Ad’~’. Space Red. Vol.4, No.4, pp.3—18, 1984 0273—1177/85 $0.00 + .50Printed in Great Britain. All rights reserved. Copyright © COSPAR

WINDS AND WAVES IN THEMIDDLE ATMOSPHERE ASOBSERVED BY GROUND-BASEDRADARS

R. Ruster

Max-Planck-Jnstjtutfür Aeronomie,3411 Katlenburg.Lindau,F.R.G.

ABSTRACT

Ground—based radars have proved to be powerful instruments for studying dynamical processesin the middle atmosphere. They have been used successfully in the last few years duringPre—MAP and MAP projects. This paper briefly reviews the following ground—based radarmethods: the new MST radar technique for remote sensing of the mesosphere, stratosphere andtroposphere, and the well known techniques for mesospheric measurements such as the iono-spheric drift experiment, the meteor radar and the MF radar experiments. A survey of ob-servational results obtained with the various techniques is presented. Particular emphasisis directed to winds and waves as well as their interaction, all of which play an importantrole in the structure and dynamics of the middle atmosphere.

1. INTRODUCTION

The project MAP is a scientific program designed to improve our knowledge of the atmospherebetween about 15 and 100 km. The various scientific objectives of MAP include investigationsof the structure and composition of the middle atmosphere, studies of the interaction ofsolar radiation with the middle atmosphere and investigations of atmospheric motions at allscales. Various techniques are available for carrying out this kind of research. These tech-niques involve ground—based, balloon—, rocket— and satellite—borne experiments.

It is the purpose of this paper to review observational results of winds and waves in themiddle atmosphere which have been obtained during the recent Pre-MAP and MAP projects byusing ground—based radars. These radars have proved to be powerful tools for studying atmo-spheric dynamics. The most relevant radar techniques are introduced briefly (Table 1). Asurvey of observational results concerning atmospheric motions on different temporal andspatial scales is presented.

TABLE 1 Radar Techniques (values given are averages for middle latitudes).

RADAR HEIGHT HEIGHT OBSERVATION TIME MEASURED

TECHNIQUE ‘WAVELENGTH) RANGE RESOLUTION TIME RESOLUTION QUANTITY

METEOR 30 MHZ 80-110 KM 1-5 KM DAY AND 30-50 MIN U. V

RADAR (10 N) NIGHT

IONOSPHERIC 200 KHZ 90—100 KM (1 HEIGHT~ NIGHT 1 MEN U, V

DRIFT (1500 (V UNCERTAINTY

FF A 50—90 KM 1—3 KM DAYDI EN 3MHZADSORPTION (100 (V 1-5 (‘11(1 Nt. (v)

________________ __________ 85-100 KM 1-3 KM (lIGHT _____________ _____________

NP 3 MHZ 60-100/110 KM 1-3 KM DAY 1-5 MEN U. V

RADAR (100 M) 80-100/105 KM 1—3 KM NIGHT

1-35 KM DAY

MST 50 MHZ 60—90 KM 150 N 30 S U. V. W

RADAR (6 (‘1) 1-35 KM NIGHT

4 R. Rüster

2.1 METEORRADAR TECHNIQUE

Many observational results concerning motions in the upper mesosphere and lower thermospherehave been provided by the meteor radar technique. It is one of the oldest radar methodsstill used successfully for atmospheric research. First wind measurements were made byManning et al. /1/. Meteors entering the earth’s atmosphere produce a trail of ionized gasesbehind them, usually at heights between about 80 and 110 km. This ionized wake is emb~dd~din the atmosphere and since the ion—neutral collision frequency is high (about 3 x 10 s ),hence becomes a tracer of neutral air motions at these levels. Radars operating at frequen-cies of about 30 MHz are used to measure the Doppler shift of the echo signals specularlyreflected from these trails, thus yielding information on the radial velocity at the respec-tive altitudes. To obtain the complete horizontal wind vector, two antenna beam directions,at right angles to each other, are normally used, assuming the vertical velocity to benegligible. This kind of observation can be carried out 24 hours a day. They are, however,restricted to an altitude range of about 80 km to 110 km. The temporal and spatial reso-lution of the measured wind velocities, depending on the adopted averaging intervals, are ofthe order of 30 mm and several km, respectively. However many radars do not yet haveheight—ranging, and wind values are assigned to an altitude near 95 km. Wind measurements inthe meteor zone have been performed at many stations in the world (e.g. Portnyagin et al./2/, Sch~ning and Weiss /3/, Muller and Kingsley /4/, Bernard et al. /5/, Cevolani et al./6/, Clark /7/, Ahmed and Roper /8/, Aso et al. /9/, Roper /10/, Hess and Geller /11/, Averyet al. /12/, Elford /119/).

2.2 IONOSPHERIC DRIFT TECHNIQUE

Radio measurements in the LF range (30—300 kHz), making use of commercial broadcastingstations, have been used in the past to study the atmospheric circulation at heights betweenabout 90 - 100 km (e.g. Schminder and Kiirschner /13/, /14/, Taubenheim et al. /15/,Kazimirovsky and Kokourov /16/, Kazimirovsky /17/).

The first systematic applications were made more than 30 years ago by Krautkr~mer /18/,Mitra /19/ and Briggs et al. /24/. Sky waves scattered at 0-region altitudes are recorded,using spaced receivers, and wind velocities are then deduced from the cross correlationfunctions resulting from the different fading records. This method normally permits reliablemonitoring of the prevailing wind as well as the semidiurnal tidal wind components in theupper mesosphere region. The technique has the advantages of being comparatively simple and,using different transmission paths, the zonal and meridional wind components can be measuredat the same time over large areas in space. Due to the high daytime ionospheric absorption,however, these measurements may only be made at night.

