Figure 1: Height-time cross-section of the zonal prevailing wind over Collm.
Dierk Kürschner
Christoph Jacobi
Profiles | QTDW | Long-period oscillations |
The wind field of the mesopause region has regularly been monitored by the LF D1 measurements over Collm. In this paper an overview of the measurements in 2000 will be presented. The data are available from the monthly reports on http://www.uni-leipzig.de/~jacobi/collm/reports/2000. A description of the measurements is found on http://www.uni-leipzig.de/~jacobi/collm/win_e.htm or in Jacobi et al. (1997).
Monthly mean prevailing winds and semidiurnal tides
The height-time cross-secions of wind parameter profiles are found in Figures 1 - 4. The profiles are calculated as monthly means, except for those months where the wind field changes were too strong, so that these months have been divided into two parts. These months were March, September and October, when rapid wind field transitions in connection with the spring and autumn wind field changes occur.
The monthly mean profiles show the general features regularly seen in mesosphere/lower thermosphere (MLT) winds, namely the westerly/easterly jets in winter/summer and the wind reversal above (Figure 1). The latter is only indicated in the decrease of the mean winds in winter. Compared with other years, the late spring/summer easterlies are extraordinarily strong. Wind values up to 35 m/s are found on a monthly mean time scale.
Figure 1: Height-time cross-section of the zonal prevailing wind over
Collm.
The meridonal winds show the southward (negative) jet in summer, and
also southward winds at most heights during most of the months. This is
somewhat different when compared with long-term means, where in winter
the northerlies reach up to more than 90 km (see
long-term mean here, please use "back" button to return to here). Another
interesting feature is the extension of the northward winds up to 100 km
in October/November, which is also not a general feature.
Figure 2: Height-time cross-section of the meridional prevailing wind
over Collm.
The semidiurnal tidal amplitudes (Figure 3) show the late summer/early autumn maximum as well as the strong amplitudes in the upper layers in winter. This is a very regular feature and found in nerly every year. The same is the case for the semidiurnal tidal amplitude (Figure 4) which shows the two phases (winter and summer), with partly rapid transitions.
Figure 3: Height-time cross-section of the zonal semidiurnal amplitude
over Collm.
Figure 4: Height-time cross-section of the zonal semidiurnal phase
(in LMT) over Collm.
The QTDW during summer 2000 has already been described on http://www.uni-leipzig.de/~jacobi/collm/results/qtdw2000/qtdw2000.htm.However, although the strong bursts of the QTDW, with amplitudes of up to 30 m/s and more, are regularly found only in summer, the wave itself can be present throughout the year. The amplitudes in the course of the year are shown in Figure 5. The strong burst is found in summer, please note the rapid decay in August, which is a feature found in many years. A secondary burst is found in October/November. During the rest of the year the amplitude is small with values around 5 m/s. The QTDW amplitudes in 2000 are relatively small compared to previous years.
Figure 5: Amplitudes of the 48-h oscillation during 2000. Data are
calculated for 93 km from 7-day databases. The sold curves are 30-day averages;
red - zonal component, lower black curve - merdional component, upper black
curve - total amplitude.
Figure 6: Zonal (red) and meridional (blue) phases of the 48-h oscillation
in 2000. Click on the figure for enlargement. Please use "back" button
to return to this page.
The QTDW phases show a rather complicated behaviour. During the time intervals of the bursts the phases are nearly in quadrature, supporting the theory of the QTDW being a normal mode. The phase position changes (e.g. around days 180 - 200) indicating a period that is not exactly 48 h. During the periods of small amplitudes the phases generally are not in quadratude, but there is rather a tendency for them being in phase. However, during many intervals the day-to-day behaviour of the zonal and meridional phases show similar behaviour, indicating that the wave is still present.
The zonal amplitude of the QTDW is generally stronger than the meridional one. The ratio is shown in Figure 7. During most times the ratio is close to 2 , only during few time intervals it falls below unity.
Figure 7: Ratio zonal/meridonal amplitude of the QTDW.
Long-period oscillations in the period range above 2 days are often due to planetary waves propagating from the troposphere/stratosphere to the MLT. Because these waves are westward propagating they cannot tunel through the summer stratospheric easterlies so that the wave activity in the summer MLT is strongly reduced. This can be seen, for example, from the long-term mean spectra on http://www.uni-leipzig.de/~jacobi/spec8396.gif (please use "back" button to return).
The high-pass filtered zonal and meridional prevailing winds are shown in Figure 8. The data were calculated by subtracting the 30-day mean from the time series of daily winds calculated from a 7-day data base each. It can be seen that the winter amplitudes are stronger than the summer amplitudes. During part of the year (particularly in January) the zonal amplitudes are stronger than the meridional ones, which is frequently found in MLT wind measurements. However, this feature is not prevailing throughout the year.
Figure 8: High-pass filtered prevailing winds in 2000. Altitude is
93 km.
The total amplitude Vo = sqrt(Vz^2 + Vm^2) of the high-pass filtered wind is shown in Figure 9. It can be seen that, according to theory, the summer long-period variance is much weaker than the winter one. Note the secondary maximum in March in connection with the winter-spring transition.
Figure 9: Total high-pass filtered wind in 2000.
Figure 10: High-pass filtered semidiurnal tidal amplitude over Collm
in 2000.
Figure 11: Amplitude spectra of the semidiurnal tidal amplitudes during
4 intervals indicated in Figure 10. Please click on the thumbnail for enlargement.
The variability of the semidiurnal tidal amplitude is shown in Figure 10. The data were produced in the same manner as the ones for the prevailing winds in Figure 8. It can be seen that the amplitude is variable in the long-period range throughout the year, and there is no significant difference between summer and winter months. This can also be seen from the spectra in Figure 11. The amplitudes are strong during each month. There is a slight tendency for longer periods during summer.
Tidal amplitude variability is often attibuted to non-linear intercation between the tide and a planetary wave. This interaction not necessarily needs to occur in the measuring volume itself. The results from Figs. 10 and 11 show that, if wave-tidal interaction should be the source for tidal variability, the interaction process in summer cannot occur in the mesopause region because the original planetary wave is absent. However, from the wind measurements at Collm alone, a definitive conlusion on this point cannot be drawn here.
References
Jacobi, Ch., R. Schminder, and D. Kürschner, 1997: Measurements
of mesopause region winds over Central Europe from 1983 through 1995 at
Collm, Germany.
Contrib. Atmos. Phys. 70, 189 - 190.
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Last modification: 27.2.2001
