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Some direct comparisons of mesospheric winds observed at Kyoto and Adelaide
Jountaio/Afmusphericand
Trrrmtriol
Physics, Vol. 44, No. 3, pp. 267-280,
002 I-91 69/82/03026714 %03.00/O 0 1982 Pergamon Press Ltd.
1982.
Printed in Great Britain.
Some direct ~ornp~so~ of mesospherie winds observed at Kyoto and Adelaide T. Aso Radio Atmospheric Science Center, Kyoto University, Gokanosho, Uji, Kyoto 611, Japan and R. A.
VINCENT
Physics Department, University of Adelaide, Adelaide 5001, S. Australia (Received
11 November 1981)
infinalfirm
Abstract-Direct comparisons have been made of the prevailing and tidal wind fields observed in the 80-100 km height region using data obtained with a meteor radar at Kyoto (35”N, 136”E) and a partial reflection spaced antenna system at Adelaide (35”S, 138”E). Data taken with a partial reflection system at Townsville( 19”S, 147%) has also been included so that the latitudinal variations qfthe tidal structures could be taken into account. The comparisons extend over periods of up to one month duration centered on the equinoxes of 1979 and the January solstice of 1980.They show that there are often significant differences in the tidal amplitudes and phases observed at Kyoto and Adelaide, despite their near geographic conjugacy, probably indicating the presence of antisymmetrical tidal modes. The diurnal tide is appreciably stronger at Adelaide on the average, than at Kyoto, whereas the semi-diurnal amplitudes are on the average greater at Kyoto.
1. INTRODUCTION
Over the last two decades there have been many observations made at various locations of atmospheric tides in the upper mesosphere. Tidal ‘modes’have been identified using single station me~urements by comparing the observations with theoretical predictions of the vertical structure relating to particular modes. By analyzing the data that are available from all stations attempts have been made to give an average ‘picture’ofthe tidalstructurein~luding thelongitudinal wavenumbers, dominant Hough modes and the effects of superposed local variations. This approach has usually faced difficulties due to differences in the availability and length of the data sets and to ambiguities due to different methods of analysis. To mitigate these problems the Cooperative Tidal Observation Program (CTOP) has been coordinated by TAGA for the atmospheric radar community, and observing campaigns have been organized for specific periods in recent years (e.g. ROPERand SALAH,1978). The present study attempts to exploit the nearly conjugate geographic locations of the meteor radar of Kyoto University (35”N, 136”E) (Aso et al., 1979) and the partial reflection spaced antenna system of the University of Adelaide (35”S, 138”E) (BRIGGS et al., 1969),sited at Buckland Park,40 km north of Adelaide. The aim of the study was to compare the amplitudes of the tides at the two sites and to find out whether the tides are symmetrical with respect to thegeographicequator.
Possible ambiguities have been minimized by using the same program to analyze both data sets for specified overlapping observation periods. The analyses were made for March, September and October 1979, and January and March 1980 in which months the observations coincide for periods varying between 2-9 consecutive days. Wherever possible, use was also made of partial reflection wind data taken at the University of Adelaide’s temporary field station which was located near Townsville (19% 147”E) (VINCENT and BALL, 1981). This data was included to help delineate the latitudinal structure of the observed tides and so improve theestimates ofwhich tidal modes were present during the observing periods. 2. TECHNlQUEs AND DATA REDUCTlON 2.1. Partial rejlection winds (Adelaide and Townsville) The partial reflection spaced antenna wind method (hereinafter referred to as the PR method) gives a set of well defined horizontal velocity components from the so-called Full Correlation Method of analysis of the moving diffraction pattern associated with scatter from irregularities in the lower ionosphere. Observations at both Adelaide and Townsville were usually made over the height range 60-98 km during thedayand 80-98 km at night. Partly for technical reasons and partly because of the occurrence of suitable echoing regions the maximum number of wind observations were normally
267
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T. Aso and R. A. VINCENT
obtained between 86 and 90 km. Data were routinely taken for 4 min in every 10, at 2 km height intervals. There was a typical daily rate of between 500 and 1000 wind observations with a rather unifo~ distribution with time. 2.2. Meteor winds (Kyoto) The Kyoto meteor radar (MR) observes the line of sight velocity of drifting meteor trails over the 80-105 km height region. The height distribution of echoes for this radar was approximately Gaussian with a peak at 94 km and a variance of about 5 km(e.g. Aso et al., 1980). The meteor radar beam points due north and has a wide beam. The first reported observations from the radar made use of the echo decay rates to determine the height of the echoes but, with the installation and calibration ofa phase sensitive interferometer (PSI), the echoes are now located in both height and direction and in this paper only PSI measurements are used. Because not all the observed echoes are suitable for PSI direction finding the average daily rate of usable echoes is about 500. The echo occurrence rate shows the well known diurnal variation with a maximum rate of 70 echoes per hour occurring in the early morning, and falling to one tenth of that value in the evening. The meteor trials, of course, occur randomly in height, but for the purpose of analysis are grouped in either 2 or 4 km height intervals. 2.3. Data reduction Both the Adelaide PR and Kyoto MR data were analyzed with the now well established Groves method (GROVES,1959). It should be pointed out that because the heights of maximum observations are at 86-88 km and 94 km for the PRand MR methods respectively, the most reliable region for comparison is near 90 km. Accordingly the most detailed comparisons of the tidal parameters presented in the next section have been made for this interme~ate layer. A minims of about 800 meteors is required for a reasonable delineation of the tidal structure with the Kyoto system (TSUDAet al., 1980). However, since an average of 500 usable echoes per day are achieved with the PSI system, we have chosen to analyze both the Kyoto and Adelaide data in intervals of at least two consecutive days. Each PR measurement gives the EW and NS wind components simultaneously, whereas the MR method only gives the line of sight velocity; consequently the estimation error for the PR is on the average a factor of -0.7-0.5 smaller than that of the MR provided all other factors are equal. Furthermore, because the main pointing direction ofthe Kyoto radar is northwards, the EW velocities are less accurately
obtained than the NS components. On the other hand the indi~dual velocity and height errors for the Kyoto MR are somewhat less (1 m s- ’ and 2 km) than for the Adelaide PR (N IO m s- ’ and 24 km). Adelaide has a long history of meteor radar observations dating back to the 1950s. However, at the time of the present comparison the meteor facility was being upgraded which was why only the PR results are presented for Adelaide. A comparison of the present PR tidal amplitudes and phases with the synoptic features ofMR observations taken over six yearS(ELFORD,1974) shows that they are within the normal year to year variability. 3. RESULTS Contour plots of the annual variation ofthemonthly mean meridional and zonal winds (positive northwards and eastwards respectively) are shown for Kyoto and Adelaide in Fig. 1 for the period March 1979-May 1980. Because both stations were run on a campaign basis there were gaps in the data, especially at Adelaide. Nevertheless, the winds conform to simple circulation models and there is general agreement with the CIRA (1972) models which are the results of averages of data taken over many years and at many stations, mainly in the northern hemisphere.There do not appear to be any periods when the mean winds in the observation periods with which we are concerned depart in any exceptional way from the expected behavior. The March and September equinoxes appear to be particularly suitable times for comparing the tidal structure. At these times the solar declination is about zero, thereby minimizing any asymmetries due to solar forcing and, as Fig. 1 shows, the prevailing winds were generally at their weakest. In particular it is noticeable that the meridional flows were symmetrical about the equator, being weakly poleward at both stations at most heights at both equinoxes. A detailed comparison was therefore made of data taken in these periods and the results are compared with ob~rvations made for January 1980. In January, under solsticial conditions, any effects due to asymmetries in the solar forcing and thestrong background windsmight beexpected to beat their maximum. Before examining the observations in detail it is worth considering what the comparisons would show in the event that only a single tidal mode dominated the observations. Calculations based on classical tidal theory suggest that symmetric modes are more important than antisymmetric modes (CHAPMANand LINDZEN, 1970, see also review by FORBES and GARRETT, 1979) and consequently we would expect that the tidal amplitudes at Kyoto and Adelaide should
Some direct comparisons of mesosphe~c winds observed at Kyoto and Adelaide
269
98
90
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80
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Fig. 1. Time-height cross sections of the prevailing winds at Kyoto (top) and Adelaide (bottom). The shaded areas indicate regions of negative (westward and southward) winds.
be similar in magnitude and behave similarly with height. For symmetric modes the EW oscillations should be inphase and the NS oscillations should be in antiphase, i.e. there would be a difference of 12 and 6 h in the times ofmaximum northward winds for diurnal and semi-diurnal tides respectively. For the antisymmetric modes it would be the NS oscillations which should be inphase at the two locations and the EW oscillations which should be in antiphase.
