at Christchurch and Scott Base made during 1983–1984. At Christchurch, the solar tide has maximum amplitudes in April and December, while at Scott Base, only the December maximum is present at all heights. Phases at Scott base mostly agree with those measured earlier at Mawson, but vertical wavelengths are always long. The lunar tide was difficult to isolate at Christchurch, but the winter to summer phase reversal was clear. This was also seen at Scott Base.     

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.     

First summer results on winds in the upper mesosphere derived from the 843 nm hydroxyl emissions measured from the Bear Lake Observatory,Utah

The Imaging Fabry-Perot Interferometer (IFPI) at the Bear Lake Observatory (BLO), Utah (41.9°N, 111.4°W) is used for studies of the aeronomy of the middle and upper atmosphere. Wind and temperature structure can be determined from observations of the Doppler shift and Doppler broadening of the airglow and auroral emissions from the mesosphere and thermosphere. The mesospheric winds recorded at the end of August, September and early October 1992 are consistent with a semi-diurnal tidal variation. The amplitude of this variation is approximately 30 ms⁻¹ at the end of August and early September and approximately 20 ms⁻¹ at the end of September and early October. However, during June and July, the semi-diurnal tidal variation, if present, is weak, with amplitude < 5 ms⁻¹. No consistent semi-diurnal tidal variation is observed during late October 1992. During the solstice period, antisymmetric tidal components may be preferentially generated in such a way that they can result in destructive interference with the normally dominant symmetric modes, resulting in a decrease of tidal variation. This is consistent with the observed decrease in tides during the June, July and late October periods. Near the equinoxes, however, the excitation of these antisymmetric modes is expected to be weaker, possibly explaining why a pronounced and consistent semi-diurnal tidal variation has been observed during the August, September and early October periods. In contrast, the mesospheric winds derived from the Sheffield Meteor Wind Radar (53.4°N, 1.5°W) reveal a clear semi-diurnal tidal variation throughout the year, with an amplitude that may vary between 15 ms⁻¹ and 50 ms⁻¹, being about 25 ms⁻¹ on average. The IFPI records winds from a region of the atmosphere centred at 87 km, whereas the Sheffield Meteor Wind Radar measures winds centred at 95 km. Therefore, the two regions may experience different tidal modes due to the different latitude, longitude and altitude of the observed regions and/or the different topography of the observing sites. Some proposed reasons for these differences are presented.     

Solar and lunar variations in TEC at low latitudes in India

Total electron content data at Ootacamund (dip 6°N) during the second phase of the ATS-6 program are analysed for solar and lunar variations. Both the lunar semi-monthly and lunar semi-diurnal tides at Ootacamund are smaller in magnitude than at stations near the Appleton anomaly crest. The phases of the lunar oscillations however are almost the same as that at tropical latitudes. Thus the lunar tides in total electron content and in maximum F2-region electron density at the equatorial station are not in phase and present problems for the simple electrodynamic theory.     

Non-migrating tides

Recently, two interesting advancements in the study of non-migrating tides have occurred. There are currently two distinct approaches to this subject. One. based on mechanistic models that consider heating due to non-uniform global distribution of water vapour or heating only over lands, solves the primitive equations. One model of this approach that shows insolution absorption of the non-uniformly distributed water vapour produces non-migrating tides with 15% of the migrating ones in the lower thermosphere. The planetary boundary layer heating can explain very enhanced tides over land masses and those with short vertical wavelengths in the stratosphere. The other approach uses a general circulation model (GCM) simulation. This model can produce tides globally and in many details. The model predicts enhancement of the non-migrating modes as eastwards traveling modes with a wave number 3 and westwards traveling modes with a wave number 5 that is in a surprisingly good agreement with observation at sea-level, at 700 mb and at 300 mb.Whilst the GCM simulation is to be developed so as to include thermospheric tides, the mechanistic model should consider more realistic situations so as to include winds. Observation of tides at many heights and locations is essential in future studies.     

Effects of geomagnetic storms in the lower ionosphere,middle atmosphere and troposphere

Geomagnetic storm effects at heights of about 0–100 km are briefly (not comprehensively) reviewed, with emphasis being paid to middle latitudes, particularly to Europe. Effects of galactic cosmic rays, solar particle events, relativistic and highly relativistic electrons, and IMF sector boundary crossings are briefly mentioned as well. Geomagnetic storms disturb the lower ionosphere heavily at high latitudes and very significantly also at middle latitudes. The effect is almost simultaneous at high latitudes, while an after-effect dominates at middle latitudes. The lower thermosphere is disturbed significantly. In the mesosphere and stratosphere, the effects become weaker and eventually non-detectable. There is an effect in total ozone but only under special conditions. Surprisingly enough, correlations with geomagnetic storms seem to reappear in the troposphere, particularly in the Northern Hemisphere. Atmospheric electricity is affected by geomagnetic storms, as well. We essentially understand the effects of geomagnetic storms in the lower ionosphere, but there is a lack of mechanisms to explain correlations found deeper in the atmosphere, particularly in the troposphere. There seem to be two different groups of effects with possibly different mechanisms—those observed in the lower ionosphere, lower thermosphere and mesosphere, and those observed in the troposphere.