Strengths and limitations of MST radar measurementsof middle-atmosphere winds

W. K. Hocking

Department of Physics and Astronomy, University of Western Ontario, Canada

Received: 18 November 1996 /Revised: 20 February 1997 /Accepted: 24 February 1997

Abstract. Radars have been used successfully for manyyears to measure atmospheric motions over a wide rangeof altitudes, from ground level up to heights of severalhundred kilometres into the ionosphere. In this paper weparticularly wish to concentrate on the accuracy of thesemeasurements for winds in the middle atmosphere (i.e.10±100-km altitude). We begin by brie¯y reviewing theliterature relating to comparisons between radar meth-ods and other techniques. We demonstrate where theradar data are most and least reliable and then, inparallel with a discussion about the basic principles ofthe method, discuss why these di�erent regimes have thedi�erent accuracies and precisions they do. This discus-sion is used to highlight the strengths and weaknesses ofradar methods. Issues like radar volume, aspect sensi-tivity, gravity wave e�ects and scatterer intermittency inproducing wind biases, and the degree by which theintermittent generation of scatterers at quasi-randompoints in space could skew the radar measurements, areall considered. We also investigate the possibility thatMF radar techniques can be contaminated by E-regionscatter to heights as low as 92±95-km altitude (i.e. up to8±10 km below the ionospheric peak echo). Within allthese comments, however, we also recognize that radarmethods still represent powerful techniques which havean important future at all levels of the atmosphere.

1 Introduction

Radars used for atmospheric wind measurements covera broad range of styles and applications, from dish-likesystems as used for meteorological work (e.g. Atlas,1990) and ionospheric work (e.g. Mathews, 1984) tophased array systems (e.g. Woodman and Guillen, 1974;RoÈ ttger et al., 1978; Fukao et al., 1985a,b) and simplerspaced-antenna systems (e.g. Vincent et al., 1987). Inaddition to a multiplicity of antenna types, there are

di�erent hardware arrangements, which involve varioustypes of pulse compression (e.g. Farley, 1985) andintegration procedures. We will not dwell on theseaspects, but will try broadly to classify di�erent radarsaccording to their methods of data analysis, since as arule it is the di�erent techniques which are mostimportant in our discussions, rather than the technicaldetails of the radars being used.

Several di�erent techniques are applied in radarmeasurements of atmospheric winds. These include theDoppler method (Woodman, 1985; Zrnic, 1979; Hock-ing, 1989), the spaced-antenna method (e.g. Briggs,1984; Hocking et al., 1989), and variations on thespaced-antenna method, which act in an interferometricmanner (e.g. Adams et al., 1985; RoÈ ttger and Ierkic,1985; RoÈ ttger et al., 1990). Figure 1 demonstratesschematically some of these techniques. We will notdwell in detail on the methods, save to give a briefoverview. The reader is referred to the references givenfor more detailed discussions. However, in brief themethods can be summarized as follows.

The Doppler method uses a large (often steerable)antenna to form a narrow beam, and then directs a largefraction of the available transmitter power along this``pencil beam''. The radio waves scatter from irregular-ities within the beam, and are scattered to a receivingarray which collects the signal. In many cases thereceiver array is the same one as the transmitter array,and this arrangement is called a monostatic radar(Fig. 1a). The returning signal is Doppler shifted relativeto the transmitted one, due to the motions of thescatterers, and the radar then measures this Dopplershift. Typical values for the Doppler shift are in therange 0.01±10 Hz. The value of the Doppler shift is thenused to determine the mean radial velocity of thescatterers within the beam, and by using combinationsof radial velocities obtained with di�erent beam point-ing directions, it is possible to make measurements ofthe nett motion of the atmospheric scatterers.

The spaced-antenna method generally utilizes di�er-ent antennas for transmission and reception, and rather

Ann. Geophysicae 15, 1111±1122 (1997) Ó EGS ± Springer-Verlag 1997

than directly measuring the scatterer's radial velocities,it measures the drift of the di�raction pattern of thescatterers across the ground (Fig. 1b). At least three

groups of antennas, spatially separated over the ground,are used to record the backscattered signals, andcross-correlation functions calculated between pairs ofreceivers are used to infer the drift speed of thedi�raction pattern. The drift speed of the scatterersthemselves are then found as one half of the drift speedof the di�raction pattern (e.g. Briggs, 1992), and thedirection of drift is the same for both the scatterers andthe di�raction pattern. Although the method seems inprinciple to be di�erent to the Doppler method, Briggs(1980) has shown that they are actually very similar inbasic principle. However, a major di�erence is the factthat the spaced-antenna method often utilizes smallgroups of antennas (for either transmission or receptionor even both), with correspondingly larger beam widths,and we will see later that this broader beam-width canhave important implications for the method. Thespaced-antenna method also tends to make more useof scatterers closer to overhead of the radar, and thispoint will also prove important later on.

Interferometric methods use a combination of theseprocedures, but are limited by the requirement that thescattering region must contain only a few discretescatterers. By using complex correlation functionsdetermined between spaced receiving antennas, it ispossible to ®nd the directions of discrete scattererswithin the sky, and then determine Doppler shiftsassociated with each individual scatterer (Fig. 1c). Bycombining many such measurements of scatterer loca-tions and Doppler shifts, it is then possible to determinemean velocities as a function of height which closelyapproximate the mean values over the radar. However,the method breaks down if there are too many scattererswithin the beam; for example, in the case of volumescatter (many scatterers distributed almost continuouslyin space), it is not possible to unambiguously identifydiscrete scattering entities.

