A. H. Manson

Empirical wind model for the upper, middle and lower atmosphere

Abstract The HWM90 thermospheric wind model has been revised in the lower thermosphere and extended into the mesosphere, stratosphere and lower atmosphere to provide a single analytic model for calculating zonal and meridional wind profiles representative of the climatological average for various geophysical conditions. Gradient winds from CIRA-86 plus rocket soundings, incoherent scatter radar, MF radar, and meteor radar provide the data base and are supplemented by previous data driven model summaries. Low-order spherical harmonics and Fourier series are used to describe the major variations throughout the atmosphere including latitude, annual, semiannual, local time (tides), and longitude (stationary wave 1), with a cubic spline interpolation in altitude. The model represents a smoothed compromise between the original data sources. Although agreement between various data sources is generally good, some systematic differences are noted, particularly near the mesopause. Overall root mean square differences between dar.a and model values are on the order of 15 m/s in the mesosphere and 10 m/s in the stratosphere for zonal winds, and 10 m/s and 5 m/s respectively for meridional winds.

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.

Validation of mesosphere and lower thermosphere winds from the high resolution Doppler imager on UARS

Horizontal wind fields in the mesosphere and lower thermosphere are obtained with the high resolution Doppler imager (HRDI) on the Upper Atmosphere Research Satellite (UARS) by observing the Doppler shifts of emission lines in the O2 atmospheric band. The validity of the derived winds depends on an accurate knowledge of the positions on the detector of the observed lines in the absence of a wind-induced Doppler shift. Relative changes in these positions are readily identified in the routine measurements of onboard calibration lines. The determination of the absolute values relies on the comparison of HRDI observations with those obtained by MF radars and rockets. In addition, the degrees of horizontal and vertical smoothing of the recovered wind profiles have been optimized by examining the effects of these parameters both on the amplitude of the HRDI-derived diurnal tidal amplitude and on the variance of the wind differences with correlative measurements. This paper describes these validation procedures and presents comparisons with correlative data. The main discrepancy appears to be in the relative magnitudes measured by HRDI and by the MF radar technique. Specifically, HRDI generally observes larger winds than the MF radars, but the size of the discrepancy varies significantly between different stations. HRDI wind magnitudes are found to be somewhat more consistent with measurements obtained by the rocket launched falling sphere technique and are in very good agreement with the wind imaging interferometer (WINDII), also flown on UARS.

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

Abstract 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.

High-latitude tidal behavior in the mesosphere and lower thermosphere

Abstract 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.

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.

Variations of the gravity wave characteristics with height, season and latitude revealed by comparative observations

Abstract This paper reviews some recent observations of gravity wave characteristics in the middle atmosphere, revealed by co-ordinated observations with the MU radar in Shigaraki (35°N, 136°E) and nearby rocketsonde experiments at Uchinoura (31°N, 131°E). We further summarize the results of comparative studies on the latitudinal variations of the gravity wave activity, which were detected by additionally employing data obtained with MF radars at Adelaide (35°S, 139°E) and Saskatoon (52, 107W) and lidar observations at Haute Provence (44, 6E). The seasonal variation of gravity wave activity detected with the MU radar in the lower stratosphere showed a clear annual variation with a maximum in winter, and coincided with that for the jet-stream intensity, indicating a close relation between the excitation of gravity waves and jet-stream activity at middle latitudes. The long-period (2–21 h) gravity waves seemed to be excited near the ground, presumably due to the interaction of flow with topography, and the short-period (5 min 2 h) components had the largest kinetic energy around the peak of jet-stream. We found an increase with height in the vertical scales of dominant gravity waves, which can be explained in terms of a saturation of upward propagating gravity waves. The values of the horizontal wind velocity variance generally increased in the stratosphere and lower mesosphere, but they became fairly constant above about 65 km due to the wave saturation, resulting in the active production of turbulent layers. Although the gravity wave energy showed an annual variation in the lower atmosphere, it exhibited a semiannual variation in the mesosphere, with a large peak in summer and a minor enhancement in winter. Lidar observations reasonably interpolated the seasonal variations in the intermediate height regions. The gravity wave energy in the mesosphere, with periods less than about 2 h, was consistently larger in summer than in winter at all the stations, i.e. at 35N, 44N,52 N and 35 S. However, the values were generally larger at 35 N than at 52 N. which was found from a comparison of l-yr observations at Shigaraki and Saskatoon. Furthermore, a comparison between Shigaraki and Adelaide, located at the conjugate points relative to the equator, revealed that the gravity-wave energy in the mesosphere was found to be fairly similar, when we compared the values in summer/winter in each hemisphere.

Global study of northern hemisphere quasi-2-day wave events in recent summers near 90 km altitude

Abstract We attempt to find the northern hemisphere zonal wavenumber for a striking quasi-2-day wave “event” or “burst” observed near 90 km altitude in the summer of 1992. A unique set of data on the upper atmosphere from nine radar sites is analysed (spacings ∼400– ∼ 12,000 km), and compared with expectations from models. The 2-day wave phase comparison, which finds zonal wavenumber m = 4, is conclusive. Determination of n, which defines the meridional wave amplitude structure, is not attempted, as the sites here have only a small latitude spread (21°N to 55°N). Also the amplitude seems to be unstable showing some sort of modulation which is not simultaneous at all sites. Finally, the radars have not been “calibrated” against each other in terms of wind speed. This calibration would have to be done before small differences in wave amplitude could be believed. A similar event in 1991 for which fewer sites are available is also discussed. Here the choice between m = 3 and 4 is not as clear.