Investigation of the upper mesospheric dynamics under late polar summer conditions by EISCAT and lidar

Journal of Atmospheric and Terrestrial Physics, Vol. 58, NO. 1~,, pp. 317-335, 1996 Elsevier Science Ltd Printed in Great Britain 0021-9169/96 $15.00+0.00

~ ) Pergamon

0021-9169(95)00039-9

Investigation of the upper mesospheric dynamics under late polar summer conditions by EISCAT and lidar Georg Hansen ~* and Ulf-Peter Hoppe 2. tThe Auroral Observatory, Tromso, Norway; 2Norwegian Defence Research Establishment, Kjeller, Norway (Received 29 November 1994; accepted 12 December 1994) Abstract--In August 1989, a strong polar cap absorption event allowed extended measurements of both horizontal and vertical wind components in the upper mesosphere and lower thermosphere by means of the EISCAT UHF radar. A distance (130 km) away from the radar site, a Na lidar instrument simultaneously measured temperature profiles over several hours in the altitude range of 85-100 km. This combination of measurements gives detailed information about the behaviour of the polar mesopause region in the transition period from summer to autumn conditions. Below 87 kin, the wind measurements reveal typical summer profiles of the mean horizontal components and a significantlong-period wave which we interpret as the semi-di.urnaltide. Between 87 and 90 km, we find a strong wind shear; there, we observe a strong damping of the tide. Higher up, the mean horizontal wind is nearly zero, which represents autumn rather than summer conditions. In particular the vertical wind, but also the horizontal components reveal shortperiod oscillations with a small or vanishing phase progression, mainly in the 75-85 km altitude region. Most probably these are gravity waves with very long vertical wavelengths as predicted by theory to be prominent in the vertical wind. The temperature measurementsconfirm the typical polar summer conditions with a marked temperature minimum of 120-130 K at around 87 km and partially very large positive temperature gradients higher up, especially before local midnight. The measurements suggest that this peculiar thermal and dynamical structure in late summer acts as a major barrier for tidal waves and shortperiod waves leading to strong turbulence slightly above the mesopause temperature minimum.

1. INTRODUCTION The mesopause regjion is one of the least understood parts of the Earth's atmosphere. This holds in particular for polar latitudes where a number of peculiar features such as polar mesosphere summer echoes (PMSEs), noctilucent clouds (NLCs), sporadic metal layers and double temperature minima are still waiting for better explanation. On the other hand, the interest in this region has increased considerably in recent years, as features like NLC might be very sensitive indicators for changes of the climate system (e.g. Thomas et al., 1989'). Significant progress has been made in the investigation of polar mesopause dynamics in the last two decades by means of long-term radar measurements (e.g. Carter and Balsley, 1982; Raster et al., 1988; Manson et al., 1991). The recording of polar mesosphere summer echoes (PMSEs) has provided a number of vertical wind measurements (e.g. Fritts et al.,

*Now at the Norwegian Institute for Air Research, Tromso, Norway.

1990) with so far unknown accuracy. In addition, a considerable number of temperature measurements has been gathered since the mid-eighties by means of the falling sphere technique (von Zahn and Meyer, 1989) and Na lidar (Neuber et al., 1988; Kurzawa and von Zahn, 1990; Hansen and von Zahn, 1994). A summary of various temperature measurements from the mesopause over northern Scandinavia is given by Ltibken and von Zahn (1991). The lidar measurements in particular have revealed that the transition period from polar winter to summer conditions in April/May, and back to autumn/winter conditions in August and September can establish very special conditions in the mesopause region, which deserve a closer investigation. Already in the late sixties, Theon and Smith found some evidence that the transition from the summer to the autumn regime propagates downward through the mesosphere in September (Theon and Smith, 1970). Does this process also take place in the lower thermosphere and at the mesopause, but some weeks earlier? What effect does this have on the behaviour of tidal and gravity waves which deposit a significant amount

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of the total energy and momentum in this part of the atmosphere? Do we find equivalent features in the vertical wind and in the thermal structure? Due to an extraordinary geophysical constellation in the period of 12-15 August 1989, we were able to take a closer look at these questions. A strong polar cap absorption (PCA) event provided very high electron densities even below 70 km altitude, allowing the recording of the complete wind field from 67 to 93 km altitude over a total of 22.5 h. In the nights of 12-13 and 14-15 August, temperature profiles from the 8598 km region were also gathered by means of a lidar instrument which was placed 130 km west-southwest of the radar.