3.1. March 1979 data PR observations were obtained at Adelaide in the period 14-29 March, while at Kyoto data were obtained in the periods 1417 and 24-27 March. Figures 2 and 3 illustrate the tidal structures observed in these periods for successive 48 h intervals, the number against each curve signifying the first day in each interval.
210
T. Aso and R. A. ADELAIDE
NS
MAR
VINCENT
1979
DIURNAL 100
100
90
90
E 1 ; P I”
80 0
80 10
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LO
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SEMI
Amplitude
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- DIURNAL
Phase
ms-’ ADELAIDE
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MAR
I
hr
1979
DIURNAL 100
100
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90
;
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CJl
c
I
60
80 0
10
20
30
LO
50
SEMI
Amplitude
I
0
6
12
18
- DIURNAL
ms-’
Phase
/
hr
Fig. 2. Height profiles of the diurnal and semi-diurnal tidal parameters observed for 2-day intervals at Adelaide in the period 1429 March 1979. The number against each curve signifies the starting date for each interval.
At Adelaide the diurnal tide was well resolved in both the EW and the NS wind components. The amplitudes were large (-30 m s-l) and relatively stable phases were obtained at most heights. The EW wind component leads the NS by about 6 h so that the tidal winds rotated in an anticlockwise manner and the phase gradients were both negative and of the same slope. The negative gradient implies energy propagating up from below and the slope indicates a vertical wavelength of - 3540 km. The behavior of the semidiurnal tide was more irregular than the diurnal in both height and time. The amplitudes were particularly weak in the NS components but even the EW components were less than half the amplitudes of the corresponding diurnal oscillations. A clear pattern in
the phase structure is not particularly evident and a simple phase quadrature relationship between the EW and NS components does not always hold. Figure 3 shows that the diurnal tide at Kyoto was weaker than at Adelaide and the phase variation as a function of height was appreciably different. At 90 km, the NS amplitudes were only about one third the corresponding values at Adelaide and the general structure often indicated interference effects ; the phases tended to remain constant with height or to show transitions at the amplitude minima. Although the EW amplitudes were somewhat larger than the NS amplitudes, the fihases again showed considerable variability. Because of the irregular phase progression evident, particularly in the EW components, it is
Some direct comparisons of mesospheric winds observed al Kyoto and Adelaide KYOTO
NS
MAR.
271
1979
DIURNAL
SEMI
Amplitude
- DIURNAL
Phase
’
I ms
KYOTO
E W
MAR.
I
hr
1979
DIURNAL
SEMI-
Amplitude
DIURNAL
I ms-’
Phase
/
hr
Fig. 3. As for Fig. 2 for Kyoto.
difficult to generalize about the phase differences between the components but, very approximately, the NS component led the EW component by 9 h. In order to bring out the differences between the tidal parameters for Kyoto and Adelaide a detailed comparison was made for the period 26-28 March. Results for Townsville were also included and excellent data rates were obtained at all three stations. The comparison is shown in Fig. 4. The two southern hemisphere stations show excellent agreement for the diurnal oscillations. A steady phase change with height is evident and the EW and NS components are approximately inphase quadrature. The NS component is larger in amplitude at the low latitude site (Townsville) but the EW amplitudes are comparable. For Kyoto, on the other hand, although the amplitude of the EW component is comparable in magnitude and behavior with height to
that at Adelaide and Townsville, the phase remained almost constant with height within the limits of error. The meridional amplitude of the diurnal oscillation at Kyoto is, at most heights, less than 10 m s- ’ which is close to the noise level, and again the phase changes only slowly with height. The agreement of the phase slope found near 90 km at Kyoto to that observed at the other locations is possibly fortuitous. In contrast to the behavior of the diurnal tide in this period, the semi-diurnal oscillation was found to be stronger at Kyoto or at least comparable to the values obtained at the other stations. The phase structure is more regular at Kyoto with the NS component leading by about 3 h (i.e. clockwise rotation). The irregular amplitude structure and corresponding irregularities in the phase observed at Adelaide and Townsville indicate interference between either a number ofdifferent modes or upgoing and downgoing waves. It is interesting that
T. Aso and R. A. VINCENT
272
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26-28
MAR. 1979
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Amplitude
I ms-’
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Phase
--k
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Fig. 4. Height profiles of the tidal amplitudes and phases at Adelaide, Town&lie and Kyoto in the period 2&28 March 1979.
the meridional components at Kyoto and Adelaide differ by about 3 h in phase with the phase at Townsville having intermediate values. Since the phases of the EW components at Kyoto and Adelaide were about 6 h apart this would suggest that an antisymmetric mode may have been present on this occasion. The data shown in Figs 2,3 and 4 indicate very clearly that the tidal amplitudes and phases at Adelaide and Kyoto can differ quite markedly even in a season when disturbances due to background winds etc, should be minimal.~causesomeofthevariationscould bedue to transient effects all the available data for March 1979 were averaged for each station so that the basic structure of the diurnal and semi-diurnal tides could be examined. The results are shown in Fig. 5 where the Adelaide and Townsville data span over 16 and 10 days
respectively. At Kyoto the data comes from 10 days in the 14-29 March interval when satisfactory interferometer information was available. As might be expected the averaging has produced more regularity in the tidal parameters and particularly in the phase. At Adelaide and Townsville the amplitudes of all diurnal components generally exceed 20 m s- ’ at all heights and they reach maximum values near 90 km. The phases also show smooth trends with the phase at Adelaide tending to lead that at Townsville by l-2 h ; the inferred vertical wavelength at both stations is about 35 km. The ~plitude of the NS component is larger at Townsville than at Adelaide, while the EW components areequal. The diurnal tide at Kyoto is less well determined, particularly in the NS component. Even the EW component has a mean
Some direct comparisons of mesospheric winds observed at Kyoto and Adelaide
NS
MARCH
273
1979
DIURNAL
0
10
20
30
LO
50
0
6
12
16
24
6
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SEMI- DIURNAL
Amplitude
Phase
I ms-t EW
MARCH
I hr
1979
DIURNAL
0
10
20
30
60
50
0
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SEMI-DIURNAL
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Amplitude
,
,
30
LO
,'~~/.~~~,
50
0
I ms-l
,
6
,;:$rf:-;,
6
12
Phase
12
I hr
Fig. 5.Height profiles of the average tidal amplitudes and phases observed at Adelaide,Townsville and Kyoto for the period 14-29 March 1979.
~pIitudeofabout 13ms- ’ which isonly halfthemean amplitude found at Adelaide and, rather surprisingly, the EW phase had a positive phase gradient which indicates downward energy progression. In contrast to the ‘classical’ 6 h phase difference between the NS and EW phases observed in the southern hemisphere stations, the components at Kyoto are more nearly in antiphase near 90 km. For the mean semi-diurnal oscillations we find that only at Townsville does the mean amplitude consistently exceed 10 m sP1 and then only in the NS component. This component showed a well behaved phase progression with an inferred vertical wavelength of about 60 km. At heights where the EW component had a significant amplitude at Townsville, it led the NS
by about 3 h in phase. The behavior at Adelaide was irregular with a node appearing at 90 km and irregular phase changes above this height. The Kyoto results indicate small meridonal amplitudes with the phase lagging by 3-5 h behind the zonal component at the upper heights. This contradicts theoretical predictions and is probably attributable to the very weak tidal amplitudes and the associated estimation errors in determining the zonal amplitude at Kyoto. The March observations discussed above indicate that, contrary to what might be expected, the tidal oscillations at Kyoto and Adelaide show quite different properties. The comparison between Adelaide and Towns~lle shows that the diurnal tide in the low to midlatitudes of the southern hemisphere can be
214
T. Aso and R. A. VINCENT
ascribed to the (1, 1) mode since the observed features are in good agreement with theoretical predictions for this mode (e.g. CHAPMANand LINDZEN, 1970). The (1,1) mode is the propagating mode which will give rise to larger NS oscillations at low latitudes than at midlatitudes and although the observed vertical wavelength is somewhat larger than the predicted value of - 25 km this is probably due to the effects of viscous damping (VINCENT and BALL, 1981). At Kyoto, conversely, the observed diurnal oscillation, both in the mean and in the individual observations, often seems to be due to an evanescent mode or modes which give rise to oscillations with a constant phase with height. During the March 1979 period therefore, we cannot identify simple tidal modes which are dominant in both hemispheres simultaneously, rather we appear to have the situation where different types of modes (propagating and evanescent) dominate separately in the two hemispheres. For the semi-diurnal oscillations we also find a quite complicated situation. On some occasions at a single station a relatively simple situation is found, for example, for the interval 26-28 March (Fig. 4) at Kyoto. The large vertical wavelength suggests the dominance of the (2,2) or possibly the antisymmetric (2,3) mode. Certainly there is a general lack of agreement between the phases observed at Kyoto and Adelaide both in the individual and the mean tidal structures. This, together with the rather variable structure evident in the amplitudes, indicates the presence of several modes, both symmetric and antisymmetric. Further comparisons between observations and theoretical calculations are considered in section 4. 3.2. September 1979 data Observations suitable for comparison studies were also available for the September equinox in 1979. At both Adelaide and Townsville continuous PR observations were made in the period 11 September-6 October while at Kyoto MR observations were restricted to four separate runs, each of 48 h duration. Figure 6 shows the comparison of the tidal structure for the three stations for the 48 h interval starting at noon on the 19 September and ending on the 21 September. This period had the best data rates of the four intervals available for study. For the meridional component of the diurnal oscillation the phases agree well, within the limits of error, at all stations and the amplitude-height profiles are also quite similar with the largest amplitudes at most heights occurring at Townsville. For the EW component however, the amplitudes at Townsville are relatively small, although still significant, and only about one half the amplitude observed at the two midlatitude stations.
