D 1 LF Wind Measurements in Autumn and Early Winter

144
D 1 LF Wind Measurements in Autumn and Early Winter 1994 at
Collm, Germany
(Presentation at the CRISTA/MAHRSI Workshop, May 30/31, 1996, Wuppertal)
Ch. Jacobi, R. Schminder, D. Kürschner
Summary:
Results of total reflection mesopause wind measurements, carried out at the Collm Observatory of the University of Leipzig in 1994 are presented with special regard to the CRISTA period in November 1994. Height-time cross-sections of the prevailing wind and the semidiumal
tidal amplitude and phase show the variation of these parameters during the course of the year.
In autumn and early winter oscillations of the prevailing wind with a period of about 2 - 3
weeks are found. They are partly due to an overshooting of the parameters while the
circulation adjusts to its winter conditions. A comparison of the zonal prevailing wind in 1994
with the values measured since 1982 shows a solar cycle dependence.
Zusammenfassung:
Ergebnisse von Windmessungen in der Mesopausenregion mit Hilfe der Reflexion von
Funkwellen, die am Observatorium Collm der Universität Leipzig durchgeführt wurden,
werden vorgestellt, wobei besonderes Gewicht auf die Zeit der CRISTA-Satellitenmission im
November 1994 gelegt wird. Höhen-Zeit-Schnitte des Grundwindes und der Amplitude des
halbtägigen Gezeitenwindes zeigen die Änderung dieser Parameter im Lauf des Jahres. Im
Herbst und frühen Winter werden Oszillationen mit Perioden von 2 - 3 Wochen festgestellt.
Diese sind zu Teil durch ein Überschießen bedingt, daß mit der Anpassung an die
Winterbedingungen zusammenhängt. Ein Vergleich des zonalen Grundwindes 1994 mit des
Werten aus anderen Jahren gibt Hinweise auf eine Abhängigkeit von der solaren Aktivität.
1. Introduction
Since 1982, continuous wind and reflection height measurements in the upper mesopause region have been carried out using low frequency (LF) radio transmitters at the Collm Observatory of the University of Leipzig (e.g. Schminder and Kürschner, 1984; Schminder, 1995).
These measurements provided monthly mean profiles of the prevailing and the semidiumal tidal
wind at heights between about 85 and 105 km. In addition, daily values of the wind parameters
can be obtained, but without exact height resolution, so that these measurements are attached
to a height level of about 95 km, which is the mean nighttime reflection height of low frequency radio waves. Since these measurements are carried out daily and with high reliability,
they can be used for monitoring mesopause winds for example in comparison with other systems or to provide background measurements for special experiments. Here the wind data
measured at Collm during the CRISTA (Cryogenic lnfrared Spectrometers and Telescopes for
the Atmosphere-Shuttle Pallet Satellite) satellite campaign in November 1994 (see Offermann
and Conway, 1995; Offermann and Grossmann, 1996) are shown in relation to their variability
during the course of the year and also to their interannual variability.
2. Description of Measurements and Data Analysis
The wind field of the upper mesopause region is continually observed by daily D 1 radio wind
measurements in the LF range, using the ionospherically reflected sky wave of three commer-
145
cial radio transmitters on 177, 225 and 270 kHz. The measurements are carried out according
to the closely-spaced receiver technique. A modified form of the similar-fade method is used to
interpret the wind measurements (Schminder and Kürschner, 1992, 1994; Schminder, 1995).
The data are combined to half-hourly mean values on each frequency and from these mean
values are calculated, referring to a reflection point at 52°N, l5°E. Since during the day the
absorption of the sky wave is too large, the daily measuring period is restricted to night and
twilight.
The reflection height is measured on 177 kHz using travel time differences between the
ground wave and the sky wave. The differences are obtained using side-band phase comparisons of both wave components in the frequency range near 1.8 kHz (Kürschner et al., 1987).
A multiple regression analysis with height-dependent coefficients is used to determine
the half-monthly and monthly prevailing wind, as well as the tidal wind field components using
the half-hourly mean values of the measured zonal and meridional wind components. The spectral selectivity of the separation of prevailing and tidal wind was improved through fitting the
measured values for the two horizontal wind components as a vector, assuming clockwise circularly polarized tidal wind components (Kürschner, 1991). The height profiles range roughly
between 85 and 105 km, depending on the height distribution of the individual half-hourly
means which varies through the year. Therefore, especially in summer, measurements can only
be taken over a smaller height range.
For further investigations, also daily harmonic analyses can be performed, but in this
case the height dependence cannot be taken into account. Data of these type of analyses are
used, for example, to complete yearly descriptions of the atmospheric processes in the respective winter (e.g. Naujokat et al., 1995).
