R. Schminder

Solar cycle dependence and long-term trends in the wind field of the mesosphere/lower thermosphere

Abstract Ground-based observations of the wind field in the mesosphere/lower thermosphere have been used to investigate long-lasting wind variations at mid-latitudes. Besides small solar activity induced variations (11 yr cycle), non-solar induced long-term trends were clearly detected, which could be a hint of anthropogenically caused variations in the wind field near the mesopause region. Comparisons with other observations qualitatively confirm the derived trends, thus indicating common long-term global changes of the dynamical state of the middle atmosphere.

Climatology of the semidiurnal tide at 52–56°N from ground-based radar wind measurements 1985–1995

Abstract Long-term wind measurements carried out at 6 northern midlatitude sites (Saskatoon, Sheffield, Juliusruh, Collm, Obninsk, Kazan) are investigated to establish a climatology of the semidiurnal tide in the mesopause region for the narrow latitudinal range between 52°N and 56°N. Comparison of zonal and meridional components shows that in general the horizontal components are circularly polarized. Intercomparison of amplitudes and phases generally shows good agreement between the results from the different measuring systems. The results are compared with an empirical model of the semidiurnal tide. The longitudinal variation of the semidiurnal tide is small in summer, but the tidal amplitudes in winter are larger at Saskatoon and Kazan, compared with the results from the other sites. The possible influence of wave–tidal interaction in the stratosphere on the interannual variability of this difference is discussed.

Planetary wave activity obtained from long-period (2–18 days) variations of mesopause region winds over Central Europe (52 °N, 15 °E)

Abstract Daily analyses of the zonal and meridional prevailing wind at the mesopause region around 95 km height are investigated with respect to long-period variations with periods ranging between 2 and 18 d that may with some restrictions be interpreted as planetary wave activity. A mean climatology of these variations taken from the years 1983–1995 is presented. The mean annual variability of the signal shows good correspondence with some results known from literature, but a strong interannual variability of the wave activity is found. Time series of the zonal component of the planetary wave activity show an increase during the regarded period, which is in accordance with results from literature.

Tidal winds from the MLT global radar network during the first LTCS campaign—September 1987

Abstract Winds and tides were measured by a number of MLT (Mesosphere, Lower Thermosphere) radars with locations varying from 43–70°N, 35–68°S, during the first LTCS (Lower Thermosphere Coupling Study) Campaign, 21–25 September 1987. The mean winds were globally westerly, consistent with early winter-like (NH) and late winter (SH) circulations. The semi-diurnal tide had vertical wavelengths near or less than 100 km at most locations, with some latitudinal variation (longer/shorter at lower latitudes in the NH/SH)—amplitudes decreased at high latitudes. The global tide was closer to anti-symmetric, with northward components being in phase at 90 km. Numerical model calculations [ Forbes and Vial (1989), J. atmos. lerr. Phys . 51 , 649] for September have rather similar wavelengths and amplitudes; however, the global tide was closer to symmetric, and detailed latitudinal trends differed from observed. The diurnal tide had similar wavelengths in each hemisphere, with short values (~30 km) at 35°, long (evanescence) at 68–70°, and irregular phase structures at mid-latitudes. The tide was neither symmetric nor anti-symmetric. Model calculations for the equinox [ Forbes. S and Hagan (1988), Planet. Space Sci . 36 , 579] were by nature symmetric, and showed the short wavelengths extending to mid-latitudes (43–52°). Southern hemisphere phases were significantly (6–8 h) different from observations. Amplitudes decreased at high latitudes in model and observation profiles.

Semidiurnal tide in the 80–150 km region: an assimilative data analysis

Abstract A set of tabulated functions called ‘Hough Mode Extensions’ (HMEs), which represent numerical extensions of classical Hough modes into the viscous regime of the thermosphere, are used to least-squares fit a climatological data base of tidal measurements. The data base consists of monthly average vertical profiles of semidiurnal amplitudes and phases at 17 radar sites accessing some part of the 80–150 km height region. The radars are distributed between 78 S and 70 N latitude, and each one provides measurements of one or more of the following: eastward wind, southward wind, perturbation temperature. As a result of the fitting process, a single complex normalizing coefficient is derived for each month and for each of the four HMEs, designated (2,2), (2,3), (2,4) and (2,5) after their classical Hough function designations. Once the complex coefficients are derived, reconstruction by weighted superposition of the HMEs results in globally continuous specifications of semidiurnal horizontal and vertical wind, temperature, pressure, and density throughout the 80–150 km height region. The tidal variations in density, in particular, provide greater accuracy for several aerospace applications. The methodology developed here can also be utilized to derive tidal lower boundary conditions for Thermospheric General Circulation Models (TGCMs), or as a basis for future empirical model development. Comparisons are also made with HME coefficients and global tidal fields from the Forbes and Vial [(1989) J. atmos. terr. Phys. 51 , 649] numerical tidal model.