The most interesting feature of the diurnal tides is the close agreement in phase observed for all stations in the NS component. This together with the 5-9 h phase difference observed between Kyoto and Adelaide in the EW component suggests a significant contribution from an antisymmetric mode in the diurnal oscillation in both hemispheres which is in contrast to the March results. The gravest diurnal antisymmetric mode is the (1,2) mode which has a vertical wavelength of about 15 km in the mesosphere. This value is close to the inferred vertical wavelength of the EW component at Townsville. For the semi-diurnal oscillation, the meridional component at Adelaide is small and has an irregular phase variation with height. The amplitudes at Kyoto and Townsville are of about the same magnitude and the oscillations are definitely in antiphase. The zonal component at Kyoto lags almost 3 h behind the meridional component, consistent with simple tidal theory and the inferred vertical wavelength is about 120 km. One noticeable feature of the Kyoto observations is the large amplitude of the semi-diurnal tide. The more irregular phase and amplitude behavior at both Adelaide and Townsville indicated interference effects produced by the presence of a number of high order modes. 3.3. January 1980 data
It is to be expected that antisymmetric modes might be even more evident in the data taken near the solstices because of the asymmetry of the solar forcing at these times and the strong and oppositely directed zonal winds in the middle atmospheres of the northern and southern hemispheres. A long sequence of observations made in January 1980 at Adelaide and Townsville and to a lesser extent at Kyoto, enabled comparisons to be made under these circumstances. Some care had to be taken when analyzing the southern hemisphere data because of the presence of a quasi 2-day oscillation which had large amplitudes especially in the meridional components. CRAIG er al. (1980) show that the mean period of the wave was 48.8 (+ 1) h and the mean meridional amplitude (-30-35 m s-r) was significantly larger than the amplitudes of either the diurnal or semi-diurnal tides. No evidence for this wave was found in the Kyoto data, which is consistent with the wave being a late summer phenomenon (e.g. MULLER and NELSON, 1978 ; CRAIG and ELFORD, 1981). This is an example of asymmetry in what is probably a free mode planetary wave due possibly to the strong asymmetry in the background zonal winds in the middle atmosphere. To reduce any influence that this quasi 2-day wave could have had on the tidal parameters, especially for
Some direct comparisons of mesospheric winds observed at Kyoto and Adelaide NS
275
19-21 SEP. 1979 DIURNAL
SEMI - DIURNAL
EW
19-21
SEP. 1979
DIURNAL
SEMI - DIURNAL
~‘~~~~~
0
10
,
L
20
30
Amplitude
1
LO
'if
50
'q;;::_.sbs
0
I ms-’
6
I 12
~~~~~~~
t
I 6
Phase
r
I,,,, 12
6
12
I hr
Fig. 6. As for Fig. 4 for the period 19-21 September 1979.
thediurnal tide,asinewaveofperiod48.8hwasfittedto the data at each height using a nonlinear regression technique and then subtracted from the raw data. This did not entirely remove the inllueace of the wave since, as CRAIG et al. (1980) show, the amplitude did fluctuate with time, although the phase was very consistent. To reduce even further any influence due to the amplitude fluctuations, we analyzed the detrended data in sliding 4-day intervals, each interval being displaced by 2 days from the previous one. Tidal parameters for the period 15-19 January 1980, are shown in Fig. 7 where it is seen that there is an appreciable degree of irregularity, especially in the diurnal oscillations. The meridional components are somewhat stronger at Adelaide and Townsville than at Kyoto. In general the diurnal amplitudes at the
Australian sites are appreciably less than the corresponding amplitudes in the observations taken at the equinoxes, especially at Townsville. Despite the weak amplitudes and variations in the phases, it is apparent that the phases at all three sites are nearly in phase in the NS component and Kyoto and Adelaide are in approximate antiphase in the EW component with Townsville being at an intermediate value. These results again indicate the presence of an antisymmetric mode but this time an evanescent mode because there is little evidence of a phase progression with height. The lowest order evanescent antisymmetric mode is the (1, - 1) mode. The semi-diurnal tide is better behaved than the diurnal and the comparison of Kyoto and Adelaide indicates inphase characteristics for the meridional and
T. Aso and R. A.
216
NS
15-19
VINCENT
JAN. 1980
DIURNAL 100 i ;
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0;
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Amplitude
/ ms-’
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6
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I hr
Fig. 7. As for Fig. 4 for the 4-day period 15-19 January 1980.
antiphase for the zonal components at most heights. The phase progressions indicate a mean vertical wavelength of about 70 km suggesting the dominance of the antisymmetric (2, 3) mode. At Townsville this mode has a relatively small amplitude so that a combination of the (2,2) and (2,4) symmetric modes may combine to give a phase shift of about 2 h relative to Adelaide. 4. DISCUSSION
In the preceding section we have shown that the tides observed at geographically conjugate locations frequently do not indicate well defined symmetry, even at the equinoxes. One way of conveniently summariz-
ing the degree of symmetry or asymmetry between the two sets of observations is to plot the results in a harmonic dial form, where the distance from the origin indicates the ratio (AA/AK) where A, and A, are the relevant tidal amplitudes at Adelaide and Kyoto and the angular displacement represents the phase difference (I$~ - 4A) in hours. In these plots the point (1,0) represents symmetry, i.e. the amplitudes are identical and there is a zero phase difference, while the points (1, 12) and (1,6) represent antisymmetry for the diurnal and semi-diurnal oscillations respectively. Observations for a mean height of 90 km are plotted in this harmonic dial form in Fig. 8 for equinoctial observations taken in March and September/October 1979 and March 1980. The results are plotted not only
277
Some direct comparisons of mesospheric winds observed at Kyoto and Adelaide DIURNAL
MAR I SEP
1979
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considerably{ N 1.5-9 times)larger than those observed at Kyoto and the points tend to cluster at locations some hours earlier than the antiphase hour. In the case of the semi-diurnal oscillations, the meridional components are on the average approximately equal in magnitudeand they tend to cluster about a point which indicates that Kyoto lags by a few hours from the expected value. The zonal component, however, has quite large amplitudes at Kyoto and relatively small amplitudes at Adelaide, consequently the zonal phase differences show a high degree of scatter. On the average the amplitude ratio A,/& -0.5 and the mean point is located closer to the antiphase (1,6) position than the inphase (1,O) point. Figure 9 shows similar results for the January 1980 data. Each point corresponds to four days of
t
12hr
S~MI~DIURNAL
MAR.1 SW
DIURNAL
1979
Ohr
Adelaide
Ohr
JAN 1980
Ypds
I
\
/
,
\
, 0
r
\
SEMI-DIURNAL
JAN
1980
Fig. 8. Harmonic dial plot of the amplitude ratios (Adelaide to Kyoto) and phase differences (q& - 4”) of the tidal parameters observed at 90 km. The numbers against each point indicate the first day in the 2-day intervals in the period 14-29 March 1979, and 4-6 March 1980 (A EW and 0 NS) and 19-24 September 1979 (LAA EW and ?:$ NS). The mean values are indicated by a (EW) and 0 (NS).
for individual 2-day intervals but averages, making use of all the data available in a given period, are also shown. For the diurnal tide the zonal components are comparable in ma~itude and tend to scatter around the (1,O) point, although on average, the amplitudes at Adelaide are l-2 times larger than at Kyoto. For the meridional component the amplitudes at Adelaide are
a’ ,
i \
Fig. 9. As for Fig. 8 for period 1L-27 January 1980.