3. The Wind Field During the CRISTA Campaign in Relation to its Variability in 1994
In Figure 1 cross-sections of the prevailing wind and the semidiumal tidal amplitude and phase
are shown for the height region of 85 to 102.5 km. Similar representations for mean values
from 1982 - 1994 are given in Jacobi et al. ( 1996). A comparison of their results with Figure 1
shows, that the characteristic annual variations are qualitatively very regular, and only few
deviations from the long-term mean are found in 1994. These are conceming above all the meridional prevailing wind, where the transition from the lower thermospheric easterlies to the
mesopause westerlies in January and February 1994 is found at comparatively low heights.
The term of the CRISTA campaign is marked in Figure 1. The profiles during this period are
given in Figure 2. The gradients of the zonal (v 0 z) and meridional (v 0 m) component of the prevailing wind are both negative in the lower region; the meridional wind is negative (i.e. directed
from the east) at heights above 90 km. The amplitude of the semidiumal tide is small in the
lower levels but rises strongly with height. This is typical for November. The zonal phase has
taken up its winter position with a very pronounced vertical gradient. lt results in a vertical
wavelength of about 40 km below 90 km height, but above 90 km it is more than twice that
large.
The determination of the diumal tidal amplitude and phase is more difficult from the measurements, since the daily data gaps lead to a larger error than the one of the semidiumal tidal parameters. Therefore the diumal tide is only calculated for a height of 95 km, where the maximum density of measured half-hourly means is found. During the CRISTA campaign the amplitude amounts to 8 ms· 1, the phase is found at 14.7 LMT.
146
102.
102.
100.
100.
97.
97.
E
"".5
E
""
.5
95.
r.
95 .
r.
92.
92.
90.
90.
87.
87.
85 . ll+'-~~~
Jan
Feb Mar Apr May
Jun Jul Aug
Sep Oe!
Nov
Dee
Feb Mar Apr May
Jun Jul Aug
Sep Oe!
Nov
Dee
(2)
(1)
102.
100.
97.
~
E
95.
""
.5
r.
.5
r.
92.
90.
87.
Jan
Feb Mar Apr May
Jun
(3)
Jul Aug
Sep Oe!
Nov
Dee
Feb Mar Apr May
Jun Jul Aug
Sep
Oe!
Nov
Dec
(4)
Fig. 1: Height-time cross-sections in 1994 ofmesopause wind parameters. 1: Zonal prevailing wind. 2: meridional prevailing wind. 3: Semidiurnal tidal amplitude. 4: Zonal semidiurnal phase, given in LMT. The
other parameters are given in ms· 1. The period of the CRJSTA campaign is shaded.
4. The Variability of the Wind Field in Autumn and Early Winter 1994
The variability of the wind field in autumn and early winter 1994 is shown in Figure 3. Here
time series of the daily analysis, referring to a height of about 95 km are depicted, together
with their low-pass filtered values (Lanczos Filter, 30 weights, cut-off period 15 days). The
CRISTA campaign is indicated by the dotted vertical lines. The days are counted beginning
with October 1. In October the autumn transition with negative zonal prevailing winds and
small semidiumal amplitudes v2 z takes place. During that time the phase T 2 z shifts to later times
up to 18 LMT. Note that this time is depicted as 6 LMT, and the filtered values taken from
these data show a variation to earlier phase positions that did not appear. After October 20, the
winter conditions are reached, but there was a sort of overshooting with particularly strong
winds both to be seen within the zonal and the meridional component. During the CRISTA
campaign this overshooting was completed, and normal winter conditions were found. Later,
in early December, again particularly strong westerly winds are found. These are connected
with a very strong polar stratospheric vortex (see Naujokat et al., 1995). At the end of December an upper stratospheric warming occurred, and the zonal prevailing wind at mesopause
heights became weaker again.
Especially from the time series of the zonal prevailing wind and the zonal semidiumal amplitude an oscillation with a period of about 2 weeks can be inferred. The energy spectra of the
components of the prevailing wind, given in Figure 4, show these variations clearly. The spectrum of the zonal components shows two peaks near 13 and 23 days, while the spectrum of the
meridional components exhibits only one peak near 18 days. However, it is doubtful whether
147
these variations are due to a propagating wave as the quasi 16-day wave (e.g. Forbes et al.,
1995), since most of the variance influencing the spectrum is due to the overshooting of the
wind field after the autumn transition and the effect during the period of the very cold stratospheric vortex. During the other days (from day 63 to day 90) this oscillation is much weaker,
as it can be seen from Figure 3. The meridional prevailing wind exhibits no oscillation in the
regarded period range at all. An additional peak is found within the spectrum of the zonal prevailing wind, belonging to a period of 7 days. This variation, however, is not found within the
spectrum of the meridional component.
105....-~~~~~~~~~~~~.~~~~.~~~--.
100
E
.:::i::.
c
..c
uuuuuum•uunu uu,muuun
u!nuuU1~v0,
1-
········-~·········
····-~············· __.__vom ...
...
.
90 ·················:············„„.:„ ············ ················:·················
...
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85 ·················:··················:·················:·····
...
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95
·················:„···
..
0
80-+--.-..--...--.--r·-.--,--.---r-T-r--r--r--r-~·-,......,-..,._-,-.,._..,,........-..-,.-~
-10
0
-5
5
10
Velocity in ms-
15
1
105
.
