Atmospheric dynamics observed during the Energy Budget Campaign

Journalo~Atmospherrc and Printed m Great i3rits.m

Twrrsrrrol

Physxs,

Vol 41. No

0021 L)l69~85$3iXl+ 00 Pergamon Press Ltd

I 3. pp 233 -241, 1985.

Atmospheric dynamics observed during the Energy Budget Campaign J. L. FELLOUS* CNET/CRPE, Issy les Moulineaux, France G. CEVOLANI CNR-FISBAT, Bologna, Italy S. P. KINGSLEY and H. G. MULLER University of Sheffield, U.K. (Receioedfor

publication

29 August

1984)

Ahstract-UsingdataofseveralmeteorradarexperimentsduringtheEnergy BudgetCampaign,asynthesisof the main components of the upper mesospheric wind field is presented. Tides, long period waves and prevailing wind, as well as small scale fluctuations, are studied. The prominent feature of this comparison relates to the semidiurnal tide variability.

1. INTRODUCTION Winds and turbulence play major roles in the energy budget of the upper mesosphere. Continuous observations of neutral air motions are necessary, in addition to the wind profiles measured in situ by means of rockets, in order to study energetic processes. Hence, four meteor radar stations were operated during the EBC, at Monpazier, France (45”N, l”E), Bologna, Italy (45”N, 12”E), Sheffield, U.K. (53”N, 2”W), and Stornoway,U.K.(5S”N,6”W),givingsomeinformation on the time and latitude variability of the main tidal modes and planetary waves, as well as on the small scale wind structure. Results of these simultaneous observations are presented and compared with some wind data deduced from partial reflection drifts.

2. EQUIPMENT AND DATA PROCESSING

The equipment situated at Budrio. near Bologna, is a coherent pulse Doppler radar and essentially consists of a transmitter with a directive antenna, a receiving system with an interferometric antenna and a real-time echo processor. The radar transmitter operates at 42.7 MHz, with a peak power of about 200 kW, pulse duration of IO ps and repetition frequency of 140 Hz (VERNIANI et al., 1974). Calibration of the interferometric receiving system suggests that the uncertainties in echo height and radial velocity are approximately k 2.5 km and 3 m s- ’ for individual echoes. * Present affiliation : CNES, Paris, France.

The equipment located at Sheffield is also a phase coherent pulse radar whose transmitter operates at 36.3 MHz with a peak power of 200 kW. Pulses of 25 PCS length are transmitted at a rate of 300 Hz continuously and simultaneously from two matched antenna systems directed NW and SW, respectively. A smaller transmitter, identical to the drive unit at Sheffield. of 20 kW maximum peak power, is currently located at Stornoway. Individual meteor echo azimuths are obtained at Sheffield by using a spaced antenna interferometer system directed due W (angular ambiguities beyond _+30” off its axis resolved by an additional system consisting of three antennae directed SW, W and NW. respectively). The basic equipment parameters at Stornoway are identical with those used in the Sheffield radar. The same type of antenna is used for transmitting, but because of the lou-er transmitter power (13 kW) they are not energised simultaneously but operatealternately NW and SW at 30 min intervals. Separate antennae are used for receiving, but no provision is made for interferometric azimuth and altitude measurements. An important feature of the Bologna-Sheffield cooperative exercise is that both radar systems measure the same parameters and analyse echoes essentially in the same way, except that whereas et Sheffield the data are analysed on line, the data at Bologna are digitized on line but recorded on magnetic tape and processed off line later. The method used to process meteor wind data assumes periodic models and fits the models to the data in some minimum error sense. In order to ensure a reasonable fit, the raw wind samples for each meteor 233