218
T. Aso and R. A. VINCENI
observations beginning on the day indicated and the averages correspond to all data taken in two periods centered on early to mid-January (10-19 January} and the other on late January to early February (21 January-7 February). For the diurnal tide the zonal points cluster near the antiphase position (1, 12) with a tendency for Kyoto to lag whereas the meridional results indicate larger amplitudes at Adelaide with the phases showing that Adelaide leads by a few hours on average. Both the zonal and meridional results are consistent with a strong antisymmetric component. The data for the semi-diurnal tide shows that on average the semi-diurnal tide was larger in magnitude at Adelaide than at Kyoto which is in contrast to equinox results. Also the points indicating the average results are located near the (1,6) position for the zonal component, which also indicates a high degree of asymmetry in the tidal behavior. Some care must be taken in interpreting the results shown in Figs 8 and 9 simply in terms of either the symmetric or antisymmetric properties of tides. Those interpretations only have meaning if the tidal oscillations at Kyoto and Adelaide are due to similar combinations of tidal modes. That this is not always the case has been indicated by the March 1979 data for the diurnal tide which indicates the prevalence of evanescent modes at Kyoto and the propagating (1,1) mode at Adelaide. In general, the semi-diurnal tides showed better agreement as far as the height structure was concerned and a significant degree of antisymmetric behavior is shown. It should be pointed out that these conclusions are not influenced by a consideration of the errors involved in our analysis. The averaged errors at 90 km for the Adelaide March data are about 1.7m s- ’ in amplitude and the phase errors are about 0.3 and 0.4 h for the diurnal and semi-diurnal components respectively. These are typical values for the other periods analyzed although the errors depend on the particular Groves model used and the data density. At Kyoto, the averages of the estimated errors are 4.0 and 1.6 m s- ’ for the zonal and meridional wind components and the phase errors are 1.1 and 0.9 h on average for the diurnal and semi-diurnal components. There are a number of possible reasons why the tidal structure may be more complex than is predicted by classical tidal theory and may include higher order and antis~metri~ modes. Amongst the factors recently considered are asymmetries or local effects in ozone and other forcings, including topography, asymmetries in the background temperature profile which can act as an atmospheric filter for some modes, and asymmetries in the background winds which give rise to higher order modes by mode coupling (e.g. TEI~L~A~M and COT, 1979 ; LINDZEN and HONG, 1974; WALTERSHEIDet al.,
1980). It is possible to simulate these effects in a computer and one such numerical simulation has been carried out for the semi-diurnal tide by Aso et al. (1981). They make use of background temperature and wind profiles taken from the CIRA (1972) model and proper models of the solar thermal forcing due to insolation absorption by ozone and water vapor. At this stage only the semi-diurnal tide has been modelled because the vertical wavelengths of the modes of interest are of the order of tens of kilometers or larger and consequently a moderate height resolution may be used. For the diurnal tide on the other hand, the propagating modes have short wavelengths and consequently a high resolution and a large computer storage is required. Figure 10 illustrates a latitudinal plot of the observed
DIURNAL
90-N
00
90°N
90-s
OY
90-s
SEMI-OIURNAL -r
MERIDIONAL
LO
ZONAL
LO
t
9OoN
0"
90'S
9O=N
00
90%
Fig. 10. Latjtudinal plot of average tidal amplitudes observed at 90 km at Kyoto (K), Townsville (T) and Adelaide (A) for 14-29 March (0) and September/October (A) 1979. The theoreticai curves are from Aso ef al., 1981 for cases of with (-) and without (----) equino~tia1 background winds included.
Some direct comparisons of mesospheric winds observed at Kyoto and Adelaide and phases of the diurnal and semi-diurnal tides at 90 km in the March and September/O~tober equinoxes and, for the semi-diurna1 tide, the numerically simulated parameters by the ASO et al. (1981) model are also shown for the cases with and without background wind. For both sets of observations, the diurnal components indicate a large asymmetry between the hemispheres, in that the amplitudes, especially in the meridional components, are much larger in the southern hemisphere. However, the phases indicate a moderate degree of symmetry, especially in the September/October data. The phase tilt between Townsville and Adelaide could be reproduced theoretically by the appropriate inclusion of higher order modes, not necessarily antisymmetric modes. For the semi-diurnal tide, the zonal amplitude in September at Kyoto seems large in comparison with theory, whereas the other amphtudes at Kyoto and at Townsville are in quite good agreement, especially with the simulation results including the effects of the background wind. The amplitudes at Adelaide seem small in all cases. As far as the phases are concerned, there is good agreement with theory at Kyoto in most cases but this agreement is less evident for the southern hemisphere stations, although the simulation with the winds included does show the same general trend in phase between Townsv~le and Adelaide. These comparisons do show that the presence of higher order modes and/or antisymmetric modes are required in the theoretical models to simulate the observed latitudinal variations in amplitude and phase and that further work along the lines indicated must be done to improve the agreement between theory and observations. amplitudes
Aso T., NONOYA~AT. and Kent S. Aso T., TSUDAT. and KATZ S. Aso T., TSUDAT., TAKASHIMA Y ., ITO R. and KATZ S. BRICCSB. H., ELFORDW. G., FELGATED. G., GOLLEYM. G., ROSSITER D. E. and SMITHJ. W. CHAPMANS. and LINDZENR. S. CIRA
CONCLUSIONS
The atmospheric tidal structures observed at meteor heights at the geographically conjugate locations of Kyoto and Adelaide, have been compared with a short term ( - 2 day) and on an average ( - 2 week) basis. The study reveals an asymmetry between the tidal parameters even at the equinoxes when the asymmetries in the forcing and due to the background winds are a minimum ; the diurnal tidal amplitudes at Adelaide are appreciably larger than those at Kyoto, but the semi-diurnal amplitudes are on the whole smaller. At the January solstice an even more well defined antisymmetry is found which to some extent is due to asymmetries in the forcing and the background winds. However, it is possible that still more compli~ted agencies, such as nonlinear interactions between tides and the mean ffow, will need to be invoked to explain the differences between the observations and theory. The dissipation of the diurnal and semi-diurnal tidal energies can make significant contributions to the energy budgets of the mesosphere and thermosphere and consequently further comparisons are required to establish whether the differences reported here are of regular occurrence or not. ~cknow~e~ge~nts-This work wascarried out white one of us (TA) was a Visiting Research Fellow at the University of Adelaide. The help given hi by Dr W. G. ELFORD,Chairman of the Physics Department, to facilitate his grant in aid from the University of Adelaide, is gratefully acknowledged as are the comments of and useful discussions with our colleagues at Kyoto and Adelaide, Professor S. KATZand Dr B. H BRIGGS. The provision of the site for the Townsville experiment by the Ionospheric Prediction Service (Australian Department of Science and the Environment) is also acknowledged. Part of this research was supported by the Australian Research Grants Committee.
1981 1979 1980
J. geophys. Res. (in press). J. atmos. terr. Phys. 41,517. J. geophys. Res. 85,117.
1969
Nature 223, 1321.
1970 1972
Atmospheric Tides D. Reidel, Dordrecht, Holland. Cospar International Reference Atmosphere, Akademie,
1980
Nature 281, 3 19.
1981 1979 1959 1974 1978
J. atmos. terr. Phys. 43, 1051. Reu. geophys. Space Phys. 17,19X J. aunos. terr. Phys. 16,344. J. atmos. Ski. 31, 1421. .I. atmos. terr. Phys. 40,761.
Berlin. CRUZ R. L., VINCENTR. A., FRASERG. J. and SMITHM. J. CRAIGR. L. and ELFORDW. G.
FORBESJ. M. and GARRETTH. B. GROVESG. V. LINDZ~ R. S. and HONGS.-S. MULLER H. G. and NELKIN L.
219
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T. Aso and R. A. VINCENT
ROPERR. G. and SALAHJ. E. TEITELBAUM H. and COT C. TSUDAT., Aso T., TAKASHIMA Y., ITO R. and KATOS. VINCENTR. A. and BALLS. M. WALTERSCHEIU R. L., DE VOKEJ. G. and VENKATFSWAREN S. V.
1978 1979 1980
J. atmos. terr. Phys. 40,879. J. atmos. terr. Phys. 41,33. J. atmos. terr. Phys. 42,461.
1981 1980
J. geophys. Res. (in press). J. atmos. Sci. 31.455.
1974
Proc. Int. Conf. on Structure, Composition and General Circulation of the Upper and Lower Atmospheres and Possible Anthropogenic Perturbation II, 624 Melbourne.