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.
.
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.
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c 90
.
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.
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....
...
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..c 85 --·····-· ·········1··········1·······-··1·····--··-,········--„··········i.··········
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...
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...
100
.
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....
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95
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•
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- ..... - ..
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- - .... - ..
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„ •• "\ •••••••••• '
•
•
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•
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„ •••••••
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~
~.
'
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'
'
80
2
4
1
1
•
'
1
•
6
8
'
'
10
•
•
•
•
1
•
14
16
12
Amplitude in ms·
18
1
105
100
.
.
..
..
..
c 90 ··························----·····J····
..
.. -·············----------·······················
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..c
.
.
.
85 ....................,................. „.....•.............. ..., ....... .
...
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E 95
.:::i::.
.
.
·····-·············-······
...........................
,, ..................,................ .
.
.
•
1
•
•
1
•
10
11
12
•
•
80
9
13
14
Phase in LMT
Fig. 2: Profiles of mesopause wind parameters, measured at Collm during the CR1STA satellite campaign 2.14.11.1994.
148
30
20
';"(/)
10
E
c
0
N
> 0 -10
-20
-30
0
';"(/)
E
c
10
20
30
40
50
60
70
80
90
0
10
20
30
40
50
60
70
80
90
0
10
20
30
40
50
60
70
80
90
0
10
20
30
40
50
60
70
80
90
30
20
10
0
-
E -10
>o
-20
-30
30
';"(/)
E
c
N
20
10
>"'
0
f-
:2
....J
c
N
f-"'
14
12
10
8
6
4
2
0
Running days beginning October 1 st
Fig. 3: Time series of mesopause wind parameters, beginning October 1st, 1994.
5. The Interannual Variability of the Zonal Prevailing Wind
Since profiles of the mesospheric parameters are available for a period of 13 years so far, longterm investigations with respect to the interannual variability of the mesopause dynamics are
possible with the aid of the measurements at Collm. Such an investigation has been done by
Jacobi et al. (1996). One result was a strong solar cycle dependence of the zonal prevailing
wind in November, such that during solar maximum v0 z is larger than during solar minimum.
This is also confirmed by the results of Bremer at al. (1996). In Figure 5, the monthly mean
values of the zonal prevailing wind, taken from the profiles at a height of 95 km, are shown in
dependence of the 13-monthly smoothed Zürich relative sunspot number R. The correlation
coefficient is r = 0.76, and the positive dependence is given at a significance level of 99% (ttest). The zonal prevailing wind in November 1994, denoted by the filled circle, is
comparatively low, in correspondence with the low solar activity during this year.
149
--zonal
------meridional
October- December 1994
23d 18d
7d
13d
4
o+-~~~..,..-~-.-~......-....-~.,......,'"""""T""'--_,_._"""--"-,.........""""'"'"'+-.:....:."-""1
0,01
0,1
Frequency in 1/day
Fig. 4: Energy spectra of the zanal and meridional prevailing wind, obtained from daily values near 95 km
altitude, based on the data from October to December. The respective 95% significance level (t-test) is
added. The vertical lines denote some characteristic periods.
16
November
1982 - 1995
12
1994
';"
(/)
E
c
0
j
8
N
0
0
>
0
4
h
= 95 km
v 02 = 3.55 + 0.05 R
0
r = 0.73
0
0-4-~~~~~~~~~-.-~-.-~--.....~--.....~--.....~~~~....-l
0
25
50
100
75
125
150
175
R
Fig. 5: Zonal prevailing ·wind at 95 km altitude, in dependence of the 13-monthly smoothed Zürich relative
sunspot nwnber R.
6. Conclusions
The mesopause wind field in autumn and early winter 1994 was observed by the D 1 LF wind
measurements at Collm. Thus the wind parameters during the CRIST A campaign, which was
carried out November 2-14, are given. The mean wind field did not show distinct differences to
the results of the long-term measurements that are carried out since 1982. An oscillation in the
period range of the quasi 16-day wave was measured, although it is doubtful, whether this is
150
due to a planetary wave, and probably variations in connection with the autumn transition of
the wind field and influences of the variability of the stratospheric polar vortex contribute to
this oscillation. The interannual variability of the November zonal prevailing wind consists
mostly of a solar cycle dependence.
Acknow ledgements
This research was partly supported by the "Deutsche Forschungsgemeinschaft" (German Science Foundation) under contract Schm 981/2-1.
References
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Solar cycle dependence and long-term trends in the wind field of the mesosphere/lower
thermosphere. J. Annas. Terr. Phys„ accepted.
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Y.I„ 1995: Quasi 16-day oscillation in the mesosphere and lower thermosphere. J.
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Addresses of Authors:
Ch. Jacobi, R. Schminder, Institut für Meteorologie, Universität Leipzig, Stephanstr. 3,
D-04103 Leipzig
D. Kürschner, Institut für Geophysik und Geologie, Universität Leipzig, Observatorium Collm,
D-04758 Collm

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