J. L. FELL~US et a/

234

traii are used to produce a uniform timeseries. Averages of the wind are computed for every hour (or half-hour) from the values of the wind and assigned a weight. This series is frequency analysed to determine which components are appropriate to be included in the model. Amplitudes and phases of the different wind components are calculated by using a weighted leastsquares method and the frequencies of these waves by using the conventional spectral analysis (discrete Fourier transform). The same program, although referred to as a spectral analysis method, produces an amplitude periodogram. The resolution of the discrete Fourier transform is essentially equal to the reciprocal of the length of the total time sample. Thus, a fairly long time sample is needed to determine the shape of a given spectral component. In order to study in closer detail the amplitude and phase variation of the periodic components, an inverse Fourier transform is then calculated using a window

SHEFFIELD 1 21 Nov.-l Dec.198C

centred on significant periods of appropriate bandwidth (MULLER AND KINGSLEY, 1981; CEVOLANI and Dmr, 1981). Theinverse transform thus indicates how the amplitude varied throughout the recording period. The meteor radar located at Monpazier since 1976 was previously used at Garchy, France (47’N. 3”E), since 1965 and has been well described as rt was developed (SPIZZICHINO, 1971; FELLOUS et 01.. 1974; GtAsset al., 1978). It is a bistatic CW radar operating at 30 MHz with 3 kW transmitted power. In 1979-1980 the radar was run in a modified version including an additional transmitting station (FELLOUS et al.. 1981). This system was designed to obtain measurements of the wind at two different points on each meteor trail. This is achieved with the help of a real time computer on-line with the receiving station : the back transmitter operates continuously until a meteor echo is recognized and digitized, it is then switched off and the front transmitter is driven ; asubsequentecho is then received and measured from the same meteor trail. Stations are

5 BOLOGNA 4

Ia)

7Nov.-1

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Dec.1980

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24

12

8

6

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MONPAZIER V-27

November

1960

I

36 x 10” m2s’

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Fig. 1. (a) Amplitude spectrum of the zonal wind component at an average height of 95 km at Bologna for the 7 November-l December period. (b) As Fig. la at Sheffield for the 21 November-17 December period. (c) Power spectrum of the all height-averaged zonal wind at Monpazier for the 17-27 November period.

Atmospheric dynamics

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236

FELLOUS er al

located a few kilometers apart and the mean vertical separation between the reflection points is about 1 km. On average, 200-600 meteor trails per day can be used to obtain as many couples of wind measurements that fulfil the accuracy requirements (k 1 km on the average altitude, +0.3 km on the radial and vertical separations, f2 m s-r on the wind velocity). In addition, some 300 meteor trails per day give rise to only one wind measurement on a single point of the trail. The wind profiles are reconstructed on a regular space-time array (1 km x 1 h) and Fourier analysed or numerically filtered (GLASS et al., 1978) to study the regular motions (tides, long period waves, prevailing wind). Structure functions of the wind fluctuations are computed using the pairs of wind determinations.

3.

OBSERVATIONS

AND RESULTS

Continuous observations at Budrio extended from 7 November at 1200 UT to 1 December at 1100 UT. Twenty-one thousand usable data were collected, providing a 580 h time series of zonal wind values averaged over three height intervals (75590,9&100 and 100-l 15 km) and an all-height average (ascribed to a mean height of 95 km). The normalized amplitude

spectrum of the zonal wind at 95 km for the whole period is shown in Fig. la. The Sheffield continuous observations cover the period from 20 November at 2300 UT to 18 December at 1000 UT. Forty-five thousand usable echoes were recorded, giving a 13 18 point (659 h) time series of wind values averaged every l/2 h for each aerial direction and for each of the height intervals 75-90, 90-96 and 96 115 km, as well as an all-height average. The amphtude spectrum of the zonal wind for this all-height average is shown in Fig. lb. The Stornoway recording session was restricted to three night periods (6 Nov. at 1900 UT-7 Nov. at 1000 UT; 8 Nov. at 1800-9 Nov. at 0900; 10 Nov. at 1800-l 1 Nov. at 1300), due to severe ionospheric noise caused by the intense solar activity during the EBC consequently, day-time meteor rates were generally well below the seasonal means at all meteor radar stations). The Monpazier station was operated from 17 November at 1500 UT to 27 November at 1600 UT, but several long interruptions (power failures) affected the data on the first four days. Six thousand three hundred usable echoes were collected, including 1750 couples of wind determinations. The power spectra of the zonal wind show similar features over the whole height range.