Reference is also made to the following unpublished material:
ELFORDW. G.
Trrrmtriol
Physics, Vol. 44, No. 3, pp. 267-280,
002 I-91 69/82/03026714 %03.00/O 0 1982 Pergamon Press Ltd.
1982.
Printed in Great Britain.
Some direct ~ornp~so~ of mesospherie winds observed at Kyoto and Adelaide T. Aso Radio Atmospheric Science Center, Kyoto University, Gokanosho, Uji, Kyoto 611, Japan and R. A.
VINCENT
Physics Department, University of Adelaide, Adelaide 5001, S. Australia (Received
11 November 1981)
infinalfirm
Abstract-Direct comparisons have been made of the prevailing and tidal wind fields observed in the 80-100 km height region using data obtained with a meteor radar at Kyoto (35”N, 136”E) and a partial reflection spaced antenna system at Adelaide (35”S, 138”E). Data taken with a partial reflection system at Townsville( 19”S, 147%) has also been included so that the latitudinal variations qfthe tidal structures could be taken into account. The comparisons extend over periods of up to one month duration centered on the equinoxes of 1979 and the January solstice of 1980.They show that there are often significant differences in the tidal amplitudes and phases observed at Kyoto and Adelaide, despite their near geographic conjugacy, probably indicating the presence of antisymmetrical tidal modes. The diurnal tide is appreciably stronger at Adelaide on the average, than at Kyoto, whereas the semi-diurnal amplitudes are on the average greater at Kyoto.
1. INTRODUCTION
Over the last two decades there have been many observations made at various locations of atmospheric tides in the upper mesosphere. Tidal ‘modes’have been identified using single station me~urements by comparing the observations with theoretical predictions of the vertical structure relating to particular modes. By analyzing the data that are available from all stations attempts have been made to give an average ‘picture’ofthe tidalstructurein~luding thelongitudinal wavenumbers, dominant Hough modes and the effects of superposed local variations. This approach has usually faced difficulties due to differences in the availability and length of the data sets and to ambiguities due to different methods of analysis. To mitigate these problems the Cooperative Tidal Observation Program (CTOP) has been coordinated by TAGA for the atmospheric radar community, and observing campaigns have been organized for specific periods in recent years (e.g. ROPERand SALAH,1978). The present study attempts to exploit the nearly conjugate geographic locations of the meteor radar of Kyoto University (35”N, 136”E) (Aso et al., 1979) and the partial reflection spaced antenna system of the University of Adelaide (35”S, 138”E) (BRIGGS et al., 1969),sited at Buckland Park,40 km north of Adelaide. The aim of the study was to compare the amplitudes of the tides at the two sites and to find out whether the tides are symmetrical with respect to thegeographicequator.
Possible ambiguities have been minimized by using the same program to analyze both data sets for specified overlapping observation periods. The analyses were made for March, September and October 1979, and January and March 1980 in which months the observations coincide for periods varying between 2-9 consecutive days. Wherever possible, use was also made of partial reflection wind data taken at the University of Adelaide’s temporary field station which was located near Townsville (19% 147”E) (VINCENT and BALL, 1981). This data was included to help delineate the latitudinal structure of the observed tides and so improve theestimates ofwhich tidal modes were present during the observing periods. 2. TECHNlQUEs AND DATA REDUCTlON 2.1. Partial rejlection winds (Adelaide and Townsville) The partial reflection spaced antenna wind method (hereinafter referred to as the PR method) gives a set of well defined horizontal velocity components from the so-called Full Correlation Method of analysis of the moving diffraction pattern associated with scatter from irregularities in the lower ionosphere. Observations at both Adelaide and Townsville were usually made over the height range 60-98 km during thedayand 80-98 km at night. Partly for technical reasons and partly because of the occurrence of suitable echoing regions the maximum number of wind observations were normally
267
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T. Aso and R. A. VINCENT
obtained between 86 and 90 km. Data were routinely taken for 4 min in every 10, at 2 km height intervals. There was a typical daily rate of between 500 and 1000 wind observations with a rather unifo~ distribution with time. 2.2. Meteor winds (Kyoto) The Kyoto meteor radar (MR) observes the line of sight velocity of drifting meteor trails over the 80-105 km height region. The height distribution of echoes for this radar was approximately Gaussian with a peak at 94 km and a variance of about 5 km(e.g. Aso et al., 1980). The meteor radar beam points due north and has a wide beam. The first reported observations from the radar made use of the echo decay rates to determine the height of the echoes but, with the installation and calibration ofa phase sensitive interferometer (PSI), the echoes are now located in both height and direction and in this paper only PSI measurements are used. Because not all the observed echoes are suitable for PSI direction finding the average daily rate of usable echoes is about 500. The echo occurrence rate shows the well known diurnal variation with a maximum rate of 70 echoes per hour occurring in the early morning, and falling to one tenth of that value in the evening. The meteor trials, of course, occur randomly in height, but for the purpose of analysis are grouped in either 2 or 4 km height intervals. 2.3. Data reduction Both the Adelaide PR and Kyoto MR data were analyzed with the now well established Groves method (GROVES,1959). It should be pointed out that because the heights of maximum observations are at 86-88 km and 94 km for the PRand MR methods respectively, the most reliable region for comparison is near 90 km. Accordingly the most detailed comparisons of the tidal parameters presented in the next section have been made for this interme~ate layer. A minims of about 800 meteors is required for a reasonable delineation of the tidal structure with the Kyoto system (TSUDAet al., 1980). However, since an average of 500 usable echoes per day are achieved with the PSI system, we have chosen to analyze both the Kyoto and Adelaide data in intervals of at least two consecutive days. Each PR measurement gives the EW and NS wind components simultaneously, whereas the MR method only gives the line of sight velocity; consequently the estimation error for the PR is on the average a factor of -0.7-0.5 smaller than that of the MR provided all other factors are equal. Furthermore, because the main pointing direction ofthe Kyoto radar is northwards, the EW velocities are less accurately
obtained than the NS components. On the other hand the indi~dual velocity and height errors for the Kyoto MR are somewhat less (1 m s- ’ and 2 km) than for the Adelaide PR (N IO m s- ’ and 24 km). Adelaide has a long history of meteor radar observations dating back to the 1950s. However, at the time of the present comparison the meteor facility was being upgraded which was why only the PR results are presented for Adelaide. A comparison of the present PR tidal amplitudes and phases with the synoptic features ofMR observations taken over six yearS(ELFORD,1974) shows that they are within the normal year to year variability. 3. RESULTS Contour plots of the annual variation ofthemonthly mean meridional and zonal winds (positive northwards and eastwards respectively) are shown for Kyoto and Adelaide in Fig. 1 for the period March 1979-May 1980. Because both stations were run on a campaign basis there were gaps in the data, especially at Adelaide. Nevertheless, the winds conform to simple circulation models and there is general agreement with the CIRA (1972) models which are the results of averages of data taken over many years and at many stations, mainly in the northern hemisphere.There do not appear to be any periods when the mean winds in the observation periods with which we are concerned depart in any exceptional way from the expected behavior. The March and September equinoxes appear to be particularly suitable times for comparing the tidal structure. At these times the solar declination is about zero, thereby minimizing any asymmetries due to solar forcing and, as Fig. 1 shows, the prevailing winds were generally at their weakest. In particular it is noticeable that the meridional flows were symmetrical about the equator, being weakly poleward at both stations at most heights at both equinoxes. A detailed comparison was therefore made of data taken in these periods and the results are compared with ob~rvations made for January 1980. In January, under solsticial conditions, any effects due to asymmetries in the solar forcing and thestrong background windsmight beexpected to beat their maximum. Before examining the observations in detail it is worth considering what the comparisons would show in the event that only a single tidal mode dominated the observations. Calculations based on classical tidal theory suggest that symmetric modes are more important than antisymmetric modes (CHAPMANand LINDZEN, 1970, see also review by FORBES and GARRETT, 1979) and consequently we would expect that the tidal amplitudes at Kyoto and Adelaide should
Some direct comparisons of mesosphe~c winds observed at Kyoto and Adelaide
269
98
90
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80
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2 90
__ I
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I
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1980
Fig. 1. Time-height cross sections of the prevailing winds at Kyoto (top) and Adelaide (bottom). The shaded areas indicate regions of negative (westward and southward) winds.
be similar in magnitude and behave similarly with height. For symmetric modes the EW oscillations should be inphase and the NS oscillations should be in antiphase, i.e. there would be a difference of 12 and 6 h in the times ofmaximum northward winds for diurnal and semi-diurnal tides respectively. For the antisymmetric modes it would be the NS oscillations which should be inphase at the two locations and the EW oscillations which should be in antiphase.