Tidal Winds from the Mesosphere, Lower Thermosphere Global Radar Network during the second LTCS Campaign: December 1988

Winds and tides were measured by nine MLT (mesosphere, lower thermosphere) radars with locations between 70°N and 78°S, including an equatorial station at Christmas Island, 2°N (Avery et al., 1990). The mean winds were eastward (westward) in the northern (southern) hemisphere mesosphere, consistent with midwinter circulations. For the 12-hour (semidiurnal) tide, observations and the model of Forbes and Vial (1989) were in generally good agreement: in both cases northward components were closer to being in phase in the two hemispheres, and winter wavelengths were shorter than those of the mid-latitude summer. Major differences were large (small) amplitudes at 70°N for model (observations); and poor agreement of equatorial tidal profiles. For the 24-hour (diurnal tide), the radar observations and model of Forbes and Hagan (1988) were in useful agreement in the summer hemisphere. However, the short (long) wavelengths at mid (high) latitudes of the models winter hemisphere were not observed during LTCS (Lower Thermosphere Coupling Study) 2, nor in climatologies for December. Suggestions as to the reasons for this disparity are presented.

Long-period variations of wind parameters in the mesopause region and the solar cycle dependence

Abstract A solar dependence of wind parameters below 100 km was found by Sprenger and Schminder on the basis of long-term continuous ionospheric drift measurements (D1) in the l.f. range. For winter they obtained for the prevailing wind a positive correlation with solar activity and for the amplitude of the semi-diurnal tidal wind a negative correlation. Later on this result was confirmed by radar meteor wind measurements (D2) at Obninsk and further D1 measurements at KUhlungsborn and Collm. However, after the years 1973–1974 a change in the behaviour of the zonal prevailing wind was observed. At this time we found a significant negative correlation with solar activity with an indication of a new change after 1983. This was obtained from D1 results in Collm and D2 results in Kuhlungsborn not only for winter, but also for summer and even for annual averages. We conclude that this long-term behaviour points rather to a climatic variation with an internal atmospheric cause than to a direct solar control. The negative correlation with solar activity of the semi-diurnal tidal wind in winter remained unchanged (up to 1984) and also proved to be the same in summer and for annual averages. Recent satellite data of the solar u.v. radiation and the upper stratospheric ozone have shown that the possible variation of the thermal tidal excitation during the solar cycle amounts to only a few per cent. This is, therefore, insufficient to account for the 40–70% variation of the tidal amplitudes. Some other possibilities of explaining this result are discussed.

Long-term trends in the mesopause wind field obtained from LF D1 wind measurements at Collm, Germany

Abstract The wind field of the upper mesopause region over Central Europe (52° N, 15°E) has been recorded automatically since the 1970s by D1 LF wind measurements at the Collm Observatory of the University of Leipzig. The measured time series are analysed with respect to long-term trends and solar cycle dependencies. Using data from different periods, the results partly differ, which is possibly due to oscillations at very long time scales.

Geomagnetic influences upon tides—winds from MLT radars

Abstract The influence of geomagnetic storms on the wind field of the mesosphere and lower thermosphere was investigated by means of ground-based observations with two MF radars, four meteor radars and two LF wind profilers in middle and high latitudes of the northern hemisphere. As the wind field in the height range between about 70 and 110 km is markedly controlled by internal atmospheric processes (e.g. stratospheric warming events, seasonal transitions, atmospheric waves), the individual geomagnetic effects were often masked by such atmospheric variations. Superposed-epoch analyses demonstrate that there were enhanced westward directed winds at middle latitudes and an eastward directed component at higher latitudes (Saskatoon, Canada), whereas the effects in the meridional component as well as in the amplitudes of the tidal waves were markedly smaller.

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

Abstract 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.