of 80-100 km) and the neutral temperature. The temperature can be determined because of the direct dependence of the Na D2 line shape on this parameter (Fricke and von Zahn, 1985). However, in summer this technique is inhibited by three factors: (1) a high background intensity (midnight sun), (2) a narrow Na layer (85-95 km) and (3) a very low Na column density. For these reasons temperature measurements were only possible from the beginning of August, and then just for a few hours around midnight. Further information about the lidar experiment in general is given by Fricke and von Zahn (1985) and Neuber et al. (1988); temperature measurements under polar summer conditions are discussed in detail by Kurzawa and yon Zahn (1990).

2. DESCRIPTION OF THE EXPERIMENTS

The EISCAT U H F system, which is operated at a frequency of 933 MHz, consists of a transmitter and a receiver located at Ramfjordmoen near Tromso, Norway, and two additional receivers in Kiruna, Sweden and Sodankyl~, Finland. A detailed description of the system is given by Folkestad et al. (1983). The data presented in this paper were gathered in a monostatic mode with the system at Ramfjordmoen by means of the GEN- 11 experiment. This modulation pattern is especially designed for D-region studies and uses 13-bit Barker-coded double-pulses with a length of 91/~s each, with a time delay between the two pulses varying between 7 and 35/~s. One transmission cycle contains 70 double-pulses at a time interval of 2.222 ms. The 22 lags used in the autocorrelation function are calculated by pulse-to-pulse correlation due to the large correlation time in this altitude region. There are 42 range gates of 1.05 kin, covering the ranges from 70- 113 km. With the U H F system, usable spectra are obtained only up to an altitude of about 93 km. The special advantage of the Gen- 11 modulation is that background noise, 50-Hz hum, and F-region clutter are removed from the signal automatically. More comprehensive information about GEN-11 is given by Turunen (1986) and Pollari et al. (1989). In order to get information about the complete wind field, the radar beam was moved between three positions: vertical, to the west and to the south, the last two with an elevation of 72.5 ° . One sequence consists of four measurement positions: vertical, to the south, vertical, to the west, each measurement lasting 5 min. West-southwest (130 km) of the EISCAT site, a Na lidar experiment was operated by the University of Bonn, recording both the Na density in the atmospheric Na layer (which is located in the altitude range

3. OBSERVATIONS

3.1. Winds

The measurements presented in this paper were made during the nights of 12-13 August (8 h), 13-14 August (8.5 h) and 14-15 August (6 h), in 1989. For most of the data the signal-to-noise ratio was so high that the 5-min intervals in the different measurement positions could be split up, yielding two values every 10 min in the case of the vertical component, and two values every 20 min for the horizontal component. This was not meaningful, however, between 67 and about 74 km around local midnight when the electron density dropped significantly due to the formation of negative ions. This effect causing reduced data quality is increased as negative ions lead to larger spectral widths and thus steeper autocorrelation functions, which in turn leads to larger errors in the fit procedure. The large spectral width becomes a general problem above, say, 86 km, and causes a significant increase of error bars there. Also, in this altitude region, a data integration time of 5 min is more appropriate. Surprisingly, the highest data quality is found on 13-14 August when the ionization rate was rather moderate compared, for example, to 12-13 August. However, on this day the ionization was also very homogeneous, while the other two days were characterized by drastic changes both in altitude and with time. Figure 1a and b shows two single altitude profiles (2.5 min integration time) with typical error bars from 12-13 and 13-14 August: they vary between less than + 0.1 m/s at around 75 km and several m/s above 86 km. Figure 2 gives a time series of vertical wind data from 12-13 August at an altitude of 79.57 km; a marked reduction of the error bars occurs after 01:00 UT due to the onset of the main PCA.