3.1. March 1979 data PR observations were obtained at Adelaide in the period 14-29 March, while at Kyoto data were obtained in the periods 1417 and 24-27 March. Figures 2 and 3 illustrate the tidal structures observed in these periods for successive 48 h intervals, the number against each curve signifying the first day in each interval.
210
T. Aso and R. A. ADELAIDE
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MAR
VINCENT
1979
DIURNAL 100
100
90
90
E 1 ; P I”
80 0
80 10
20
30
LO
50
SEMI
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12
2L
6
12
- DIURNAL
Phase
ms-’ ADELAIDE
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MAR
I
hr
1979
DIURNAL 100
100
E 1
90
;
90
CJl
c
I
60
80 0
10
20
30
LO
50
SEMI
Amplitude
I
0
6
12
18
- DIURNAL
ms-’
Phase
/
hr
Fig. 2. Height profiles of the diurnal and semi-diurnal tidal parameters observed for 2-day intervals at Adelaide in the period 1429 March 1979. The number against each curve signifies the starting date for each interval.
At Adelaide the diurnal tide was well resolved in both the EW and the NS wind components. The amplitudes were large (-30 m s-l) and relatively stable phases were obtained at most heights. The EW wind component leads the NS by about 6 h so that the tidal winds rotated in an anticlockwise manner and the phase gradients were both negative and of the same slope. The negative gradient implies energy propagating up from below and the slope indicates a vertical wavelength of - 3540 km. The behavior of the semidiurnal tide was more irregular than the diurnal in both height and time. The amplitudes were particularly weak in the NS components but even the EW components were less than half the amplitudes of the corresponding diurnal oscillations. A clear pattern in
the phase structure is not particularly evident and a simple phase quadrature relationship between the EW and NS components does not always hold. Figure 3 shows that the diurnal tide at Kyoto was weaker than at Adelaide and the phase variation as a function of height was appreciably different. At 90 km, the NS amplitudes were only about one third the corresponding values at Adelaide and the general structure often indicated interference effects ; the phases tended to remain constant with height or to show transitions at the amplitude minima. Although the EW amplitudes were somewhat larger than the NS amplitudes, the fihases again showed considerable variability. Because of the irregular phase progression evident, particularly in the EW components, it is
Some direct comparisons of mesospheric winds observed al Kyoto and Adelaide KYOTO
NS
MAR.
271
1979
DIURNAL
SEMI
Amplitude
- DIURNAL
Phase
’
I ms
KYOTO
E W
MAR.
I
hr
1979
DIURNAL
SEMI-
Amplitude
DIURNAL
I ms-’
Phase
/
hr
Fig. 3. As for Fig. 2 for Kyoto.
difficult to generalize about the phase differences between the components but, very approximately, the NS component led the EW component by 9 h. In order to bring out the differences between the tidal parameters for Kyoto and Adelaide a detailed comparison was made for the period 26-28 March. Results for Townsville were also included and excellent data rates were obtained at all three stations. The comparison is shown in Fig. 4. The two southern hemisphere stations show excellent agreement for the diurnal oscillations. A steady phase change with height is evident and the EW and NS components are approximately inphase quadrature. The NS component is larger in amplitude at the low latitude site (Townsville) but the EW amplitudes are comparable. For Kyoto, on the other hand, although the amplitude of the EW component is comparable in magnitude and behavior with height to
that at Adelaide and Townsville, the phase remained almost constant with height within the limits of error. The meridional amplitude of the diurnal oscillation at Kyoto is, at most heights, less than 10 m s- ’ which is close to the noise level, and again the phase changes only slowly with height. The agreement of the phase slope found near 90 km at Kyoto to that observed at the other locations is possibly fortuitous. In contrast to the behavior of the diurnal tide in this period, the semi-diurnal oscillation was found to be stronger at Kyoto or at least comparable to the values obtained at the other stations. The phase structure is more regular at Kyoto with the NS component leading by about 3 h (i.e. clockwise rotation). The irregular amplitude structure and corresponding irregularities in the phase observed at Adelaide and Townsville indicate interference between either a number ofdifferent modes or upgoing and downgoing waves. It is interesting that
T. Aso and R. A. VINCENT
272
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26-28
MAR. 1979
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Fig. 4. Height profiles of the tidal amplitudes and phases at Adelaide, Town&lie and Kyoto in the period 2&28 March 1979.
the meridional components at Kyoto and Adelaide differ by about 3 h in phase with the phase at Townsville having intermediate values. Since the phases of the EW components at Kyoto and Adelaide were about 6 h apart this would suggest that an antisymmetric mode may have been present on this occasion. The data shown in Figs 2,3 and 4 indicate very clearly that the tidal amplitudes and phases at Adelaide and Kyoto can differ quite markedly even in a season when disturbances due to background winds etc, should be minimal.~causesomeofthevariationscould bedue to transient effects all the available data for March 1979 were averaged for each station so that the basic structure of the diurnal and semi-diurnal tides could be examined. The results are shown in Fig. 5 where the Adelaide and Townsville data span over 16 and 10 days
respectively. At Kyoto the data comes from 10 days in the 14-29 March interval when satisfactory interferometer information was available. As might be expected the averaging has produced more regularity in the tidal parameters and particularly in the phase. At Adelaide and Townsville the amplitudes of all diurnal components generally exceed 20 m s- ’ at all heights and they reach maximum values near 90 km. The phases also show smooth trends with the phase at Adelaide tending to lead that at Townsville by l-2 h ; the inferred vertical wavelength at both stations is about 35 km. The ~plitude of the NS component is larger at Townsville than at Adelaide, while the EW components areequal. The diurnal tide at Kyoto is less well determined, particularly in the NS component. Even the EW component has a mean
Some direct comparisons of mesospheric winds observed at Kyoto and Adelaide
NS
MARCH
273
1979
DIURNAL
0
10
20
30
LO
50
0
6
12
16
24
6
12
6
12
6
12
SEMI- DIURNAL
Amplitude
Phase
I ms-t EW
MARCH
I hr
1979
DIURNAL
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10
20
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60
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,
,
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,
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Fig. 5.Height profiles of the average tidal amplitudes and phases observed at Adelaide,Townsville and Kyoto for the period 14-29 March 1979.
~pIitudeofabout 13ms- ’ which isonly halfthemean amplitude found at Adelaide and, rather surprisingly, the EW phase had a positive phase gradient which indicates downward energy progression. In contrast to the ‘classical’ 6 h phase difference between the NS and EW phases observed in the southern hemisphere stations, the components at Kyoto are more nearly in antiphase near 90 km. For the mean semi-diurnal oscillations we find that only at Townsville does the mean amplitude consistently exceed 10 m sP1 and then only in the NS component. This component showed a well behaved phase progression with an inferred vertical wavelength of about 60 km. At heights where the EW component had a significant amplitude at Townsville, it led the NS
by about 3 h in phase. The behavior at Adelaide was irregular with a node appearing at 90 km and irregular phase changes above this height. The Kyoto results indicate small meridonal amplitudes with the phase lagging by 3-5 h behind the zonal component at the upper heights. This contradicts theoretical predictions and is probably attributable to the very weak tidal amplitudes and the associated estimation errors in determining the zonal amplitude at Kyoto. The March observations discussed above indicate that, contrary to what might be expected, the tidal oscillations at Kyoto and Adelaide show quite different properties. The comparison between Adelaide and Towns~lle shows that the diurnal tide in the low to midlatitudes of the southern hemisphere can be
214
T. Aso and R. A. VINCENT
ascribed to the (1, 1) mode since the observed features are in good agreement with theoretical predictions for this mode (e.g. CHAPMANand LINDZEN, 1970). The (1,1) mode is the propagating mode which will give rise to larger NS oscillations at low latitudes than at midlatitudes and although the observed vertical wavelength is somewhat larger than the predicted value of - 25 km this is probably due to the effects of viscous damping (VINCENT and BALL, 1981). At Kyoto, conversely, the observed diurnal oscillation, both in the mean and in the individual observations, often seems to be due to an evanescent mode or modes which give rise to oscillations with a constant phase with height. During the March 1979 period therefore, we cannot identify simple tidal modes which are dominant in both hemispheres simultaneously, rather we appear to have the situation where different types of modes (propagating and evanescent) dominate separately in the two hemispheres. For the semi-diurnal oscillations we also find a quite complicated situation. On some occasions at a single station a relatively simple situation is found, for example, for the interval 26-28 March (Fig. 4) at Kyoto. The large vertical wavelength suggests the dominance of the (2,2) or possibly the antisymmetric (2,3) mode. Certainly there is a general lack of agreement between the phases observed at Kyoto and Adelaide both in the individual and the mean tidal structures. This, together with the rather variable structure evident in the amplitudes, indicates the presence of several modes, both symmetric and antisymmetric. Further comparisons between observations and theoretical calculations are considered in section 4. 3.2. September 1979 data Observations suitable for comparison studies were also available for the September equinox in 1979. At both Adelaide and Townsville continuous PR observations were made in the period 11 September-6 October while at Kyoto MR observations were restricted to four separate runs, each of 48 h duration. Figure 6 shows the comparison of the tidal structure for the three stations for the 48 h interval starting at noon on the 19 September and ending on the 21 September. This period had the best data rates of the four intervals available for study. For the meridional component of the diurnal oscillation the phases agree well, within the limits of error, at all stations and the amplitude-height profiles are also quite similar with the largest amplitudes at most heights occurring at Townsville. For the EW component however, the amplitudes at Townsville are relatively small, although still significant, and only about one half the amplitude observed at the two midlatitude stations.