2.3 MF RADAR TECHNIQUES

A most valuable technique for studying the lower ionosphere at heights between about 60 to100 km (0-region) is the differential absorption experiment, which originally was developedby Gardner and Pawsey /20/. Due to the earth’s magnetic field, the D-region becomes doublyrefracting and absorbing for MF (0.3 — 3 MHz) and HF (3 — 30 MHz) radio waves. The complexradio refractive index, therefore, is different for the two characteristic circularly—

o~arizeci propagating modes. Assuming a height profile of the electron collision frequency,tt.~ observed difference in the absorption of the two magneto-ionic components can be used toestimate the electron density (e.g. Holt et al. /21/, Thrane et al. /22/, Rastogi et al./23/).

The MF winds technique, which was established by Fraser /77/ is based on field strengthrecordings of radio waves in the mediwi frequency range which have been partially reflectedin the lower ionosphere. Similar to the ionospheric drift experiment (Section 2.2), measure-ments of the moving and varying diffraction pattern on the ground, formed by the reflectedradio waves, are carried out simultaneously at a series of locations and from a correlationanalysis of the different data sequences in space and time, wind vectors can be derived(e.g. Gregory at al. /25/, Meek /26/, Vincent et al. /27/, Fraser /88/). Complex integrationand Doppler measurements have recently allowed turbulence (Hocking /120/, /121/) and gravitywaves studies (Vincent and Reid /122/) to be made. The radars in general provide profiles ofbackground wind, tides and longer period (> 15 mm) gravity waves.

2.4 MST RADAR TECHNIQUE

In the past ten years, a new generation of sensitive Doppler radars has been developed forremote sensing of the lower and middle atmosphere between about 1 and 100 km. The firstradar investigations of this kind were carried out at Jicainarca by Woodman and Guillen/28/. These radars are monostatic and operate in the VHF band (30—300 MHz) usually atfrequencies near 50 MHz, corresponding to a wavelength of 6 m. The radar echoes result fromvariations in the refractive index structure of the atmosphere with scale sizes of half theradar wavelength. Refractive index variations, in turn, are due to changes in humidity,

Winds and Waves in the Middle Atmosphere 5

pressure, temperature and free electron concentration. Atmospheric scatterers are advectedwith the background air motions, so that the 3—dimensional velocity vector can be directlydeduced from the Doppler shifts of radar echoes received in three independent beam di-rections. The optirnu1~ temporal and spatial resolutions are about 1 mm and 150 m, res-pectively. MST (mesosphere-, stratosphere-, troposphere-) radars can measure height profilesof background wind, tides, atmospheric gravity waves and turbulence; although the seasonaland diurnal variability of reflecting structures makes tidal and mean wind measurements lessregular than MF and Meteor Radars. Their ability to observe simultaneously large and smallscale processes with high temporal and spatial resolution makes them unique instruments forstudying not only each process separately but also their nonlinear interactions. The newMST-radars currently active are described by Green et al. /29/, Balsley et al. /30/, Milleret al. /31/, Schmidt et al. /32/, Czechowsky et al. /33/.

3. ATMOSPHERICMOTIONS

One useful method of classifying atmospheric motions is to decompose the motions into meanzonal flow, by longitudinal averaging, and into the deviations from this mean, which finallyrepresent atmospheric waves of different space and time scales (e.g. Holton /34/, Wallaceand Hobbs /35/, Gossard and Hooke /36/).

Atmospheric motionsS

lod P(ane)ary

~ Synaptic scale sturb~mce...cTidis

: :i _______ ,,~#ret1wa.s

~ 1e~n ~

104~Trbul Ce

MicrO— Mesa— (1cc~o—sc5le

1km 100km 10000km

1 2 ~ ~ ~ ~ ~ Fig. 1. Characteristic temporal and1 0 10 m spatial scales of motions in the middleChoracteristic length atmosphere.

Figure 1 gives a rough survey and guide line for the subsequent parts of this paper. It pre-sents a sketch of characteristic scales of some atmospheric motions and, in general, itshows that large space scales are associated with long time scales. Planetary waves are de-fined as disturbances of global scale (10,000 km) with periods greater than one day, wherethe Coriolis force and its latitudinal variations dominate the motions. Cyclones and anti—cyclones with length scales of the order of 1000 km are of major importance in troposphericweather systems and arise from synoptic scale instabilities. Atmospheric tides are mainlyrepresented by the diurnal, semidiurnal and terdiurnal components with vertical wavelengthsof • 30 to over 100 km. Since tides comprise both evanescent and vertically propagatingmodes, their wavelengths extend over three orders of magnitude. At periods of the order of5 mm to several hours, gravity becomes an important force in governing atmospheric dynami-cal processes. The characteristic period of about 5 mm is limited by the V~is~l~—Bruntperiod in the respective height range. Vertically propagating gravity waves have horizontalwavelengths of the order of 100 km. At periods of minutes down to seconds there is a con-tinuous transition from internal waves to turbulence, generated by wave instabilities orwave—wave interactions, finally cascading down to small scale—eddies due to nonlinear pro-cesses. Sound waves associated with longitudinal pressure oscillations at time scales of theorder of seconds play no role in atmospheric dynamics.

Using Fig. 1 as a guide line, some of the recent results obtained since the beginning of1979, by applying the above mentioned radar techniques, during Pre—MAP and MAP projects,will be discussed.

3.1 MEAN FLOW

Knowledge of the mean flow is essential for studies of atmospheric dynamics since the meanwinds represent the so called “basic state’. The dominant feature is the radiatively con—

6 R. Riister

trolled annual cycle of summer westward and winter eastward winds in the middle atmosphere.This seasonal variation of the circulation is observed mainly at stratospheric and lowermesospheric heights. At altitudes larger than 80 — 90 km the annual circulation patternchanges.