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Two examples of horizontal wind fields are given in Fig. 3a and b (meridional wind on 13-14 August; zonal wind on 12-113August). In both cases, the most significant feature is. a strong variation between 77 + 2 and 87 + 2 km with a time distance from maximum to minimum of 6 + 1 It. Above ~ 86 km this large-scale variation almost disappears within a few range gates. Furthermore, periodic structures in both time and altitude with periods mainly less than 1 h and typical vertical scales of ~ 7 km are present. They are characterized by a very small or even vanishing phase progression. Figure 4a and b shows the vertical wind fields on 13-14 August (2.5 min resolution) and 14-15 August (5 min resolution). In both cases, below 85+2 km, the vertical wind field is dominated by quite regular periodic variations with an almost vanishing vertical phase progression, which at times extend below the lower edge of the altitude region investigated. Oscillation periods vary between ~ 20 and -~90 min. The velocity variations on this time scale can be dramatic. The example shown in Fig. 5 reveals a change of the vertical wind by as much as 10 m s - ' within 7 min. From Fig. 4, one can see that the oscillations depend

on the time of day both in intensity and in vertical extent; in all three measurement runs they are comparatively weak before 21:30 UT, intensifying considerably afterwards and spreading from higher to lower altitudes. Between 85 + 2 and 89 km the vertical wind field is much more irregular than below, with the amplitude of the wind varying strongly during a single measurement run and from day to day. On 12-13 and 14-t5 August, the velocity amplitudes in this region are much larger and more variable with time than on 1314 August. Above 89 km the pattern of the vertical wind again tends to become more regular, but with longer periods than below 85 km. The strong variations above 85 km may be partially due to the decreasing data quality, as the error bars in Fig. 1 show. The decrease of variability above 89 km, however, indicates that there is also a real physical effect. 3.2.

Temperatures

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solar depression angles) temperature measurements were limited to local midnight + 2 h, i.e. about 21:00 UT to 1:00 UT. Besides the background intensity, the atmospheric sodium density determines the quality of the temperature data. In case of very low Na densities, as for example on 12-13 August, only half-hour mean profiles yield reliable data. On 14-15 August, a time resolution of 10 min was achieved. As an example, the temperature profiles measured by the lidar on 12-13 August are shown in Fig. 6a. The variations due to atmospheric wave motions are better visible if one subtracts the monthly mean profile; the result is shown in Fig. 6b. The mean profile was taken from Ltibken et aL (1991), by taking the average of the values from July ( + CIRA 86 above 92 km) and August; this is most appropriate for the first half of August, as their Fig. 13a shows. The profiles in Fig. 6b reveal two maxima moving downward with a speed of approximately 2 km/h. A similar behaviour is observed on 14-15 August. The relative variation of the temperature on 14-15 August amounts to 14% exceeding that on 12-13 August by a factor of about 1.5. In both cases the dominant

vertical wavelength is in the order of l0 km. The period of the dominant wave is estimated to about 6 h on 12-13 August, and somewhat shorter on 14-15 August. On 14-15 August we find a persistent temperature minimum at 87 km, while on 12-13 August this minimum is only seen until 0:00 UT at 86 km. The mean temperature profiles are shown in Fig. 7. The 14-15 August profile seems to contain a considerable wave residual, which might be due to the shortness of the measurement window. The minimum mean temperature value of about 135 K at 86 and 87 km, respectively, on both occasions is remarkably low for this time of the year; it is rather typical of polar summer conditions usually found in June and July. On the other hand, both profiles agree with reference atmosphere August temperatures as given in the CIRA 86 (Fleming et al., 1988) above 93 km. 4. DECOMPOSITION OF THE WIND FIELD

4.1. Introduction

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ponents present in our data, the decomposition of the wind into mean components, long-period and shortperiod variations is desirable. Already from the total wind field it is obvious that the horizontal components contain a variation whose period is of the order of or longer than the duration of the single measurement runs. It becomes even clearer when one averages over several range gates; the result for the ranges 73--78 km and 79-87 km is shown in Fig. 8. Of course, these profiles suffer a smearing effect if we are dealing with gravity waves of short vertical wavelength, but in the case of long-period waves it can be assumed that the vertical wavelength is significantly larger than 7 km. Figure 8 reveals characteristic tidal wave features: - - O n all three days both horizontal components indicate a period of about 12 h; in addition, the phase is almost the same for all measurement runs. - - T h e amplitude of the long-period variation increases significantly from the lower to the higher altitude range. - - A given phase of the variation is first seen in the upper and then in the lower range, i.e. it moves downward with time.