Fig. 3. Maximum amplitude variation with time of the 12 h tide at Bologna (zonal), Sheffield (zonal and meridional), Monpazier (zonal) and Collm (zonal). (Meridional values at Collm are not represented for the sake of legibility.)

Atmospheric

dynamics

237

recorded at Bologna, which appear consistently lower An all-height average power spectrum is represented in than those recorded at Sheffield and Monpazier. Fig. lc. Since reasonable agreement was observed at other (a) The semidiurnal oscillation exceeds in amplitude times between Sheflield and Bologna (CEVOLANIet al., any other harmoniccomponent in all these spectra. The 1982), thiseffect is thought to be only partly due to data structure of this oscillation observed at Bologna in the 7 unreliability in connection with low day-time meteor November-l December 198Operiodisclearlyshownin rates. Wind measurements at Budrio in November Fig. 2a. The inverse Fourier transform (IFT) drawn for 1978 also showed a small semidiurnal amplitude, while a narrow band of frequencies about that of the peak in November 1976 it reached 30 m s”” ’ (CEVOLANIand indicates that this tide is modulated in amplitude DARDI, 1981). On the contrary, a remarkably stable throughout the observing period. Similarly, Figs 2b behavior is illustrated in Fig. 4a, which shows the and 2c show the time variation of the sem~d~~rnal vertical variationoftheamplitudeandphaseofthe t?h oscillation, respectively observed at Sheffield in the 21 component recorded at Monpazier in November I980 November-17 December period, as computed by the and in November 1979 during a similar campaign. IFT, and at Monpazier, as computed with a numerical However, local effects could play a major role in the bandpass (10-14 h) filter. Figure 3 summarizes the variations of maximum amplitudes of the semidiurnal diflerence of behaviour observed between stauons tide at Bologna, Sheffield and Monpazier. Their time located on either side of the Alps. Attention is deserved by a number of oscillations scales generally exceed 5 days, Also represented are the amplitudes of the semi-diurnal oscillation reported at evident in the Bologna and Sheffield spectra, while Collm, G.D.R. (52”N, 15”E), from 1X drift measureabsent in that of Monpazier, whose period is close to ments during the EBC campaign by SCH~~~ND~R et al. that of the semidiurnal tide. One of these has a period at (1982). the two stations of about 9.5 h and exhibits a The variability is possibly due to: (i) variations in modulatian similar to that of the semidiurnal tide, but time and longitude of the main tidal modes 5; and S$ of much greater depth at Shefheld (MULLER and over time scales greater than 5-6 days, related either to KIXGSLEY, t ‘38 1). change in the source (ozone and water vapor (b)Thedjurnal tiderepres~~tsasmall~arto~thetotal distribution} or of propagation conditions (BERNARD, windenergy in themeteorzoneat alistationsduring the 1981); (ii) coupling of energy from one mode into quoted periods. The amplitudes derived from the another or into the background flow (SPIZZKXINO, velocity spectra are very small. Two modes, whose i 970). periods are about 20and 24 h, areofsimilar importance A gross agreemeot is found far the semidiurnal tidal at Bologna and Sheffield and exhibit phases which phase at the average height, as can be seen from Table 1. disagree strongly at the two stations. It is possible that A salient feature is presented by the amplitudes different interfering modes of approximately equal

J. L. FELLOW

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Fig. 4. (a) Amplitude and phase variation vs height of the semi-diurnal tide at Monpazier for the 12-23 November 1979 and 17-27 November 1980 periods. (b) As Fig. 4a for the diurnal tide. (c) As Fig. 4a for the prevailing wind. (Confinued over.)