Zonal

J FMA M J ~ A SON 0/I FM AM982 983

Meridional

~ )~~ii—I~I~Ji~F’J ~

AMIII ~

~ ft_sz~

1~\C ~ a

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\

~ ~ Fig. 2. Contours of mean zonal (positive.1 F M A M .j .i A s o N D/J F M M eastward) and meridional (positive northward)

982 983 winds (from Manson and Meek /37/).

Figure 2 presents an instructive plot of the annual variation of the 10-day averaged zonaland meridional mean winds in the altitude range between about 60 and 110 km obtained withthe MF radar at Saskatoon (52°N, 1O7°W) (Manson and Meek /37/). The meridional component,which is important in accelerating the zonal flow through the Coriolis force, shows a summerequatorward motion between 80 and 95 km, and an unexpected poleward flow above. During theequinoxes (March, September) and winter the flow is poleward between 60 and 80 km, andequatorward above 80 km. Similar investigations of mean meridional winds at 90 km in themid- and high latitude summer mesosphere by Nastrom et al. /38/ also show a strong equator-ward flow. The zonal wind pattern reveals a seasonal variation below about 90 km with maxi-mum values of 40 to 60 rn/s eastwards in winter and 60 m/s westwards in summer. Above thataltitude there is a change of eastward to westward flow. Other observations (Massebeuf /39/,/40/, Clark /7/, Vincent and Ball /41/) seem to indicate that this height of changeoverwhich is associated with the height of the mesopause, varies with latitude. Figure 3 (from

KImJNA (SWEDEN) GARCHY (FRANCE) PUNTA 80RINQUEN (PUERTO-RICO)£ U J J A SO NO J FM UJ .1 £SO NO S Jr U AUJ J A SONS

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~ ~ ~5 IllillIll I

JFMAMJJAOONQ .JF U £MJ J £ SONS 0 ~ A U J .i A MONOTIUC (~OflN)

Fig. 3. Contours of mean zonal (positive eastward) wind above Kiruna (68°N), Garchy (47°N),and Puerto Rico (18°N) (from Massebeuf et al. /40/).

Winds and Waves in the Middle Atmosphere 7

Massebeuf /40/) presents a comparison of zonal wind variations at different latitudes. Thechangeover height lies at 87 km over Kiruna (68°N, 1O°E), at 81 km over Garchy (47°N, 3°E)and varies between 84 and 100 km over Puerto Rico (18°N, 66°W). Similar results have beenreported by Vincent and Ball /41/ for Townsville (19°S, 147°E) and Adelaide (35°S, 13~E),by Dolas and Roper /42/ for Atlanta (34°N, 84°W), and by Clark /7/ for Durham (43°N, 71°W).

Figure 4 shows as an example the annual variation of the mean zonal and meridional wind overEurope at a height of about 95 km observed with the ionospheric drift experiments at Collm(52°N, 15°E) by Schminder and K~irschner /44/ and Kazimirovsky et al. /17/, /45/ at Irkutsk(52°N, 104°E). Since both observations are above the changeover height, they show an

S

S

(rn~s)

i F M’ 11 J AS ~ N ~ Fig. 4. Seasonal variation of the meanzonal (positive eastward) and mean

Coils meridional (positive northward) wind at• zonal 1979-1983) 95 km above Collm (51°N, 13°E) (aftero mer,d,onol Schminder and KUrschner /44/) and abovelrkutsk Irkutsk (52°N, 1O4°E) (after KazimirovskyS zonal 11971 -1979) /17/)~ mertd,onal

eastward wind during winter and summer with two periods of westward flow during the equi—noxes. Similar observations have been reported by Cevolani and Dardi /46/ for Bologna (45°N,12°E), and by Cevolani et al. /6/ and by Portnyagin et a]. /2/. The whole mean flow patternalso shows a variability from year to year (e.g. Roper /47/, Gregory et a]. /48/). In recentinvestigations of Lindzen /49/, Holton /50/ and Dunkerton /51/ it is suggested that gravitywave transmission from the troposphere and momentum deposition in the mesosphere seem to beof major importance in closing the winter/summer mesospheric eastward/westward cells.

3.2 PLANETARY WAVES

Planetary waves can be interpreted as the first zonal Fourier components of a decompositionof the hemispheric windfield. They can be divided into extratropical and equatorial modes.The latter consist mainly of the Kelvin and mixed Rossby-gravity waves and seem to drive thequasibiennial oscillation in the mean zonal winds of the equatorial stratosphere. Labitzke/52/ suggested that the phase of the quasibiennial oscillation may control the occurrence ofstratospheric warmings. Extratropical waves of global scale, with periods greater than oneday, embrace both stationary and transient modes. One of the most spectacular and well knowneffects associated with transient planetary waves is the so—called stratospheric warming,which is mainly due to a wave—mean flow interaction. It occurs irregularly in the northernhemisphere during winter (Labitzke /53/), and is characterized by an anomalous amplificationof planetary waves with zonal wavenumbers 1 and 2 in the polar stratosphere. Associated withthe wave amplification are poleward fluxes of heat, which in turn lead to a reversal of thetemperature gradient between polar and middle latitudes. To maintain thermal wind balancethe normal eastward (cyclonic) flow in the polar night is replaced by a westward (anti—cyclonic) flow (Matsuno /54/, Holton /55/, Schoeberl and Strobe] /56/, /57/).