As the single measuring windows are shorter than the assumed period of this wave, the covered part of the tidal wave does not only contribute to the spectral component of lowest frequency in the spectrum (which is higher than the true tide frequency), but also causes a DC component which has to be removed if one wants to determine the true background wind. The size of this DC component depends on both the amplitude and the covered phase interval of the tide. In order to find the most reliable values for these parameters we tried the following approach: we calculated running mean (time) data sequences, averaged over three altitude gates (in order to reduce smallscale variations), similar to those in Fig. 8. From these profiles, both amplitude and phase were determined graphically. The result was then compared with the smallest-frequency component of the Fourier transform of the respective time series. It turned out that the graphical method was more reliable with respect to the phase, while the Fourier transform provided the best amplitudes. With the phase and the amplitude information plus the knowledge of the "time window" of the measurement, correction factors of the mean horizontal components were calculated. (The procedure was not

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applied to the vertical component, because here the graphical method does not work; the short-period oscillations are much too strong.) Then these firstorder-corrected mean winds and the tidal component were subtracted from the measured wind, and the

residual wind field was Fourier-analyzed, in order to find weak points in the first approach. If necessary, the phase, amplitude and mean wind correction were modified. By this primitive iteration, the D C component and the lowest-frequency component of the

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runs on 12-13 and 13-14 August could be removed almost completely. This gives, as we think, strong evidence that we isolated these components fairly well. There were, however, problems with the third run in the region of the strongest variations around 85 km. This is probably caused by the short measuring window which makes the corrections very sensitive to the tidal phase interval covered, and by interference from strong residual oscillations with periods between 3 and 6 h on this day. 4.2. Mean winds In order to get mean wind profiles we first calculated nightly mean profi]es after having applied a data quality filter. The latter filtered out values with very large error bars ( > _ 10 m s-~), and values calculated from low electron densities in combination with large spectral widths. The number of values included in the nightly mean profiles was then used as a weight for calculating the total mean profiles of the whole data set. In a second step we corrected for the effect due to the longest-period variation as described above. We estimate the error margin of the resulting profiles to be less than about + 5 m s -l in the worst case of 14-

15 August, and at most -t-3 m s -l in the total mean profiles. Despite this correction one should regard these mean wind profiles only as first-order or "night" mean profiles as we did not account for other longperiod waves as, for example, the diurnal tide which are not negligible at these latitudes. Figure 9a-c shows the night mean meridional, zonal and vertical wind profiles for the three measurement runs, including the semi-diurnal-tide correction, and the weighted total mean profiles. The average meridional profiles of the single runs are quite similar below 81 km, both with respect to their absolute values and their gradients. Higher up, the profiles of 1213 and 14-15 August agree fairly well, showing a minimum of about - 3 5 m s -1 at 83 km and a large positive gradient higher up. In addition, the 14-15 August measurement contains a wave component which might be due to the shorter measurement window on this day. The profile of 13-14 August reveals a wave pattern above 81 km, which seems to be in antiphase with the other two runs. The total average zonal wind profile reveals strongly negative velocities between 67 and 84 km ( - 3 0 to - 4 0 m s -1) followed by a large positive shear higher up. Again the profiles of 12-13 and 14-15 August

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agree fairly well above 80 km, while the profile of 1314 August gives the impression of containing a wave being in antiphase with the other two runs. Below 80 km, the 12-13 and 13-14 August profiles are similar, while the profile from 14-15 August deviates significantly• It should be noticed that the transient change of the zonal wind on the latter day is also found at almost the same altitude (75-76 km) in the mean vertical wind. The mean vertical velocities are close to zero, but with apparently significant deviations from zero. Between 85 and 88 kin, the vertical velocity is negative. Above 88 km, there is an approximately 3 km wide region with slightly stronger upward motion. As this pattern is seen in each of the three nights examined, we regard it as a real effect despite rather large error bars. Allowing vertical shifts of this structure of + 1 km from one day to another the deviations from zero of the different days are almost in phase. The 76--85 km altitude region is characterized by a decreasing upward wind on 12-13 and 14-15 August, while on 13-14 August the wind increases from slightly negative to small positive values• Another interesting fea-

