D. Kürschner

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.

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.

Global-scale tidal structure in the mesosphere and lower thermosphere during the PSMOS campaign of June-August 1999 and comparisons with the global-scale wave model

Observations of mean winds and semidiurnal and diurnal tides in the mesosphere/lower-thermosphere (MLT) region were made during the 3-month Planetary-Scale Mesopause Observing System Summer 1999 campaign. Data from 22 ground-based radars (and from two other instruments with measurements for the same period but in 1998) allow us to investigate the ability of the GSWM-00 to simulate the solar tides in the mesopause region (90-95 km). Here we have found that the GSWM-00 provides an increasingly reasonable estimate of most of the tidal characteristics in the MLT region. However, the representation of the 24 h tide appears superior to that of the 12 h tide. Some of these discrepancies are studied in detail. In particular, the observations reveal significant 12 h tidal amplitudes at high latitudes in the Northern Hemisphere summer. There is evidence for relation between the longitudinal variability of the mean zonal wind and the tidal characteristics seen from the radar wind measurements in the summer middle latitudes and a quasi-stationary planetary wave with zonal wave number one.

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.

Response of the mesopause region dynamics to the February 2001 stratospheric warming

Abstract The response of the mesosphere/lower-thermosphere (MLT) region to a major stratospheric warming in Europe during winter 2000/2001 has been investigated using mesopause-region winds measured by meteor radar or the LF-D1 method over three stations (Castle Eaton, 52°N; Collm, 52°N; and Esrange 68°N). The vertical wind structure measured over the three sites, and its time evolution, are found to be quite similar despite the different techniques used in the measurements. The effects of stratospheric warming are very clear, and are similar over both the mid-latitude and high-latitude sites. The warming resulted in a reversal of both the zonal and meridional wind. In the zonal component, this reversal was apparently associated with a planetary-wave oscillation with a period of ∼10 days. The effect was most conspicuous in the vertical prevailing wind gradients. The mesopause-region effects thus seem to be the results of a superposition of an intensifying planetary wave and a slow overall decrease in the strength of the zonal prevailing winds.

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.