Figure 5 presents observational results obtained with four different methods during themajor warming in the beginning of March 1980, together with an additional example of un-disturbed conditions (Smith et al. /58/, Riister et al. /59/). On the right hand side, thestratospheric radiance data obtained from the stratospheric—sounding—unit (SSU) (Met.Office, Bracknell, U.K.) are shown. The radiances, which are converted to black body tem-peratures, correspond to an altitude of about 45 km. To the left the corresponding heightprofiles of the zonal and meridional wind components in the mesosphere, as derived fromVHF—radar measurements, are plotted. During undisturbed winter conditions in November 1979the stratospheric circulation pattern above the winterpole is formed by the cyclonic polarvortex with a cold pool of stratospheric under a warmer pool of mesospheric air. The zonalrnesospheric winds are therefore directed eastwards. At the beginning of March 1980, however,a major stratospheric warming was in progress. The radiance maps from the previous daysshowed the decay of wave 1 and the simultaneous development of wave 2, which finallyresulted in a splitting of the polar vortex. The mesospheric winds changed to westward. Onthe left-hand side of the figure the corresponding wind observations obtained with the iono-spheric drift experiment at Collm (51°N, 13°E) (Taubenheim et a]. /15/) and the MF radar atSaskatoon (52°N, 107°W) (e.g. Manson at a]. /60/) are shown. In particular the Collm datareveal the same clear change of the zonal wind between March 4 and 6. The data from Canadaare slightly shifted in time possibly due to hemispheric differences.

8 R. Rüster

In addition to this irregularly occurring wave—mean flow interaction, planetary waves playan essential part in the atmospheric circulation. Several investigations have been carriedout in the past concerning the dominant periods of these waves. Results obtained with theMF radar technique by Manson et al. /60/, /61/ /62/, /63/, Manson and Meek /37/, revealprevalent periods at: 30 days, 15—20 days, near 5 days and 2.2 days. Similar results havebeen reported by Kazimirovsky /17/ using the ionospheric drift experiment at Irkutsk (52°N,1O4°E) and by v. Cossart et al., /64/, Greisinger /65/, Massebeuf et a]. /40/ obtained frommeteor radar data.

SOUSY VHF Radar (52°N, 1O°EI SSU - Rod iances“. 19Nov79

O ~ IOVN. IO7~e,I N) e~ 7

‘°/~\r~1~I 81 ~ 6 March 80

78 ~

-30 -~0 •i~ ~

(a) (b) (c)

Fig. 5. (a) Variation of zonal (positive eastward) wind over Central Europe and Saskatoon(after Smith et a]. /58/), (b) Height profiles of zonal and meridional winds at 52°N, 1O°Efor disturbed and undisturbed conditions (after Riister at ~l. /59/), (c) Correspondingstratospheric radiance data from the stratospheric—sounding—unit (Met. Office, Bracknell,U.K.).

In particular, many observations of a 2—day oscillation in the mesosphere (and upperstratosphere), mainly obtained with meteor and MF radars have been reported by severalauthors (e.g. Muller /66/, Manson et al. /67/, Kingsley at al. /68/, Coy /69/, Salby andRoper /70/, Craig et al. /71/, /72/, Rodgers and Prata /73/, Clark /7/, Cevolani at al. /6/,Muller and Kingsley /4/, Aso at a]. /74/). Recent model calculations by Salby /75/ and Hunt/76/ suggest that the oscillation is consistent with a westward travelling thirdRossby—gravity mode.

lOUSY VHF RADAR28 January 1983 11 LI - 8 February 1983 22 LI

.20-1

-2O~ ~ 2UOkm

.20 ~ 170km

.70] ~ 110km

~ Fig. 6. Temporal variation of the___________________________________ meriodional wind v at 10 consecutive29 ~o ~ heights observed with the SOUSY VHF

Inn a .1 P.~rLsr(c2°N 1n°F’l ~

Winds and Waves in the Middle Atmosphere 9

It is interesting to note that MST—radar observations at lower stratospheric heights,carried out in January and February 1983 using the SOUSY VHF Radar (52°N, 1O°E), revealoscillations with the same period. Figure 6 shows the temporal variation of the meridionalwind component for heights between 8 and 21.5 km. A low pass filter, with a 3 dB point at6 h, has been used to eliminate short period variations from the time series. The resultsindicate a 2—day period at heights below 16 km. The maximum entropy spectrum clearly revealsa peak well above noise, with a period of 52 - 54 hours. The maximum amplitude at 10 km isabout 15 m/s. This particular radar observation will be the subject of further study.

3.3 TIDES

Tides are examples of forced oscillations for which the characteristics of the sources arequite well known. Thermal tides are primarily forced by ozone heating in the stratosphereand water vapor heating in the troposphere. Of the various excited modes the diurnal and thesemidiurnal components have sufficient amplitudes to be of major importance in the dynamicsof the middle atmosphere, although there are observations, also revealing a terdiurnal and a16 h—component (e.g. Carter and Balsley /83/). The classical theory of atmospheric tides hasbeen reviewed by Chapman and Lindzen /78/, more recent very comprehensive theoreticalinvestigations and model calculations have been carried out by Forbes /79/, /80/.

The diurnal tide is composed of propagating modes, which are mainly confined to low lati-tudes, and evanescent modes, which have their maximum amplitudes at high latitudes. The pro-pagating modes have short vertical wavelengths of about 20 km and show a downward phaseprogression, indicating an upward energy flux. The main seonidiurnal oscillation has verticalwavelengths from — 30 to over 100 km and reveals quite regular behaviour with littlelatitude dependence. Most of the observations of the middle atmosphere tidal structure havebeen made using the four radar methods discussed earlier. The results will be grouped intofour main areas according to the geographical location of the respective measurements.