ture in this altitude region and below is the node structure: at 85 km, 81 km, 77 km and 73 km the mean velocities of all measurement runs are very similar, decreasing from about 10.3 m s -~ at 73 km to almost 0 at 85 km, while in between the velocities vary considerably. This holds also if one calculates mean profiles from halves of the respective measurement series, i.e. for example from the periods 20:00 to 00:00 UT and 00:00 to 04:00 UT on 12-13 August. In other words, the velocities at these altitudes are not only very similar on subsequent days, but also at shorter time scales of only a few hours. This structure very much looks like a standing wave with 2/2 = 4 km. In addition, the fact that the amplitude of this "wave" is largest on 14-15 August and smallest on 13-14 August, implies that the period is larger than 9 h. Below 76 km, all three single run mean vertical winds become positive• 4.3. The lon#-period component The best-fit phases and amplitudes of the long-period variation in the horizontal wind components are shown in Fig. 10a. In both components we find a

Investigation of the upper mesospheric dynamics

329

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4-0

330

G. Hansen and U.-P. Hoppe

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similar general structure of the altitude dependence of the amplitude: emerging from noise above 70 km, reaching a sharp maximum of 3 0 ~ 0 m s l at 82_+3 km, and decreasing at least as rapidly above. Also, the phase has a common general structure: below 75 km it is ascending with time or constant, followed by an approximately 5-km-wide region of descending phase with time, then a 7-km-wide region of an ascending phase, and above 87 km again a descending phase. Such a behaviour of the phase can be due to the fact that the direction of phase propagation of an atmospheric wave observed in the radar's frame of reference is not necessarily the same as the intrinsic direction of propagation: if a wave is propagating into a strong background wind with Cph--U< 0, it is Doppler shifted to an apparent phase progression in the opposite direction. However, the observed effect seems to be much larger than one can expect. For instance between 75 km and 80 km u ~ - 3 5 m s -~. If we are indeed observing a tide, its zonal phase speed must be Cph,, ~ 2~R cos q~/24 h 164 m s-~, (R: Earth's radius, q~= 70 ° latitude) and it is not reasonable to assume that Doppler shifting by the zonal wind inverts the observed propagation direction with respect to the intrinsic direction. Although

the meridional phase speed of a tide should be smaller, to a first approximation maximally Cph,~~ _+164 m s -l "sin23 ° g _+64 m s -1 at the solstices (due to the angle of the terminator with a meridian), it is still larger than our largest observed meridional mean wind in this season. Although the general structure of the amplitude and phase functions is always recognizable, both parameters show significant variations from day to day. An interesting detail is the fact that the day-to-day variations of the amplitude are most significant in the zonal component, while the phase functions are very similar there. On the other hand, the phase function varies considerably in the meridional component, but in this case the amplitudes agree very well. It should be noted that also detail structures, e.g. the two bumps in the meridional amplitude function of 13-14 August at 84 and 88 km had to be included to get a satisfactory removal of the DC- and the tidal component of the spectrum. We, therefore, regard them as real features, in accordance with structures of the same vertical scale in the mean wind. We also Fourier-analyzed the vertical wind, and found a weak but detectable long-period component. One example--from 12-13 August--is shown in Fig.