The most interesting feature of the diurnal tides is the close agreement in phase observed for all stations in the NS component. This together with the 5-9 h phase difference observed between Kyoto and Adelaide in the EW component suggests a significant contribution from an antisymmetric mode in the diurnal oscillation in both hemispheres which is in contrast to the March results. The gravest diurnal antisymmetric mode is the (1,2) mode which has a vertical wavelength of about 15 km in the mesosphere. This value is close to the inferred vertical wavelength of the EW component at Townsville. For the semi-diurnal oscillation, the meridional component at Adelaide is small and has an irregular phase variation with height. The amplitudes at Kyoto and Townsville are of about the same magnitude and the oscillations are definitely in antiphase. The zonal component at Kyoto lags almost 3 h behind the meridional component, consistent with simple tidal theory and the inferred vertical wavelength is about 120 km. One noticeable feature of the Kyoto observations is the large amplitude of the semi-diurnal tide. The more irregular phase and amplitude behavior at both Adelaide and Townsville indicated interference effects produced by the presence of a number of high order modes. 3.3. January 1980 data
It is to be expected that antisymmetric modes might be even more evident in the data taken near the solstices because of the asymmetry of the solar forcing at these times and the strong and oppositely directed zonal winds in the middle atmospheres of the northern and southern hemispheres. A long sequence of observations made in January 1980 at Adelaide and Townsville and to a lesser extent at Kyoto, enabled comparisons to be made under these circumstances. Some care had to be taken when analyzing the southern hemisphere data because of the presence of a quasi 2-day oscillation which had large amplitudes especially in the meridional components. CRAIG er al. (1980) show that the mean period of the wave was 48.8 (+ 1) h and the mean meridional amplitude (-30-35 m s-r) was significantly larger than the amplitudes of either the diurnal or semi-diurnal tides. No evidence for this wave was found in the Kyoto data, which is consistent with the wave being a late summer phenomenon (e.g. MULLER and NELSON, 1978 ; CRAIG and ELFORD, 1981). This is an example of asymmetry in what is probably a free mode planetary wave due possibly to the strong asymmetry in the background zonal winds in the middle atmosphere. To reduce any influence that this quasi 2-day wave could have had on the tidal parameters, especially for
Some direct comparisons of mesospheric winds observed at Kyoto and Adelaide NS
275
19-21 SEP. 1979 DIURNAL
SEMI - DIURNAL
EW
19-21
SEP. 1979
DIURNAL
SEMI - DIURNAL
~‘~~~~~
0
10
,
L
20
30
Amplitude
1
LO
'if
50
'q;;::_.sbs
0
I ms-’
6
I 12
~~~~~~~
t
I 6
Phase
r
I,,,, 12
6
12
I hr
Fig. 6. As for Fig. 4 for the period 19-21 September 1979.
thediurnal tide,asinewaveofperiod48.8hwasfittedto the data at each height using a nonlinear regression technique and then subtracted from the raw data. This did not entirely remove the inllueace of the wave since, as CRAIG et al. (1980) show, the amplitude did fluctuate with time, although the phase was very consistent. To reduce even further any influence due to the amplitude fluctuations, we analyzed the detrended data in sliding 4-day intervals, each interval being displaced by 2 days from the previous one. Tidal parameters for the period 15-19 January 1980, are shown in Fig. 7 where it is seen that there is an appreciable degree of irregularity, especially in the diurnal oscillations. The meridional components are somewhat stronger at Adelaide and Townsville than at Kyoto. In general the diurnal amplitudes at the
Australian sites are appreciably less than the corresponding amplitudes in the observations taken at the equinoxes, especially at Townsville. Despite the weak amplitudes and variations in the phases, it is apparent that the phases at all three sites are nearly in phase in the NS component and Kyoto and Adelaide are in approximate antiphase in the EW component with Townsville being at an intermediate value. These results again indicate the presence of an antisymmetric mode but this time an evanescent mode because there is little evidence of a phase progression with height. The lowest order evanescent antisymmetric mode is the (1, - 1) mode. The semi-diurnal tide is better behaved than the diurnal and the comparison of Kyoto and Adelaide indicates inphase characteristics for the meridional and
T. Aso and R. A.
216
NS
15-19
VINCENT
JAN. 1980
DIURNAL 100 i ;
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I’ ‘\ S%, ’ 10
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15-19 JAN. 1980 DIURNAL
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Lb
,
;.;
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Amplitude
/ ms-’
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6
,_E,:,
12
Phase
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12
6
I hr
Fig. 7. As for Fig. 4 for the 4-day period 15-19 January 1980.
antiphase for the zonal components at most heights. The phase progressions indicate a mean vertical wavelength of about 70 km suggesting the dominance of the antisymmetric (2, 3) mode. At Townsville this mode has a relatively small amplitude so that a combination of the (2,2) and (2,4) symmetric modes may combine to give a phase shift of about 2 h relative to Adelaide. 4. DISCUSSION
In the preceding section we have shown that the tides observed at geographically conjugate locations frequently do not indicate well defined symmetry, even at the equinoxes. One way of conveniently summariz-
ing the degree of symmetry or asymmetry between the two sets of observations is to plot the results in a harmonic dial form, where the distance from the origin indicates the ratio (AA/AK) where A, and A, are the relevant tidal amplitudes at Adelaide and Kyoto and the angular displacement represents the phase difference (I$~ - 4A) in hours. In these plots the point (1,0) represents symmetry, i.e. the amplitudes are identical and there is a zero phase difference, while the points (1, 12) and (1,6) represent antisymmetry for the diurnal and semi-diurnal oscillations respectively. Observations for a mean height of 90 km are plotted in this harmonic dial form in Fig. 8 for equinoctial observations taken in March and September/October 1979 and March 1980. The results are plotted not only
277
Some direct comparisons of mesospheric winds observed at Kyoto and Adelaide DIURNAL
MAR I SEP
1979
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8.5
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%s
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i
considerably{ N 1.5-9 times)larger than those observed at Kyoto and the points tend to cluster at locations some hours earlier than the antiphase hour. In the case of the semi-diurnal oscillations, the meridional components are on the average approximately equal in magnitudeand they tend to cluster about a point which indicates that Kyoto lags by a few hours from the expected value. The zonal component, however, has quite large amplitudes at Kyoto and relatively small amplitudes at Adelaide, consequently the zonal phase differences show a high degree of scatter. On the average the amplitude ratio A,/& -0.5 and the mean point is located closer to the antiphase (1,6) position than the inphase (1,O) point. Figure 9 shows similar results for the January 1980 data. Each point corresponds to four days of
t
12hr
S~MI~DIURNAL
MAR.1 SW
DIURNAL
1979
Ohr
Adelaide
Ohr
JAN 1980
Ypds
I
\
/
,
\
, 0
r
\
SEMI-DIURNAL
JAN
1980
Fig. 8. Harmonic dial plot of the amplitude ratios (Adelaide to Kyoto) and phase differences (q& - 4”) of the tidal parameters observed at 90 km. The numbers against each point indicate the first day in the 2-day intervals in the period 14-29 March 1979, and 4-6 March 1980 (A EW and 0 NS) and 19-24 September 1979 (LAA EW and ?:$ NS). The mean values are indicated by a (EW) and 0 (NS).
for individual 2-day intervals but averages, making use of all the data available in a given period, are also shown. For the diurnal tide the zonal components are comparable in ma~itude and tend to scatter around the (1,O) point, although on average, the amplitudes at Adelaide are l-2 times larger than at Kyoto. For the meridional component the amplitudes at Adelaide are
a’ ,
i \
Fig. 9. As for Fig. 8 for period 1L-27 January 1980.