Zona] wind measurements at lower stratospheric heights have been carried out at equatoriallatitudes using the Arecibo radar by Fukao at al. /81/. Both diurnal and semidiurnal compo-nents were detected, showing downward phase progression. The amplitudes of 1-5 m s

1 weremuch larger and the vertical wavelength much smaller than predicted by tidal theory. Theauthors suggest that the observed oscillation is not the wavenumber 1 tide, but rather anoscillation linked to regional topography.

Most of the observations of tides over North Mierica have been made by the meteor radars atAtlanta, Durham and Urbana, by the MST radar at Poker Flat and by the MF radar atSaskatoon. The average picture of the seasonal behaviour of the tidal oscillations in meteorwinds over Atlanta has been studied by Ahrned and Roper /8/. The vertical structure of thesetides is compared with similar studies over Garchy, Urbana and Adelaide. Strong latitude de-pendence of the semidiurnal tide has been found, with vertical wavelength increasing withdecreasing latitude. The average properties of the semidiurnal tide at Durham agree wellwith classical theory, showing regularity in amplitude and phase (Clark /7/), whereas ampli-tude and phase of the diurnal tide are quite irregular. Continuous measurements since 1978using the MF radar in Canada lead to a very comprehensive picture of winds and tidal os-

Zonal Wind (EWI Oscillations Saskatoon (52N l07’W)106 km 1982/83

June 982

O~r III

JUl

De~e~~

Apr~’ hAll,

M 5m/s~ Fig. 7. Spectra of the zonal wind for each month

I I of 1g82/83 over Saskatoon (52°N, 107°W) at 106 km24d Sd 2d 24k 2k ~ (from Manson and Meek /37/).

10 R. Rüster

2-Hour ComponentSummer Avera~rs

141 Amplitude Pttase 1411km) ~ Eastward Eastward ~~•___~, 1km)

88

86

84 ~ ~82 ~O5kO$O01 \~4- ~82

Forbes Model 80

I If I)i’i 1)1

90 Nortkword I Northward

82 82 Fig. 8. Height profiles

80 80 of amplitude and phase of________________________ b the semidiurnal tide (from

0 20 30 40 0 I 2 3~4 1 ~ Carter and Balsley /87/).Amplitude lnVsl Local Time of Mao (hours)

cillations at mesospheric heights over Saskatoon. In particular, their variation withdifferent years and their seasonal and height dependence are studied in detail by Manson eta]. /60/, /61/, /63/, /82/; Manson and Meek /37/. Figure 7 presents zonal wind spectraranging from 6 h to 30 d for 12 consecutive months of a year at one height. The seasonalchange of the various spectral components is quite clear. During winter the semidiurnal tideis dominant and becomes weaker during sunwner, when the diurnal tide becomes stronger. DuringJune and July a weak 2d-period seems to be present and a terdiurnal component appears to bepresent between November and January. These features are, of course, height dependent andmay vary for different years. Various height profiles of the amplitude and phase of theseinidiurnal tide in sunnier are compared in Fig. 8. The different results have been obtainedfrom MST radar measurements by Carter and Balsley /83/, from MF radar measurements by Mansonet al. /60/, /61/ and from recent model calculations by Forbes /79/. There is good genera]agreement although the theoretical phase values are shifted by about h. The amplitude ofthe semidiurnal wind increases with height ranging from about 10 m s at 80 km to 17 mat 90 km, the vertical wavelength of the upward propagating tide appears to vary from — 30to over 100 km, but over a short vertical distan~e. Corresponding observations of thediurnal tide show amplitudes of about 15—25 rn s and no phase variation with height,indicating an evanescent mode. The whole spectrum of the zonal wind variation at 83.5 kmwithin 16 days in summer obtained with the Poker Flat MST radar reveals, apart from thediurnal and semidiurnal components, a weaker terdiurnal, a 2—day, a 5-day and a very strong

a(mis)

• I) el. 0—171:143 n, • tO •/.

.~.... • 2t.t ~T .0 0, • ~.3 ~.0

C . pemn.11044clod n7. .~ . s,ntdj~,m.L tad.1 •tod

(rn/,i

a

~ O..O~6Ooo ~ Fig. 9. Temporal variation of the zonal

a . ~ ~ and meridional wind components, between 9014 el. -hi/ifS) .‘. ~ and 95 km over Central Europe (51°N, 13°E)

.__ • ‘7.? LAO ~ :. ~ ~ for undisturbed conditions (solid dots)-s~ S —— P and for the stratospheric warming inLMT February 1983 (open dots (from Schminder

17 18 19 ~ 21 72 23 2~ 01 02 03 0/ 0 06 070 and KUrschner /44/).

Winds and Waves in the Middle Atmosphere 11

16-h peak. The authors suggest this 2/3-day period may result from nonlinear interaction ofdifferent waves.

Cevolani et al. /6/ report on wind observations at 95 km in Northern Europe using three me-teor radar stations (at Bologna, Sheffield and Stornoway). The results show both the diurnaland the stronger semidiurnal tide. However systematic differences exist in the amplitudesand phases recorded at the different latitudes, possibly reflecting local variations in thestratospheric wind and temperature fields. Bernard /84/ comes to similar conclusions fromobservations of the variability of the semidiurnal tide. Seasonal variations of the tidalmotions have been reported by Kingsley et al. /68/ over Sheffield , Bernard /85/ overGarchy, Schminder and Kürschner /86/, /87/ over Collm and Kazimirovsky /17/ over Irkutsk.Figure 9 presents the temporal variation of the zonal and meridional wind components, in theupper mesopause region (90-95 km) over Central Europe (Schminder and KUrschner /44/). Thesolid dots represent the undisturbed winter conditions in February 1983 with an eastwardprevailing wind v