Investigation of the upper mesospheric dynamics 10b. It is in a way complementary to the horizontal tide: we find the largest amplitude at 87___3 km, where the horizontal tide :is already strongly damped, and in the regions where the horizontal phase function descends (below 80 and above 86 km) the vertical phase function rather ascends. 4.4. The residual wir,!dfield As described above, the residual wind field was determined by subtracting the best-fit mean wind and semi-diurnal tide as shown in Figs 9 and 10 from the measured wind field. In Fig. l la and b the residual wind field for the horizontal components on 13-14 August is shown. The most prominent feature of the meridional compor.ent is a variation of more than +30 m s l in the altitude range of 81-86 km. In the Fourier spectrum this feature appears as a strong component with a period of about 4 h and some weaker contributions with periods of about 60, 70 and 90 min. Components with the latter two periods are also found at other altitudes. Also the zonal component has the most prominent features in the 80-to-85-km range. In the Fourier spectrum again the 4-h component is strongest, but while this component is strong at 82+ 1 km in the meridional wind, it is x~eakest at this altitude in the zonal component and much stronger at higher and lower altitudes. At 82 km we find significant contributions with periods of 2 h 45 min and 1 h 25 min. As in the meridional component, there is in addition a significant component with a period of about 1 h in the altitude rage 80-85 km. Besides these very prominent features there are a number of weaker vertical structures especially below 80 km in both components which resemble the structure of the vertical wind. These may partially be caused by the fact that the "horizontal" measurements were not really horizontal, but were taken at a zenith distance of 17.5°, i.e. they contain a non-negligible vertical wind component. One might have tried to remove this contribution by subtracting interpolated values of the true vertical wind measurements, but on the other hand, this method is doubtful, as the measurement volumes are about 30 km apart and the vertical wind strucl:ures are of very small scale. We are confident that most of the structures seen in the residual horizontal wind are not caused in that way, as there is no clear correlation between the structures in the horizo~atal and in the vertical wind (positive vertical winds should appear as negative winds in the horizontal components as we measured towards west and south). The vertical component is not shown here because

331

it very much looks like the measured wind field due to the comparably small long-period component.

5. DISCUSSION As mentioned in the Introduction, many observations from the polar summer mesopause region have been published in the last two decades. These include wind data from Poker Flat at 65 ° N (e.g. Carter and Balsley, 1982), from Andoya at 69°N (ROster et al., 1988; Fritts et aL, 1990) and from Troms~ at 69° N (Manson and Meek, 1991; Manson et al., 1992; Fritts et al., 1990), as well as temperature data from Pt Barrow at 70° N (Theon et al., 1967) and from Andoya (Kurzawa and yon Zahn, 1990). Figure 12a-c shows some of the published mean wind profiles, together with our profiles (mean profiles of the whole measurement series, 12-15 August). The data from Poker Flat which are means of the period of 17 J u n ~ 1 3 July 1979 and 22 June-22 July 1980, respectively, as well as the SOUSY VHF data from Andoya, represent typical polar summer conditions: negative zonal and meridional winds up to altitudes of more than 90 km with larger wind speeds in the zonal component. Manson and Meek (1991) give monthly mean profiles of the background wind as well as of various tidal modes as measured with an MF radar close to the EISCAT site. Their mean August profiles are very different from the Poker Flat and the Andoya data, showing a meridional wind which is almost zero, and a zonal wind which is shifted by about 15 m s - ' towards positive values. This is most probably a consequence of the transition from summer to autumn conditions which usually takes place in the second half of August. Our data take an intermediate position. Above, say, 88 km, both horizontal components are close to the Tromso profile of Manson and Meek. Below 88 km, our data represent rather typical summer conditions, as seen in the Poker Flat and Andoya data. This feature can not be explained by the aliasing of our mean profiles due to the long-period components which we could not remove (e.g. the diurnal tide); both in Carter and Balsley, and in Manson and Meek, these components do not change so dramatically at this altitude. The profiles rather give the impression as if at the time of our measurements the polar summer regime was shifted downwards by about 5 + 2 km compared to June/July conditions. The mean vertical wind is much more structured than the other two profiles published by Fritts et al. (1990). Also, the EISCAT mean from 1987 shows increasing fluctuations around 86 km, but with a

G. Hansen and U.-P. Hoppe

332

reversed phase and with much smaller amplitudes. This might, on one hand, be due to the much larger data base, as the presence of PMSE allows a time resolution of 10 s. On the other hand, all these measurements were made between 09 and 13 UT, i.e. with a shift in the time of day of about 12 h. As described above, our single run mean profiles reveal standing-wave-like features, whose period then would have to exceed 9 h. If this is a real effect, it might account for the phase difference between the two EISCAT measurement sequences shown in Fig. 12c. Concerning the SOUSY radar profile the reader is referred to Fritts et al. (1990). The average temperature profile of the period 9-15 August agrees very well with the average profile of the first half of August as given by Kurzawa and yon Zahn (1990) which reveals significantly lower temperature values in the 80-90-km region than standard atmosphere profiles for August. Already in the previous section we listed some arguments why we think that the long-period variation is most probably the semi-diurnal tide. This assumption is further supported by other measurements from the same geographical location which have been published in recent years. Manson and Meek (1991) give

