C. E. Meek

Quasi 16-day oscillation in the mesosphere and lower thermosphere

A quasi-16-day wave in the mesosphere and lower thermosphere is investigated through analyses of radar data during January/February 1979 and through numerical simulations for various background wind conditions. Previous workers have examined about 19 days of tropospheric and stratospheric data during January 10–28, 1979, and present conflicting evidence as to whether a large westward propagating wavenumber 1 oscillation observed during this period can be identified in terms of the second symmetric Rossby normal mode of zonal wavenumber 1, commonly referred to as the “16-day wave.” In the present work we have applied spectral analysis techniques to meridional and zonal winds near 95 km altitude obtained from radar measurements over Obninsk, Russia (54°N, 38°E) and Saskatoon, Canada (52°N, 107°W). These data reveal oscillations of the order of ±10 m s−1 with a period near 16 days as well as waves with periods near 5 and 10 days. These periodicities all correspond to expected resonant frequencies of atmospheric disturbances associated with westward propagating free Rossby modes of zonal wavenumber 1. Numerical simulations are performed which demonstrate that the 95-km measurements of the 16-day wave are consistent with upward extension of the oscillation determined from the tropospheric and stratospheric data. Noteworthy features of the model in terms of its applicability in the mesosphere/lower thermosphere regime are explicit inclusion of eddy and molecular diffusion of heat and momentum and realistic distributions of mean winds, especially between 80 and 100 km. The latter include a westerly wind regime above the summer easterly mesospheric jet, thus providing a ducting channel enabling interhemispheric penetration of the winter planetary wave disturbance. This serves to explain the appearance of a quasi-16-day wave recently reported in the high-latitude summer mesopause (Williams and Avery, 1992). However, the efficiency of this interhemispheric coupling may be reduced by gravity wave stress. No significant penetration of the 16-day oscillation above about 100 km is predicted by the model. Reported signatures of a 16-day periodicity in ionospheric data therefore require modulation of tidal or gravity wave accessibility to the thermosphere, or perhaps in situ excitation.

An efficient method for analysing ionospheric drifts data

A fast and efficient method for analysing ionospheric drift records employing the concepts of full correlation analysis (Briggs et al., 1950) is described. A Gaussian correlation function is assumed, and only the most accurately determined experimental data, viz. the width of the mean auto correlogram, and the positions and magnitudes of the maxima in the cross correlograms are required. Because of its speed, the method is well suited for real time analysis.

Dynamics of the middle atmosphere at Saskatoon (52°N, 107°W): a spectral study during 1981, 1982

Abstract Planetary waves (2 day–30 day) and tidal wave (8 h, 12 h, 24 h) oscillations are studied by forming height (60–110 km) and time (January–December) contour cross-sections and comparing with mean winds for 1981, but also on occasion 1982. Strong seasonal variations with evident wave-mean flow couplings are demonstrated. The zonal mean wind momentum budget (80–90 km) is assessed using cross-sections of northward-eastward covariances for wind fluctuations in the planetary and tidal wave period ranges and also gravity wave cross-sections. Dominant contributions are due to coriolis torque in the meridional mean flow and vertical divergence of the gravity wave momentum flux, although planetary wave contributions may be large also in winter months.

Super Dual Auroral Radar Network observations of meteor echoes

Radar echoes from ranges less than 500 km are routinely observed by the Super Dual Auroral Radar Network (SuperDARN) on most days. Many of these echoes have properties which are markedly different from what one would expect from E or F region irregularities. We show that these unusual short-range HF echoes are due to scattering off meteor trails. This explains why, among other things, the Doppler shift from the short-range echoes taken from the SuperDARN Saskatoon antenna are consistent with the mesospheric winds observed by the Saskatoon MF radar. This means that the SuperDARN radars can be used to study neutral winds at meteor heights, a result which is especially interesting since it opens up the capability for a global coverage of mesospheric winds using the worldwide distribution of SuperDARN radars.

Fluctuations in tidal (24-, 12-h) characteristics and oscillations (8-h-5-d) in the mesosphere and lower thermosphere (70–110 km): Saskatoon (52°N, 107°W), 1979–1981

Abstract An M.F. radar (2.2 MHz) operating at Saskatoon, Canada (52°N, 107°W) has been used to produce continuous wind data (∼ 80–110km) from September 1978–April 1981. The 24-, 12-h tidal oscillations reveal regular summer-winter transitions; in particular the semi-diurnal tide demonstrates strikingly regular and rapid equinoctial changes over the three years. The vernal and autumnal equinox changes are clearly different in morphology. Shorter term tidal fluctuations (2d ≲ τ ≲ 10d) are compared with mean winds and gravity wave amplitudes, as well as with satellite-derived stratospheric temperatures. Spectral analysis of monthly data sets for 1980, from ∼ 90–105 km, reveal oscillations of the expected 8-, 12-, 24-h periods, but also of ∼ 10-, ∼16- and ∼ 2-, 5 6 d . A modulation of the “2-d” wave by the 12-h wave is suggested as a possible cause of these surprisingly regular oscillations.