0 and a dominating semidiurnal tide with an amplitude v2 and a phaseT2. The open circles represent results obtained during the Febuary 1983 stratospheric war-ming. The breakdown of the polar vortex is accompanied by a change in the prevailing windsystem from eastward to westward. These wind field disturbances extend to the upper meso-pause region. Similar observations of the effect of stratospheric warmings on the prevailingwind have been reported by Dolas and Roper /42/. The vertical phase gradient of the semi—diurnal tide has been investigated by Schminder and KUrschner /13/, /14/. They infer a ver-tical wavelength of about 60 - 70 km, in good agreement with other observations. In EasternAsia obervations of lower ionospheric winds have been made using the Kyoto meteor radar(35°N, 136°E) by Aso et a]. /9/. The result~ indicate a steady peak at the semidiurnalperiod with an amplitude around 5 — 25 m s - The less regular diurnal tide has comparablemagnitude. The variability of the observed tidal structure suggests a possible interactionbetween respective waves or winds.

MARCH 1979

DIURNAL

100 ISO ) — KyotoE J ~-:———. I .‘-. —— Townsvitle

90 ,,/ ~ 90 / -.\ —--• Adelaideç~ ~ ~ NS

ISO ~ j

80 ‘la a a I I

0 10 20 30 00 50 0 6 Ii t8 20 6 12

too too

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SEMI— DIURNAL

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180 _____________l~~ a a a a! a

0 90 20 30 00 50 0 6 13 5 12 6 12

Fig. 10. Height profiles100 ..~ too of the average tidal

-~ (\.‘~ 7 EW amplitudes and phases~o ~‘<) 90 ~ ) observed at Adelaide

~‘ ,\ ~i / (35°S, 138°E), Townsville~ so~’~t~.

1 0 ~ (19°S, 147°E) and KyotoI ha ahataha 1 f~cO 1,~O~9 (c A0 10 20 30 00 50 0 5 12 6 12 6 ~ ~ .. ~ ..~ L) ~ rom SO

Amplitude I ms~ Phase /hr and Vincent /89/).

Mesospheric winds observed at Kyoto have been compared with respective measurements inAustralia carried out with the MF radar system at Adelaide (35°S, 138°E) and Townsvi]]e(19°S, 147°E) by Aso and Vincent /89/ during equinoxes 1979 and January solstice of 1980.Figure 10 presents the average height profiles for 10 to 16 days in March 1979 of the ampli-tude and phase of the semidiurnal and diurnal tides as one example. The results show thatthere are significant differences in the tidal amplitudes and phases at the geographicallyconjugate locations of Kyoto and Adelaide, possibly indicating the presence of antisymmetr!_cal tidal modes. On average, the diurnal tidal amplitudes at Adelaide of more than 20 m s~

12 R. Rüster

are much stronger than those at Kyoto of less than 10 m s1. The semidiurnal amplitudes are,

on the whole, similar. The inferred vertical wavelength of the diurnal tide is about 35 kmat Adelaide and Townsville, for the N—S component of the semidiurnal tide it is about 60 km.

3.4 GRAVITY WAVESAND TURBULENCE

Gravity waves, having horizontal wavelengths of the order of tens to hundreds of kilometersand periods between 5 mm and hours, are generated by a wide variety of sources in thetroposphere. These vertically propagating waves transport energy upwards and therefore canhave important influences on the middle and upper atmosphere (Holton /50/; Lindzen /49/,Walterscheid /90/). Buoyancy or gravity waves have been observed in the past few years attropospheric, stratospheric and mesospheric heights by several authors (e.g. VanZandt eta]. /91/, Fukao et al. /92/, R&ttger /93/, Ecklund et a]. /94/, Klostermeyer and RUster/95/, /96/, RUster and Klostermeyer /97/, Balsley et a]. /98/; Carter and Ba]sley /83/,Baisley and Carter /99/, Balsley and Ecklund /100/, Balsley /101/, Manson et a]. /61/, /60/,/102/, Manson and Meek /103/, Vincent and Ball /41/, Riister and Czechowsky /43/, Vincent/123/).

SOUSY VHF Radar observations (RSttger /93/) in the upper troposphere and lower stratospheretaken shortly after a severe local thunderstorm showed that the convective activityassociated with the thunderstorm excited a range of gravity waves with periods less than theBrunt—V~is~l~period.

SOUSY VHF RADAR19 lay ~98~

i/k ~

~- -—~——~——--~-—--—-—-.~

~ 0.8

9.6

~

- ~ 8.4

Fig. 11. Time series of~ 7.2 vertical velocity w at

20 consecutive heightsobserved with the SOUSYVHF Radar at Arecibo

.is~ ~ 6.0 (Puerto Rico) (from0530 0600 0630 0700 0730 0800 RUster and Klostermeyer

(/Ll /97/).