239

Atmospheric dynamics IC!

ZONFiL

PREVAILING

WIND

-a-

12-23/11/!97Y

1 _-+-.. __-*.___. --\c-

Fig. 4 continued.

period are simultaneously present during this transitionalperiod(FELtousetal., 1975).At Monpazier there is a small spectral peak for a period somewhat higher than 24h.The verticalvariationoftheamplitude shows a marked decrease between 80 km, where the amplitude exceeds 30 m s - ’ and 90 km, where it cancels, while the phase fluctuates wildly. Comparison with the diurnal tide in the November I979 campaign indicates asimilar trend(Fig.4b), but a25 km vertical wavelength was clearly identified, consistent with the S: mode. SP~ZZICHINO (1970) attributed the breakdown of the diurnal tide around 85 km to non-linear interactions with the semidiurnai tide. (c) The third component in importance is the prevailing wind. As is shown in Table 1 and Fig. 4c, the amplitudes of the mean zonal and meridional flows at all stations have a low, positive value. The vertical profiles in November t979 and 1980 at Monpazier appear strikingly similar and fit very well with empiricalmid-latitudemodelsor averages(~~~~~~~~, 1975; MASSEBEUF et al., 1979; MANWN et al., 1981). Thepresenceoflongperiodoscillationsin themeteor wind with periods in the 2-10 day time-interval can be noticed in the recordings of all stations. A 5-day wave with an amplitude of a few m s- ’ has been extracted in the spectra la and lb. This oscillation has been suggested to be an atmospheric free mode and precisely to corr,espond to the greatest symmetric low tiequency external normal mode of the atmosphere with wavenumber 1 (GEISLEXand DICKINSON,1976).

At Sheffield there is evidence also for an oscillation with a ?-day period, whereas at Bologna a wind fluctuation of period near 10 days has been identified. A IO-15 day oscillation has been previously observed in E-region ionosondemeasurements and was found to be correlated with the wavenumber 1 component of stratospheric radiance during autumn and winter (CAVALIERI, 1976). The shorter duration of the Monpazier observations precludes identification of the long period oscillations still apparent in the filtered wind values (Fig. 5). The quasi 2-day oscillation observed during summer campaigns (MIJLLERand KINGSLEY,1974; GLASSet ai.. 1975 ; CEVOLANIer al., 1981) is not conspicuous in the Bologna and Sheffield spectra as predicted by the seasonal variation of this wave (MASSEBEUF.1975; MULLERand NELSON,1978). A small amount ofenergy is present in the Monpazier spectrum around 2.S-day. (d) Short period (2-8 h) wind oscillations are of very little importance according to the spectra of Fig. 1.This isdue to theshort Iifetimeofthesecomponents. Gravity waves observed at meteor heights show up at no more than a few cycles. On the other hand, the Monpazier system yields information on the smallest scale wind fluctuations. The structure function D(r) represented in Fig. 6 is computed by averaging together the squared horizontal velocity differences corresponding to a particular spatial separation along a meteor trail D(r) = <(V(*-+-r)-- V(x))Z>.

J. L. FELLCW LONG

PERIOD

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OF

et al. JHE

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1980

95.0

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using a high-pass

filter &xt-off period

30 h)

STRUCTURE FUNCTION OF THE ZONRL WIND FLUCTUATIONS AT HONPRZIER - 17”27/11/1980

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1 Fig. 6. Structure

function

of the zonal wind fluctuations

at Monpazier

in November

1980.

241

Atmospheric dynamics For values of r extending from 0.3 to 4 km, D(r) increases from 70 to 450 m* s-*. This variation is approximately linear in a log-log scalediagram, so that a fit with a power law of the type D(r)

= A@

gives the values A = 179 and k = 0.55. Theoretical estimates relative to structure functions would give a 2/3 exponent for an inertial range of isotropic turbulence (BATCHELOR, 1953). A more detailed analysis ofsuchdata is beyond the scope of this paper and will be treated elsewhere.