(a)

the semi-diurnal tide as measured by their MF radar at Tromso. At the beginning of August, the meridional amplitude has a weak maximum of about 15 m s -~ at 90 km and a phase which changes from 5 h at 80 km to 6 h at 85 km and then slowly back to earlier hours with increasing altitude. At the same time, the zonal component shows a maximum of about 27 m s ' at 90 km and a phase which changes from 9 h at 80 km to 11 h at 86 km and then back to 9 h higher up. So their phase values agree very well with ours while the amplitudes have a similar variation with altitude, but are significantlyweaker and found at somewhat higher altitudes. This can, however, be due to the larger statistical sample in the Manson and Meek (1991) data. In fact, data from the same data set, but from shorter time periods in June, show amplitudes (20 m s ] for a 10-day run, and about 30 m s 1 for a 3-day run) comparable to our data (Manson et al., 1992; their Fig. 2). The difference in the maximum altitude between our data and the June data of Manson et al. and other published data is probably due to the structure of the background wind which is shifted downwards by about 8 km in the meridional component and, say, 4 km in the zonal component when comparing our

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Fig. 12. Various measured profiles of (a) the mean meridional, (b) the mean zonal wind, and (c) the mean vertical wind at high latitudes in summer. The EISCAT profiles are identical with those in Fig. 10.

(Continued opposite.)

Investigation of the upper mesospheric dynamics (b)

333

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Fig. 12--continued.

. . . . . . . . .

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334

G. Hansen and U.-P. Hoppe

measurements with others from June/July. This downward movement of the tide maximum with the season progressing is, by the way, clearly seen in Fig. 4 of Manson and Meek (1991). From our combined wind and temperature data we conclude that the damping of the tide occurs in a region where we find both a large positive temperature gradient and a large mean wind shear. These conditions provide an abrupt change of the Brunt-Vaisala frequency ("refractive index for atmospheric waves") leading to dissipation, but also refraction and possibly reflection of the waves. The most striking features in our data set are the short-period oscillations. It might be argued that these structures are artifacts as their half-periods coincide with the time resolution of the measurements. There are, however, two strong arguments in favour of a real geophysical effect: (1) When splitting up a 5-min data sequence in five l-rain sequences at times of very high data quality, a series of consistent velocities is produced which very much supports the picture of short-period waves. (2) The vertical wind pattern as seen in our data is by no means unique. Already Fritts et al. (1990) report very similar oscillations which are coherent over an altitude range of about 6 kin, both with the EISCAT VHF radar at 224 MHz and the SOUSY radar at 50 MHz. Note that their time resolution is in the order of 10 s. Similar structures have also been observed in a number of other PMSE measurements (with a time resolution of 2 s) in recent years (G. Hansen, private communication). Fritts et al. (1990) have shown that most of the variance in the vertical velocity w is concentrated between periods of about 5 min and 30 min. The dispersion relation (Hines, 1960) together with the canonical gravity wave spectrum adopted by Fritts and VanZandt (1993) requires then that most of the w variance is concentrated at vertical wavelengths of 30 km < 2~< 60 km. Such wavelengths would appear infinitely long in our observations in a limited height range. What is unique with the 1989 measurements, is the fact that they show that these oscillations indeed can extend down to 70 km in altitude and maybe even lower. Hence, they support what is expected from theory, namely waves in the vertical wind with very large vertical wavelengths (Fritts and VanZandt, 1993). Some characteristics as strength and the vertical extension of the oscillations are related to background conditions and long-periodic variations as tides: --They are most prominent on the 13-14 August with its very special layered background horizontal wind.