Orography or weather fronts are further sources for gravity waves in the troposphere.Gravity waves may also be generated by strong wind shears, such as are associated with jetstreams at tropospheric heights. Observations of Klostermeyer and RUster /95/, /96/, madewith the SOUSY VHF Radar during the passage of a polar front jet stream, reveal velocityoscillations with a period close to the Brunt—Váis~l~—period and an amplitude of about1 m/s, which are produced by a Kelvin—Helmholtz (KH) instability in the region of strongestwind shear. During the occurrence of the KH instability the radar signal was characterizedby a sequence of power bursts having the same period as the associated velocityoscillations. Model computations showed that these power bursts are due to KH induced superadiabatic lapse rates at the critical level, where the horizontal phase velocity of the KHinstability equals the wind speed. One particularly interesting event of a KH instabilitywas observed during the passage of a subtropical jetstream with the SOUSY VHF Radar atArecibo, Puerto Rico by Riister and Klostermeyer /97/. Figure 11 presents the temporalvariation of the vertical velocity w at 20 consecutive heights. The height and timeresolution of the radar measurements are 300 m and 22 s, respectively. Oscillations with aperiod of 340 s are present for 40 mm in an altitude range of about 4 km. They startgrowing near 0525 LT and reach maximum amplitudes of the order of 1.5 m s1 around 0545 LT.In particular, the amplitude varies with height showing a maximum between 8 and 9 km. Thephase of the 340—s oscillation shows a shift of about 900 around 9 km. These propertiesindicate that a KH instability is the source of the observed oscillations. Around 0730 LT,similar oscillations appear at heights near 7.5 km. These changes in time and height of theonset of KH instabilities seem to be due to the observed lee waves which are superimposed onthe mean background wind and which, in turn, cause critical Richardson numbers at differentheights and times. Ecklund et al. /94/, using the Poker Flat MST radar, found a modulation

Winds and Waves in the Middle Atmosphere 13

in the intensity of gravity wave activity due to propagating planetary scale waves. Theirresults also indicated a correlation between the variance of the radar-derived vertical ve-locity component and the balloon—derived mean wind shear. Similar investigations are repor-ted by Nastrom and Gage /104/ and Baisley /101/.

Period

8a1 4d 2~2~h~k~h 4k h ~m 3m

\F

— 979(1

0 ~ ~

a~~ /1980

S.1/) II

a,

Poker Flat MST Radar0 Summer Mesospheric

Wind Speed Power Spectra

Zonol Component 86km Fig. 12. Power spectrum of the zonal windI fluctuations at 86 km at Poker Flat (65°N, 147°W)

io~ o~ o~ i03 0-2 for mid—sunnier 1979 (dashed lines) and 1980

Frequency 1Hz) (solid lines) (from Carter and Baisley /83/).

Investigations of the spectrum of atmospheric motions is of particular importance in under-standing atmospheric dynamics. VanZandt /105/ showed that the power spectra of mesoscale ho-rizontal velocity fluctuations as a function of frequency and horizontal and vertical wavenumber in the troposphere and lower stratosphere can be described in terms of a universalspectrum of internal gravity waves. From continuous records of wind measurements using thePoker Flat MST radar Baisley and Carter /99/ have derived power spectra of atmospheric velo-city fluctuations over the period of a few minutes to many days. The spectra reveal a slopeclose to -5/3. It is concluded that gravity waves propagating upwards from the tropospheredistribute their energy by nonlinear interaction over a wide range of frequencies and scalesizes by a turbulent-like cascade process. Figure 12 shows the mesospheric results obtainedfor a hçight of 86 km during mid—summer 1979 and 1980. The straight line, corresponding tothe f5~3 law, fits the data well up to periods of about 8 hours, indicating the fundamen-tal coupling between atmospheric motions of different scales in the spectral domain. Con-trary to the tropospheric observations, the mesospheric spectrum shows pronounced peaks atperiods greater than about 8 h. In particular, the terdiurnal, semidiurnal and diurnal tidesare superimposed on the background spectrum. At periods 9reater than 1 day the power spec-tral density for the mesosphere no longer follows the f~/3 law.

SOUSY VHF RADAR 1 FEB 1984rib ~ha a a’ I

1101—1318 LI ‘. I •

~7O ?~ ~ 3 ~ -J

65

60 ____ ~II~~0 •20 —10 .60 -80 -20 -20 .10 1200 1230 1300

u/ms’ vIm~1 tiLl

Fig. 13. Left: Height profiles of zonal (u, positive eastward) and meridional (v, positivenorthward) wind components observed at Andenes (69°N, 16°E) using the SOUSY VHF Radar.Riqht: Heiqht-time contours of received echo power.

14 R. Riister

Turbulence plays an important role in the dynamic structure and energy budget of the meso—sphere. The unstable breakup of gravity waves is thought to be a major source of turbulence(Lindzen /106/, /107/, Hodges /108/; Lindzen /109/). Long—term observations of the arcticmesosphere, carried out with the Poker Flat radar by Ecklund and Balsley /110/ and Carterand Balsley /83/, show different characteristic features during the summer and winter pe-riod. Bals]ey et a]. /98/ concluded that a seasonally dependent generation mechanism is res-ponsible for the observed mesospheric radar echoes. They suggest that the winter echoes area consequence of saturating gravity waves while the sunnier echoes are mainly due to shearinstability of atmospheric tides together with large mean temperature gradients at, andabove, the mesopause. Observational evidence that long period (— 10 h) internal gravitywaves in the mesosphere may indeed cause wave instability, and thereafter turbulence, isgiven by Klostermeyer and Rüster /111/. They report on observations made in winter 1983using the SOUSY VHF Radar (52°N, 1O°E). The measurements reveal short period (— 10 mm) ve-locity oscillations, simultaneous with observed radar echo power bursts having the same pe-riod, at heights with strong vertical shears generated by internal gravity waves. It is con-cluded that the velocity oscillations are due to a Kelvin—Helmholtz instability which gene-rates superadiabatic lapse rates resulting in strong turbulence. Similar results supportingthese conclusions have just been derived from measurements carried out in January/February1984 at Andenes/Norway during the MAP/WINE campaign using the mobile SOUSY VHF Radar. Theobserved height profiles of the zonal and meridional wind components are presented inFig. 13 on the left hand side. Superimposed on the mean background profile are wavelikepatterns with an apparent vertical wavelength of about 4 km. On the right hand side, theheight-time contour plot of the received radar echo power is shown. Several parallel echostructures are also observed having a downward slope of about 1 km/h and, interestingly, avertical separation of about half the apparent vertical wavelength of the periodic windpattern. These preliminary results already indicate that the echo power maxima may be asso-ciated with instablities generated in the maximum shear regions of propagating internal gra-vity waves. A second even more instructive event gives further support to this suggestion.Figure 14 presents a sequence of height profiles of zona] winds, observed with the EISCAT

WINE 21 JANUARY1144 EISCAI UHF-SAOAR

P

U’,,—,

SOUSY VHF RADARh~b~h•a•h’b~a•b•h•h’b’b’ ‘‘a.’.!.

