Diurnal propagating tides in the low-latitude middle atmosphere

Using an equivalent gravity wave f-plane model it is shown that longitude variations in diurnal insolation absorption by tropospheric H₂O can account for longitudinal variations of at least ± 12–15% about zonal mean values in the diurnal wind amplitude at low latitudes (0–20°) between 80 and 100 km, by virtue of the non-migrating propagating tidal modes which are excited. Phase variations of about ± 0.75 h also occur. These percentage variations are conservative estimates, since the background migrating (1,1,1) mode appears to be slightly (20–25%) overestimated in amplitude. In addition, the assumed eddy dissipation values, which appear necessary to model the breaking (1,1,1) mode, are larger than generally considered ‘reasonable’ by photochemical modellers. For a photochemically more reasonable eddy diffusion profile, estimates of longitude differences in diurnal wind amplitude are quite similar to the above values below 87 km, but increase to ± 17–25% near 100 km, with accompanying phase variations of ± 1–2 h about zonal mean values. In addition, it is shown that radiative damping by CO₂ parameterized by a scale-dependent Newtonian cooling coefficient accounts for no more than a 20% reduction in the amplitudes of diurnal propagating tides above 80 km.     

Diurnal and semi-diurnal tides in the equatorial middle atmosphere

As part of the DYANA Programme, six rocket launchings (ship-borne) were conducted on three days in the equatorial region (Indian Ocean/Arabian Sea region). Using the temperature and wind data from these launchings, the diurnal and semi-diurnal tidal components in wind and temperature in the middle atmosphere are obtained and are compared with theoretical predictions. It is found that significant departures occur between the observed and theoretical values. The results are discussed in the light of current theoretical understanding of the tides.     

Lunar tides in the middle atmosphere from NIMBUS 5 data

Lunar tides in temperature have been determined at stratospheric heights from about 2 yr of radiance measurements by the NIMBUS 5 satellite. The tides have an amplitude of order 0.1 K, and results are presented for the variations with height and latitude.     

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.     

Observations of lunar tides in upper atmosphere winds at Poker Flat,Alaska

The lunar semidiurnal tide is extracted from hourly values of winds in the 75–105 km region measured by the Poker Flat Alaska MST radar used in the meteor mode. Since year-to-year variations are apparent, detailed results for 1983 and 1984 are presented. Inferred vertical wavelengths range from 17 km in March 1983 to 46–55 km in September of 1983 and 1984. The height progression of the phase is frequently too irregular to derive a vertical wavelength. Amplitudes of 3 m s⁻¹ are common and range up to 8 m s⁻¹. Amplitudes generally are largest at the equinoxes, especially in September, with another maximum in winter sometimes occurring. Reasonable agreement is found with lunar tidal measurements at Saskatoon, and some points of similarity are found with the solar semidiurnal tide at Poker Flat.     

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.     

Middle atmosphere electrical structure during MAC/EPSILON

Extensive use of rocket-launched probes during the MAC/EPSILON campaign at the Andøya Rocket Range, Norway, has enabled the characterization of the region's electrical environment for all four flight series. The first rocket salvo was conducted during daylight (15 October 1987) and the subsequent three occurred at night (21 and 28 October and 12 November 1987), all of them during geomagnetically disturbed conditions. Measurements of polar electrical conductivity, ion mobility and number density are presented, and their associated structure is investigated for local auroral ionization effects. This is believed to be the first time that Gerdien condenser mobility measurements have indicated a heavy-ion presence (positively charged aerosols) in the auroral mesopause region.     

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.     

Middle atmosphere abundances of water vapor and ozone during MAP/WINE

Water vapor and ozone concentrations measured at mid and high latitudes above Europe during the MAP/WINE Campaign in the winter 1983/1984 are presented. Both water vapor and ozone distributions are found to be variable and structured. On two occasions strong enhancements of mesospheric ozone were recorded, which were probably caused by dynamic effects. The assumption of a photochemical equilibrium for ozone at high latitudes in winter does not appear justified according to the variations measured during MAP/WINE. Water vapor mixing ratios determined in the stratosphere and mesosphere above Scandinavia yield average mixing ratios of near 4 ppmv.     

Thermal excitation of non-migrating tides

Thermal excitation of diurnal tides is discussed. The heat source of excitation is assumed to be localized near the equator, a situation which is based on certain observations of stratospheric tides at Jicamarca, Peru. Possible heating processes would be radiative heating by water vapour and thermal conduction due to turbulent eddies near the ground; these two processes could be localized because of differences in topography. Such sources tend to generate tidal Hough modes with various zonal wavenumbers both positive and negative. It is found that the resultant perturbations have generally fairly short vertical wavelengths around 10 km. They are almost stationary and the perturbed region tends to increase horizontally with height.     

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.     

Midlatitude seasonal behaviour of tides near the mesopause level

Wind measurements near the mesopause level were provided in Badary (Eastern Siberia, U.S.S.R.) by the Dl method using a low-frequency (200kHz) radio transmitter during 1976–1986. The seasonal variations of the wind are analyzed and compared with other midlatitude wind measurements as well as some theoretical predictions. It is shown that tidal parameters depend not only on latitude but on the longitude as well.     

In search of a spiritus: Francesco Patrizi on tides

ABSTRACT

This article analyzes two chapters of Francesco Patrizi’s Nova de universis philosophia that deal with a topic widely debated in European intellectual circles at the time: the origin of tides. By deconstructing Patrizi’s views on the phenomenon of ebb and flow, this paper places these chapters of Patrizi’s opus magnum into a larger context and shows how, with the use of novel philosophical arguments, he sought to replace what he considered an outdated theory of tides.     

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.     

Lunar tides in the stratosphere and mesosphere from NIMBUS 6 data

Infra-red radiance data from the NIMBUS 6 satellite have been analysed for lunar tidal variations in temperature at heights from 45 to 80 km. The tide at 45 km agrees well with that previously found from NIMBUS 5 data. The tide at 80 km has an amplitude of about 0.4K, and shows a phase change in the expected sense relative to that at 45 km.     

Interactions between diurnal tides and gravity waves in the lower thermosphere

Recent progress on interactions between breaking gravity waves and the diurnal tide in the upper mesosphere and lower thermosphere is reviewed, mainly based on the recent results of our numerical models.     

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.     

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.     

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.