E. S. Kazimirovsky and G. V. Vergasova
Institute of Solar-Terrestrial Physics, Irkutsk, Russia
It is generally accepted that while the International Reference Models of the zonally averaged upper mesosphere/lower thermosphere wind field are still useful for many purposes, significant discrepancies exist between them and new experimental data. The latter demonstrate in addition to the seasonal and latitudinal variations also the longitude dependent variations and in addition to the tides also the planetary waves and intraseasonal quasiperiodical fluctuations. It is evident that sometimes specific regional features of the wind regime are very important.
The Badary observatory, East Siberia, (52o N, 102o E) was the only point between the Ural Mountains and Pacific Ocean, where long continuous wind measurements were conducted since 1974 up to 1997 (unfortunately this observatory was completely destroyed by the fire in May 1997).
There measurements were carried out by the closely spaced receiver technique (D1), using the time displacements of the fading of radio signals reflected from the lower ionosphere (80-100 km), which provide drift motions of electron concentration irregularities. The low frequency (200 kHz) nighttime broadcast radio signals have experienced a single oblique reflection (one hop), the ground wave being suppressed by the loop aerials, which were set up at the corners of a triangle with the sides 500 m long. The distance between the transmitter and receivers was about 150 km. The routine processing of continuous records was done by the simple similar-fade method. A particular drift vector was determined from a time displacement of each individual fade and the displacements averaged over half an hour were taken to calculate the vector and its zonal and meridional components.
A uniquely observational material on drift motions has been obtained from these measurements, though necessary restricted to the night hours (between sunset and sunrise). We believe that the ionized irregularities below about 110 km move with the neutral air. The D1 technique has been calibrated repeatedly and independently by meteor radars and by rocket wind measurements, so we may be sure that the motions which we consider are the neutral winds in the height range 80-100 km. The details of the installation and standard similar-fade analysis were published by Kazimirovsky et al. [1976].
We tried to perform a kind of harmonic analysis in order to describe the mean diurnal variation of the wind velocity in terms of at least the prevailing component ( V0 ) and semi-diurnally rotating component ( V2 ). The 24-h component ( V1 ), known to be of minor importance at our latitudes, cannot be determined from our observations, since no daytime measurements are available. Part of this component is certainly involved in the determination of both V0 and V2, but probably in most cases it does not seriously affect them.
During the last two decades we have published some results concerning the wind regime in the lower thermosphere over East Siberia [e.g., Kazimirovsky, 1981, 1994; Kazimirovsky and Kokourov, 1991; Kazimirovsky and Vergasova, 1996, 1997; Kazimirovsky and Zhovty, 1989; Kazimirovsky et al., 1982, 1988, 1994]. With respect to the above mentioned publications, the aim of this paper is to present the climatology and some details of the wind field in the East-Siberian region (prevailing wind, semi-diurnal tide amplitude, periodical structure) using the 20-year homogeneous time-series of the wind data over the Badary observatory.
The results for each month averaged over the entire period
of
observations are shown in Figure 1 (prevailing wind) and Figure 2
(semidiurnal tide amplitude). The mean zonal wind is eastward
having maxima in January and July and minima in April and
September. The mean meridional wind is southward having a maximum
in July and a minimum in January. The meridional wind as a rule is
weaker than zonal one. The mean amplitude of the zonal semidiurnal
tide is maximal in the autumn equinox (September) and minimal in
the spring equinox (March). The mean amplitude of the meridional
semidiurnal tide demonstrates one sharp maximum in
August-September and two slight minima in May and November. We
have calculated the averaged values for each day and each
month
during 20 years and selected the maximal and minimal values for
V0 and
V2.
These climatic norms of the wind regime (mean, standard
deviation, max and min) are shown in Table 1.
If, as has been demonstrated by many authors, there is a meteorological control of the lower ionosphere, we may expect the existence of a longitudinal effect in the dynamical regime due to the well known longitudinal inhomogeneity of the lower atmosphere processes and to the longitudinal differences in the conditions of upward propagation of the internal atmospheric waves from the lower atmosphere. The longitudinal effect has really been revealed on the basis of the simultaneous upper mesosphere/lower thermosphere wind measurements at two or more sites along one latitude circle [e.g., Kazimirovsky et al., 1988].
Figure 1 demonstrates monthly mean variations of the prevailing wind at Badary and Collm. Systematic climatological distinctions are evident especially for the zonal circulation. In winter, the averaged wind over Eastern Siberia is about twice stronger than over Central Europe. The seasonal variation of the zonal circulation depends on longitude as well, the autumn minimum over Siberia occurring earlier that over Europe, and spring minimum being accompanied by a wind reversal only over Europe but not over Siberia. The observed longitudinal effect may be partly interpreted as a result of the large-scale stationary planetary waves formed at lower thermosphere heights. In this case the longitudinal variation of the prevailing wind is due to the existence of such waves. To determine the sources of planetary scale perturbations in the mesosphere and lower thermosphere is one of the principal topics in the International Program PSMOS (Planetary Scale Mesopause Observing System) of SCOSTEP.
Figure 2 shows that the seasonal variations of the amplitudes of the monthly averaged semidiurnal zonal tide at both observatories are very similar with small discrepancies mainly in the summer months. The systematic climatological distinctions (but with similar character of seasonal variations) are evident for the semidiurnal meridional tide amplitudes, which are systematically larger in East Siberia than in Central Europe.