~O30uI1

7

10~E0 ul ~ ~

~~~1

Fig. 14. Height profiles of zonal wind over Andenes (69°N, 16°E) observed with the EISCATUHF radar (top) (after R~ttger /117/) and the SOUSY VHF Radar (bottom).

facility (upper part) (Rättger /117/) and with the SOUSY VHF Radar (lower the part). Thewave—like patterns show increasing amplitude with increasing height. Again a downward phaseprogression with an apparent velocity of 1 km/h is visible. The strong shears around 69 kmat 1030 UT are of special interest. Figure 15 presents the corresponding height profiles ofthe received echo power P. the spectral width o, a measure of turbulence intensity, and themagnitude of the total vertical shear 5, deduced from the zonal and meridional winds forthis period. An association between the height intervals of maximum echo power and maximumshear is evident. (The dashed line represents the shear that would result in a Richardsonnumber of 1/4, for a height independent Brunt-V~is~l~ period of 8 mm.) Additionaltemperature variations resulting in changes in the Brunt—Välsäl~ period could account,therefore, for the observed slight difference between the heights - around about 72 km - ofmaximum echo power and maximum shear. The temporal variation of the vertical velocity w andthe signal-to-noise ratio S/N for the lower shear region are presented in Fig. 16. Spectral

Winds and Waves in the Middle Atmosphere 15

SOUSY VHF RADAR

80 21 JAN 1986 1126-1163 LI

Z.nrthzo.~.. NE

— — — zo.,..,Nw

75~

SI

Fig. 15. Height profiles ofthe echo power P received

( J with the SOUSY VHF Radar in( three different beam

65 -.- I positions at Andenes (69°N,I I I I 16°E), spectral width o and

0 5 10 15 0 1 2 3 4 0 10 20 30 40 so magnitude of total verticalP/dB o/ms~ Sllats-

1Ikm) shear S.

SOUSYVHF RADAR21 JAN 84

70.9

mc ttoo moo

s/N 4

600 ~ Fig. 16. Time variation of vertical

‘l ~ velocity w and signal-to-noise ratio69.7 S/N obtained from measurements using

_________________________________ the SOUSY VHF Radar at Andenes (69°N,tILT 16°E).

analysis of these time series reveals dominant periods of 16 and 30 mm in the velocity andthe power data. This event will be studied in more detail, but it can already be concludedfrom the observations that long period gravity waves are of major importance in generatingmesospheric turbulence at middle and high latitudes in winter. Further observations ofmesospheric structures and their seasonal variation have been reported by Czechowsky et a]./74/ and Riister et a]. /118/.

Small—scale turbulence, in turn, is important in vertical mixing processes and in its energydissipation. There are different methods of estimating the energy dissipation rate c, but ingeneral most ~f the n

9merous investigations reported in the literature arrive at values ofabout 0.5’lO to 1O W kg~for lower stratospheric heights (Woodman /112/, Gage etal./113/, Sato and Woodman /114/) and 0.04 to about 0.1 W kg~for mesospheric altitudesbetween 80 and 90 km (Manson et a]. /103/, /102/, /60/, /61/, Woodman /112/, Vincent andBall /41/. Nastrom et a]. /38/. Hockina et a]. /115/). Fioure 17 oresents an examole of

16 R. Rüster

Eddy Diffusion Coefficient (Kd). m2 s

00 200 300 400 500 .~j00 200 300 400 500 600 100

a a Day Night

_______ _________ g~~~iso~ ~e0.05 o~ 0.15 0.05 01 ~ to turbulence and eddy

Energy Dissipation Rate (. W kg diffusion coefficients (fromManson et a]. /60/).

height profiles of energy dissipation rates and eddy diffusion coefficients deduced from thefading rates of radio wave signals by Manson et al. /60/.

4. CONCLUDING REMARKS

One of the most powerful tools for investigating atmospheric dynamics are ground-based ra-dars. Observational results obtained using radar techniques contributed significantly to themain goal of MAP, i.e. to improve the knowledge of dynamical processes in the middle atmo-sphere at different spatial and temporal scales, such as mean flow, planetary waves, tides,gravity waves and turbulence. In particular, observational results should be emphasizedsupporting the coupling of the lower and middle atmosphere by vertically propagating waves,their breaking and the associated generation of turbulence. There are, however, manyquestions open. The last figure presented, should be meant to lead to the subject of aplanned research project in Europe, the so—called EPSILON campaign (Thrane et a]. /116/). Itis aiming at a detailed study of upper atmosphere turbulence, in order to answer some ofthe unsolved questions: sources and spectrum of turbulent energy, temporal and spatialextent of turbulent layers, the variability of the energy dissipation rate and the corres-ponding heating rate, and the effect of turbulence on the vertical temperature structure andon the ionization balance.

ACKNOWLEDGEMENTS

Measurements during the MAP/WINE campaign have been carried out on Andoya in Northern Norwayusing the mobile SOUSY VHF Radar. The author would like to thank the staff of the AndoyaRocket Range for efficient cooperation. The antenna system and the operation of the SOUSYVHF Radar during the MAP/WINE campaign has been funded by the Deutsche Forschungsgeniein-sch aft.

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