--They are most pronounced in altitude regions where the phase of the long-period vertical wind component ascends. --Both the dominating period and the vertical extension change in the course of all measurement runs: after 0:00 UT these parameters seem to increase considerably. The data sets of 12-13 and 14-15 August suggest that the development and extension of the short-period oscillations also depend on the temperature gradient. In both cases the lidar measurements reveal a marked temperature minimum at about 86+ 1 km before 0:00 UT. This implies that we have a large negative temperature gradient some kilometers below this altitude, which might be close to the adiabatic gradient of about - 9 K km ]. After midnight the mesopause temperature increases and the thermal structure necessarily flattens. This coincides very well with the development of the vertical extension and the period of the oscillations. Consequently, the latter require stable background conditions. At the mesopause, the temperature gradient changes very sharply before 0:00 UT, leading to a similar change of the Brunt-Vaisala frequency. The consequence of this is a strong change of the vertical wavelength to smaller values and, thus, an increase of the gradients above ~ 85 km caused by the wave. This seems to be seen as the "chaotic" zone in the vertical wind field. On 12-13 August, this region is almost identical with the region of measured positive temperature gradients: as one can see in Fig. 6, the latter descends from about 88 km at 21 UT to about 84 km at 01 UT and then eventually fades away until 04 UT, so does the region with the "chaotic" vertical wind pattern. The sharp increase of the gradients necessarily leads to instabilities and the dissipation of the wave energy via turbulence slightly higher up. This, however, can only be seen in our data as a reduction of the velocity variation because of insufficient spatial resolution. A similar transition from one wave regime to another at the mesopause level has been reported by Hall et al. (1987). The question arises whether this structure of the vertical wind field is linked to the existence of polar mesospheric summer echoes (PMSE). Balsley et al. (1984) and Fritts et al. (1984) reported a close relationship between the occurrence of PMSE and the thermal structure of the mesopause region, with summer echoes tending to be strongest when temperatures are less than 170 K and gradients greater than +2.6 K km -j. These conditions are met in the altitude region where our vertical wind data suggest the dissipation of the oscillations by turbulence.

Investigation of the upper mesospheric dynamics Balsley et al. (1984) also report a variation of the signal-to-noise ratio over the day with a pronounced minimum in the early evening hours and a strong increase from about local midnight (23:00 U T in Tromso). This is in good agreement with the power in the "chaotic" zone of the vertical wind field. Finally, both radar and rocket experiments in recent years have given safe evidence of the presence of strong turbulence in the altitude range of 85-89 kin, while below 85 km features with very narrow, but rapidly changing Doppler spectra are more common. Both observations are in good agreement with the vertical wind pattern as reported above.

335

damping level. Such short-period gravity waves with very large vertical wavelengths are most prominent in the vertical wind in agreement with the dispersion relation and the observed saturation spectrum. Their characteristics are very much dependent on the local background conditions. It is highly desirable to make further wind measurements throughout the m o n t h of August and early September in order to learn how the transition to the winter regime really takes place in detail. Also, numerical simulations with realistic background conditions would be of great help. However, this would require substantially more complicated models than those used today, as they would have to contain a non-isothermal and dynamically varying background.

6. SUMMARY AND OUTLOOK Measurements of the dynamical and thermal structure of the polar mesopause region during the transition from the summer to the autumn regime reveal the mesopause as a region where the dynamical patterns mainly propagating from below are modified significantly. The combination of a major change of the temperature gradient and a large positive horizontal wind shear acts as a barrier for atmospheric waves, in particular tides, which leads to strong damping of the latter ones and possibly formation or reinforcement of short-period oscillations below the

Acknowledgements--We would like to thank the EISCAT

staff for their helpful assistance in acquiring the wind data. The EISCAT Scientific Association is funded by Norges Almenvitenskapelige Forskningsrad of Norway, the MaxPlanck-Gesellschaft of the Federal Republic of Germany, the Science and Engineering Research Council of the United Kingdom, Naturvetenskapliga Forskningsradet of Sweden, Suomen Akatemia of Finland, and Centre National de la Recherche Scientifique of France. We thank M. Serwazi of the University of Bonn for his collaboration in acquiring and evaluating the lidar data. G. Hansen was supported by a fellowship grant of the Royal Norwegian Council for Scientific and Industrial Research (NTNF), and by the Norwegian Defence Research Establishment (NDRE).

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Dynamics of the Middle Atmosphere, Holton J. R and

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1970 1989

J. atmos. Sci. 27, 173. Nature 338, 490.

1986 1989

J. atmos, terr. Phys. 48, 777. J. Geophys. Res. 94, 14, 647.

Matsuno T., Terra Scientific, Tokyo NASA Technical Memorandum 100697

Matsuno T., p. 97, Terra Scientific, Tokyo.