The seasonal variations of the observed maximal and minimal
daily
values of the prevailing wind (zonal
V0x and meridional
V0y) and semidiurnal tide amplitudes (zonal
V2x and meridional
V2y )
are shown in Figure 3 (see also Table 1).
It is evident
that for the prevailing wind the character of these seasonal
variations is similar.
The difference between the maximal and
minimal values varies from 11.4 m s-1
in May to 25.2 m s-1 in September
for the zonal wind and from 10.2 m s-1 in November
to 17.2 m s-1 in
July for the meridional wind. For the semidiurnal tide amplitudes
the difference between the maximal and minimal values varies from
6.8 m s-1 in April to 15.5 m s-1
in October for the zonal wind and from
9.2 m s-1 in February to 19.1 m s-1 in June for the meridional wind.
Long-term studies of wind parameters could be used to study the interannual variability and solar cycle dependency. We can state nothing unambiguous concerning the solar cycle dependency. The correlation between the prevailing wind V0x and solar activity (the Wolf number index R ) is statistically significant but very low. Moreover the correlation coefficients may be positive for 1975-1984 and negative for 1985-1995. The correlation may be higher (up to 0.9) for a time lag between R and V0x of about 1-3 years. As for V2x, the statistically significant high positive correlation without time lag was observed only for 1975-1995 in May (0.62) and June (0.78).
Time series of the annual mean values of the zonal/meridional
prevailing wind and semiannual tide amplitude are shown
in Figure 4.
We have high positive correlation (0.79)
between
V0y and Wolf
number with time-lag of 1 year and negative correlation ( -0.75 )
between
V2y and Wolf number with the same time-lag.
The variability of the wind regime may be described in terms of the
deviations from
the long-term mean values. The occurence of rather
small deviations ( 
10 m s-1 )
in the zonal prevailing wind is the
largest for the years of low solar activity. But the number of
large deviations (
20 m s-1 ),
although they are comparatively
rare, correlates well with the number of geomagnetic storms
( Ap > 20 ). It is visual in Figure 5.
This fact serves to support the
well-known data on the magnetic storm influence on the upper middle
atmosphere winds including the variability of the wind regime at
middle latitudes
[Lastovicka and Kazimirovsky, 1995].
Planetary waves contribute significantly to the variability of atmospheric parameters in the middle atmosphere. In the mesosphere and lower thermosphere the wave fluctuations are sufficiently large to mask often the prevailing or mean state of the atmosphere. The waves manifest themselves in wind, density, pressure, ionization and airglow fluctuations in the 80-120 km height range. Tropospherically forced planetary waves with the periods between 2 and 30 days have been observed to penetrate up to heights of about 110 km under favourable conditions. A range of the wave periods has been identified but the periods reported most frequently fall into four well defined intervals which are 14-20 days, 9-12 days, 4-7 days, and 1.6-2.2 days. These are often referred to as the "16-day", "10-day", "5-day" and "2-day" oscillations respectively, although precise determination of the periods involved is often impossible. It should be noted that waves of other periods have also been reported. It is usually assumed that these transient oscillations of the wind field in the lower thermosphere are caused by the Rossby gravity-normal modes generated in the lower atmosphere [Vincent, 1990].
We have used daily mean values of
V0x and
V0y and
correloperiodogram analysis
[Vitinsky et al., 1986] to
reveal the fluctuations
with periods of planetary waves and
intraseasonal variations. This analysis allow us to reveal the
maxima of correlation function between the real variations
and
variations with particular periods. The results are shown in
Figure 6.
One can see that the spectrum is rather wide but we can identify
only a few periods with confidence level higher than 0.95,
including the 27-day, quasi-monthly, and quasi-bimonthly
periodicities. It may be noted that periods close to 27-days are
clearly seen in the wind variations during one year or so (see
Table 2).
The origin of the 27-day planetary wave periods is still
a controversial subject. It may be connected with the solar
rotation period or with internal atmospheric processes?
There are interannual differences in the spectra of fluctuations in the wind field. Table 2 shows the most significant periods for 10 particular years.
We have analyzed the time variations of the amplitudes for the most interesting waves ( T = 16 days; T = 27 days) for the zonal prevailing wind. The spectra of these variations contain half-year and quasi-biennial periods.
Extending the earlier analysis we described in this study some upper mesosphere/lower thermosphere wind field peculiarities at the East-Siberian region. We demonstrated the nonzonality of the prevailing wind and semidiurnal tide, and also the richness and variability of the planetary scale variations in the dynamical regime. The variations may be due to the non-stationarity of the processes under consideration. Considerably more observations are required before the structure and role of the atmospheric waves in the 80-120 km region could be fully elucidated.
There is a need to organize an effective monitoring system that would be reasonably intensive and extensive. What is desired from future research is a qualitative assessment of all the significant couplings, trigger mechanisms and feedback processes.
Concluding, it should be noted that the lower thermosphere dynamics response to meteorological processes, solar and geomagnetic activity remains a stimulating challenge to scientists. Although individual investigators have much to contribute, it would seem that now is an appropriate time (for instance, in the frames of the PSMOS SRAMP project) to attempt to improve the collaboration between observationalists, theoreticians and modellers in order to maximize our knowledge of this important problem.
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