An explanation for the seasonal dependence of midlatitude sporadic E layers

[1] The midlatitude sporadic E layers form when metallic ions of meteoric origin in the lower thermosphere are converged vertically in a wind shear. The occurrence and strength of sporadic E follow a pronounced seasonal dependence marked by a conspicuous summer maximum. Although this is known since the early years of ionosonde studies, its cause has remained a mystery as it cannot be accounted for by the windshear theory of E s formation. We show here that the marked seasonal dependence of sporadic E correlates well with the annual variation of sporadic meteor deposition in the upper atmosphere. The later has been established recently from long-term measurements using meteor radar interferometers in the Northern and Southern Hemispheres. Knowing that the occurrence and strength of sporadic E layers depends directly on the metal ion content, which apparently is determined primarily by the meteoric deposition, the present study offers a cause-and-effect explanation for the long-going mystery of sporadic E layer seasonal dependence.

Long period (∼8-20 h) wind oscillations in the upper middle atmosphere at Saskatoon (52°N): evidence for non-linear tidal effects

Abstract Winds measured by the Saskatoon MF radar (2.2 MHz) have been used to study atmospheric oscillations of periods greater than 1 h (e.g. inertio-gravity waves). Perturbations in the wind field are found by subtracting the mean day of the interval studied (6–11 days) from each day to form “difference” plots. In summer, below ∼ 90 km, the oscillations (10–20 m s−1) are extremely regular and coherent with height and time—inspection of the difference plots, and spectral and harmonic analysis, show these to be of 16 h period with λz of ∼ 10 km. It is probable that these are due to non-linear mixing of the 2-day wave and 12-h tide. In winter months, the oscillations are smaller and less regular, consistent with the superposition of inertio-GW of periods 8–20 h.

Global-scale tidal variability during the PSMOS campaign of June-August 1999: interaction with planetary waves

During the PSMOS Global-scale tidal variability experiment campaign of June 1-August 31, 1999, a network of radars made measurements of winds, waves and tides in the mesosphere/lower-thermosphere region over a wide range of latitudes. Clear evidence was found that fluctuations in tidal amplitudes occur on a global scale in both hemispheres, and that at least some of these fluctuations are periodic in nature. Modulation of the amplitude of the 12 h tide was particularly evident at periods of 10 and 16 days, suggesting a non-linear interaction with planetary waves of those periods to be responsible. In selected cases, the secondary waves predicted from non-linear theory could be identified and their zonal wave numbers determined. In some, but not all, cases the longitudinal structure of the secondary waves supports the theory of planetary-wave/tidal interaction being responsible for the observed tidal modulation. It was noted also that beating between a 12.4-lunar and the solar tide could produce a near 16-day modulation of the 12 h tide amplitude that is frequently observed in late summer.

Planetary waves in coupling the stratosphere and mesosphere during the major stratospheric warming in 2003/2004

[1] The vertical coupling of the stratosphere-mesosphere system through quasi-stationary and traveling planetary waves during the major sudden stratospheric warming (SSW) in the Arctic winter of 2003/2004 has been studied using three types of data. The UK Met Office (UKMO) assimilated data set was used to examine the features of the global-scale planetary disturbances present in the winter stratosphere of the Northern Hemisphere. Sounding the Atmosphere using Broadband Emission Radiometry (SABER) satellite measurements were used as well for extracting the stationary planetary waves in the zonal and meridional winds of the stratosphere and mesosphere. Radar measurements at eight stations, four of them situated at high latitudes (63–69N) and the other four at midlatitudes (52–55N) were used to determine planetary waves in the mesosphere-lower thermosphere (MLT). The basic results show that prior to the SSW, the stratospheremesosphere system was dominated by an upward and westward propagating � 16-day wave detected simultaneously in the UKMO and MLT zonal and meridional wind data. After the onset of the SSW, longer-period (� 22–24 days) oscillations were observed in the zonal and meridional MLT winds. These likely include the upward propagation of stationary planetary waves from below and in situ generation of disturbances by the dissipation and breaking of gravity waves filtered by stratospheric winds. Citation: Pancheva, D., et al. (2008), Planetary waves in coupling the stratosphere and mesosphere during the major stratospheric warming in 2003/2004, J. Geophys. Res., 113, D12105, doi:10.1029/2007JD009011.

E_ region real heights and their implications for MF radar-derived wind and tidal climatologies

E_ region real height determinations for 2.2 MHz has been carried out using standard electron density (N(h)) profiles from Maedas rocket observations and the International Reference Ionosphere (IRI) models for application to the Saskatoon (52°N, 107°W) MF radar. A comparative study of the virtual and real heights for the totally reflected signals, for both solar maximum and minimum conditions, reveals that these heights increase from solar maximum to minimum. The group retardations from rocket N(h) are found to be lower than that of the IRI models in all the seasons. During summer the noon time virtual heights are very similar for the IRI model and MF radar data, but during the winter the IRI virtual heights are considerably lower than the observed heights. The derived group retardations are added to the mean wind and tidal climatologies for Saskatoon. This enables the prediction of the heights up to which the winds and tidal data can be taken without any correction for group retardation.