4. CONCLUDING

REMARKS

The meteor radar observations carried out during the EBC provide a comprehensive picture of upper atmosphere dynamics at middle latitude for that

period. The main wind component at all stations is outstandingly the semidiurnal tide. This oscillation is strongly modulated in amplitude and an important result of this paper is in the evidence of the large horizontal extent of this phenomenon, as comes out from the data in Fig. 3. The variations of semidiurnal amplitude over S” in latitude and 15” in longitude appear reasonably well correlated as measured by different instruments. BERNARD (1981) has proposed a global description for that variability. We recognize in our data the variation of the main tide over scales of several days and shorter time scale variations from place to place, related to local effects. It would be desirable to complete the above description of combined wind data with a global model relating the energy input in the atmosphere and its distribution through dynamical processes. At least the present data are indicative in this respect of the importance of tides. long period waves and small-scale fluctuations.

REFERENCES BATCHELOR G.

K.

BEKNARD R. CAVALIERI D. CEVOLANI G., KINGSLF~ S. P. and MULLER H. G. FELLOUS J. L., SPIZZICHINO A., GLASS M. and MA~.~EBEUF M. FELLOUS J. L., BERNARD R., GLASS M., MASEBEUF M. and SPIZZICHINO A. GEISLER J. E. and DICKIN~QN R. E. GLANDM., FELLOUS J. L., MA~~EBEUF M., SPIZZICHINO A., LYSENKO I. A. and PORNIAFHIN Yu. 1. GLASS M., BERNARD R., FELLOUS J. L. and MA~~EBEUF M. MAN~~N A. H., MEEK C. E. and GREGORY J. B. MASSEBEUF M., BERNARD R., FELLOUS J. L. and GLASS M. MULLER H. G. and KINGSLEY S. P. MULLER H. G. and NELSON L. SPIZZICHINO A. SpI%ZlcHINO A. SC.HMINIXR R. and KUKSCHNER D. VERNIANI F., SCHAFFNER M., SINIGAC;LIAG., BORTOLOTTI C., DARDI A., FORMIGCWI C.. FRANCHESCW C., GOTTAKW S. and THIVEL.LONEG.

Rejfcvence is also made to the followmy

1981 1976 1983 1974

The Theory Umversity J. atmos. [err. J. atmos. terr. J. atmos. ten. J. otmos. terr.

1975

J. nrmos. terr

1976 197s

J. utmos. SCI. 33, 632. J. atmos. terr Ph> .s. 37, 1077.

1978

./. urmos. frrr.

1981 I979

J y~opI~j.\. Rcs. 86, 9615. J dtmoc. twr Ph\,r 41, 647

1953

unpublishrd

Turbulence.

Cambridge

Pi:j,s. 37, 15I 1

PIIVS. 40, 923.

1974 1978 1970 1971 1982 1974

material

CEVOLANI G. and DAKIX A.

19x1

C~VOLANI G.. KINGSLEY S. P. and MLNI.L~KH G.

1981 19x1 1975

FKLOUS J. L. and FREZAL M. E. MASSEBEUFM. MUI.LEK H. G. and KINGSLEY S. P.

11j 1fomogetleou.s Prebs. Phj,,s. 43, 663. Ph\‘s. 38, 965. Phj,.s. 45, 275. PII!,s. 36, 386

1981

BMFT-FB-W81-052. Energy

Budget Campaign Experiment Summary. Bundesministerium Forschung und Technologie. IAGA Bulletin no. 45, p. 312. MAP Handhook. Vol 2, p. 323. These, Paris. BMFT-FB-W81-052, Energy Budget Campaign Experiment Summary. Bundesministerium Fnrschung und Tzchnologie.

1980 fiir

1980 fiir