218
T. Aso and R. A. VINCENI
observations beginning on the day indicated and the averages correspond to all data taken in two periods centered on early to mid-January (10-19 January} and the other on late January to early February (21 January-7 February). For the diurnal tide the zonal points cluster near the antiphase position (1, 12) with a tendency for Kyoto to lag whereas the meridional results indicate larger amplitudes at Adelaide with the phases showing that Adelaide leads by a few hours on average. Both the zonal and meridional results are consistent with a strong antisymmetric component. The data for the semi-diurnal tide shows that on average the semi-diurnal tide was larger in magnitude at Adelaide than at Kyoto which is in contrast to equinox results. Also the points indicating the average results are located near the (1,6) position for the zonal component, which also indicates a high degree of asymmetry in the tidal behavior. Some care must be taken in interpreting the results shown in Figs 8 and 9 simply in terms of either the symmetric or antisymmetric properties of tides. Those interpretations only have meaning if the tidal oscillations at Kyoto and Adelaide are due to similar combinations of tidal modes. That this is not always the case has been indicated by the March 1979 data for the diurnal tide which indicates the prevalence of evanescent modes at Kyoto and the propagating (1,1) mode at Adelaide. In general, the semi-diurnal tides showed better agreement as far as the height structure was concerned and a significant degree of antisymmetric behavior is shown. It should be pointed out that these conclusions are not influenced by a consideration of the errors involved in our analysis. The averaged errors at 90 km for the Adelaide March data are about 1.7m s- ’ in amplitude and the phase errors are about 0.3 and 0.4 h for the diurnal and semi-diurnal components respectively. These are typical values for the other periods analyzed although the errors depend on the particular Groves model used and the data density. At Kyoto, the averages of the estimated errors are 4.0 and 1.6 m s- ’ for the zonal and meridional wind components and the phase errors are 1.1 and 0.9 h on average for the diurnal and semi-diurnal components. There are a number of possible reasons why the tidal structure may be more complex than is predicted by classical tidal theory and may include higher order and antis~metri~ modes. Amongst the factors recently considered are asymmetries or local effects in ozone and other forcings, including topography, asymmetries in the background temperature profile which can act as an atmospheric filter for some modes, and asymmetries in the background winds which give rise to higher order modes by mode coupling (e.g. TEI~L~A~M and COT, 1979 ; LINDZEN and HONG, 1974; WALTERSHEIDet al.,
1980). It is possible to simulate these effects in a computer and one such numerical simulation has been carried out for the semi-diurnal tide by Aso et al. (1981). They make use of background temperature and wind profiles taken from the CIRA (1972) model and proper models of the solar thermal forcing due to insolation absorption by ozone and water vapor. At this stage only the semi-diurnal tide has been modelled because the vertical wavelengths of the modes of interest are of the order of tens of kilometers or larger and consequently a moderate height resolution may be used. For the diurnal tide on the other hand, the propagating modes have short wavelengths and consequently a high resolution and a large computer storage is required. Figure 10 illustrates a latitudinal plot of the observed
DIURNAL
90-N
00
90°N
90-s
OY
90-s
SEMI-OIURNAL -r
MERIDIONAL
LO
ZONAL
LO
t
9OoN
0"
90'S
9O=N
00
90%
Fig. 10. Latjtudinal plot of average tidal amplitudes observed at 90 km at Kyoto (K), Townsville (T) and Adelaide (A) for 14-29 March (0) and September/October (A) 1979. The theoreticai curves are from Aso ef al., 1981 for cases of with (-) and without (----) equino~tia1 background winds included.
Some direct comparisons of mesospheric winds observed at Kyoto and Adelaide and phases of the diurnal and semi-diurnal tides at 90 km in the March and September/O~tober equinoxes and, for the semi-diurna1 tide, the numerically simulated parameters by the ASO et al. (1981) model are also shown for the cases with and without background wind. For both sets of observations, the diurnal components indicate a large asymmetry between the hemispheres, in that the amplitudes, especially in the meridional components, are much larger in the southern hemisphere. However, the phases indicate a moderate degree of symmetry, especially in the September/October data. The phase tilt between Townsville and Adelaide could be reproduced theoretically by the appropriate inclusion of higher order modes, not necessarily antisymmetric modes. For the semi-diurnal tide, the zonal amplitude in September at Kyoto seems large in comparison with theory, whereas the other amphtudes at Kyoto and at Townsville are in quite good agreement, especially with the simulation results including the effects of the background wind. The amplitudes at Adelaide seem small in all cases. As far as the phases are concerned, there is good agreement with theory at Kyoto in most cases but this agreement is less evident for the southern hemisphere stations, although the simulation with the winds included does show the same general trend in phase between Townsv~le and Adelaide. These comparisons do show that the presence of higher order modes and/or antisymmetric modes are required in the theoretical models to simulate the observed latitudinal variations in amplitude and phase and that further work along the lines indicated must be done to improve the agreement between theory and observations. amplitudes
Aso T., NONOYA~AT. and Kent S. Aso T., TSUDAT. and KATZ S. Aso T., TSUDAT., TAKASHIMA Y ., ITO R. and KATZ S. BRICCSB. H., ELFORDW. G., FELGATED. G., GOLLEYM. G., ROSSITER D. E. and SMITHJ. W. CHAPMANS. and LINDZENR. S. CIRA
CONCLUSIONS
The atmospheric tidal structures observed at meteor heights at the geographically conjugate locations of Kyoto and Adelaide, have been compared with a short term ( - 2 day) and on an average ( - 2 week) basis. The study reveals an asymmetry between the tidal parameters even at the equinoxes when the asymmetries in the forcing and due to the background winds are a minimum ; the diurnal tidal amplitudes at Adelaide are appreciably larger than those at Kyoto, but the semi-diurnal amplitudes are on the whole smaller. At the January solstice an even more well defined antisymmetry is found which to some extent is due to asymmetries in the forcing and the background winds. However, it is possible that still more compli~ted agencies, such as nonlinear interactions between tides and the mean ffow, will need to be invoked to explain the differences between the observations and theory. The dissipation of the diurnal and semi-diurnal tidal energies can make significant contributions to the energy budgets of the mesosphere and thermosphere and consequently further comparisons are required to establish whether the differences reported here are of regular occurrence or not. ~cknow~e~ge~nts-This work wascarried out white one of us (TA) was a Visiting Research Fellow at the University of Adelaide. The help given hi by Dr W. G. ELFORD,Chairman of the Physics Department, to facilitate his grant in aid from the University of Adelaide, is gratefully acknowledged as are the comments of and useful discussions with our colleagues at Kyoto and Adelaide, Professor S. KATZand Dr B. H BRIGGS. The provision of the site for the Townsville experiment by the Ionospheric Prediction Service (Australian Department of Science and the Environment) is also acknowledged. Part of this research was supported by the Australian Research Grants Committee.
1981 1979 1980
J. geophys. Res. (in press). J. atmos. terr. Phys. 41,517. J. geophys. Res. 85,117.
1969
Nature 223, 1321.
1970 1972
Atmospheric Tides D. Reidel, Dordrecht, Holland. Cospar International Reference Atmosphere, Akademie,
1980
Nature 281, 3 19.
1981 1979 1959 1974 1978
J. atmos. terr. Phys. 43, 1051. Reu. geophys. Space Phys. 17,19X J. aunos. terr. Phys. 16,344. J. atmos. Ski. 31, 1421. .I. atmos. terr. Phys. 40,761.
Berlin. CRUZ R. L., VINCENTR. A., FRASERG. J. and SMITHM. J. CRAIGR. L. and ELFORDW. G.
FORBESJ. M. and GARRETTH. B. GROVESG. V. LINDZ~ R. S. and HONGS.-S. MULLER H. G. and NELKIN L.
219
280
T. Aso and R. A. VINCENT
ROPERR. G. and SALAHJ. E. TEITELBAUM H. and COT C. TSUDAT., Aso T., TAKASHIMA Y., ITO R. and KATOS. VINCENTR. A. and BALLS. M. WALTERSCHEIU R. L., DE VOKEJ. G. and VENKATFSWAREN S. V.
1978 1979 1980
J. atmos. terr. Phys. 40,879. J. atmos. terr. Phys. 41,33. J. atmos. terr. Phys. 42,461.
1981 1980
J. geophys. Res. (in press). J. atmos. Sci. 31.455.
1974
Proc. Int. Conf. on Structure, Composition and General Circulation of the Upper and Lower Atmospheres and Possible Anthropogenic Perturbation II, 624 Melbourne.
Reference is also made to the following unpublished material:
ELFORDW. G.