Extensive tests of these di�erent procedures havebeen applied at many altitudes, but we will begin byconsidering comparisons in the lowest altitude of theatmosphere (i.e. especially in the troposphere and lowerstratosphere). Parts a and b of Fig. 2 illustrate nicelycomparisons between the Doppler method applied withnarrow-beam radars and radiosonde measurements, andit is clear that in general the agreement between themethods is quite good. These show comparisons fromCrane (1980) and Hocking (1997). Other detailedcomparisons (not shown) have been presented by manyauthors, including particularly careful studies by Weberand Wuertz (1990) and Astin and Thomas (1992). Inaddition, RoÈ ttger and Vincent (1978), Larsen andRoÈ ttger (1982) and Vincent et al. (1987) have demon-strated the accuracy of the spaced-antenna method inthe troposhere. There is little question that Doppler andspaced-antenna techniques work quite well in thetroposphere, where the scatterers are generated relative-ly continuously in height and time, so we may concludethat there is nothing intrinsically wrong with the basictheory of the methods. However, some of the speci®cs ofimplementation can lead to problems in certain situa-tions, and these special cases will be one subject of this

Polar diagram of radar

U

Principle of doppler method

Hei

ght

Scattering layer

Ground levelradar diffraction pattern

Transmitterantenna

Receiverantennas

V

2V

Principle of spaced antenna method

Hei

ght

Scattering centres

Ground levelTransmitterantenna

Receiverantennas

VPhase-front

Principle of interferometry

a

b

c

→

→

→

→

Fig. 1a±c. Schematic diagram representing the main principles appliedin radar techniques. a demonstrates the Doppler method, b shows thespaced-antenna method and c shows the interferometric principle. Ina, the Doppler shift of the returning radiation relative to thattransmitted is indicated, and this shift is the basis of the method. In b,the scattering distribution over the sky at one altitude is shown,together with the di�raction pattern on the ground. Note that thebroken lines represent just a few of the many ``ray paths'' which canexist; the radiation propagating along these paths eventually interferesat the ground to give the resultant di�raction pattern. In c, thee�ective ray paths associated with just a few scatterers are shown. Theinset to the right shows incoming phase fronts from a single scattererin two dimensions, illustrating that a single phase front arrives atdi�erent antennas at di�erent times. The delay between phase frontsarriving at di�erent antennas is used to determine the original locationof the scatterers

1112 W. K. Hocking: Strengths and limitations of MST radar measurements of middle-atmosphere winds

article. These special cases can be especially important inthe upper atmosphere (above 60-km altitude), and it isto this region that we now turn.

The importance of determining the accuracy andreliability of radar winds in the mesosphere has alwaysbeen crucial, but is even more so now that new satellitetechniques [especially WINDII (e.g. Shepherd et al.,1993; Gault et al., 1996; McLandress et al., 1996) andHRDI (Hays et al., 1993; Burrage et al., 1996)] havebegun producing large quantities of data. Studiesincorporating data from di�erent systems like radarsand satellites will need to be carried out more frequentlyin the future, and knowledge about the relative accura-cies of each system will be important in such compar-isons. As seen, radar comparisons with other techniquesgenerally show good agreement in the lower atmo-sphere, but as the altitude of the scatterers reachesmesospheric heights, we note that the level of agreementdegrades somewhat; comparisons of radar techniques atthese heights are not always as good as in the loweratmosphere. Cases reporting moderate to good agree-ment include those due to Stubbs (1973), Stubbs andVincent (1973), Vincent et al. (1977), Vincent and

Stubbs (1977) and Phillips et al. (1994), but cases ofpoor agreement also exist, e.g. Hines et al. (1993). Eventhe cases of apparently ``good'' agreement are not aswell matched as tropospheric comparisons [e.g. Stubbs(1973), especially above 90 km], although it is also truethat some of the cases of ``bad'' agreement have been tosome extent explained (e.g. Turek et al., 1995). There aregood understandable reasons for this great degree ofvariability and discrepancy, particularly in relation tothe fact that the beam width of a radar in terms ofphysical width (kilometers) becomes quite large at largeranges, and also the fact that the entities producing thebackscatter become more intermittent in space and timeat the higher heights. In addition, there is often a greaterdegree of inherent wind variability at these upperheights, where wave activity becomes more vigorous inthe higher regions (e.g. Hines, 1960). Thus comparisonson short time-scales invariably show variability, andlonger-term averages are required to show better agree-ment. For example, Hocking and Thayaparan (1997)have shown fairly good comparisons of semidiurnaltidal amplitudes and phases using meteor and MFtechniques, but greater scatter on scales of minutes.

Fig. 2a, b. Illustration of comparisons between the Doppler methodand radiosonde measurements, showing a comparisons of magnitudesof winds using a UHF radar (Crane, 1980) and b comparisons ofmagnitudes directions between VHF radar winds and radiosondes(Hocking, 1997). Note that in the second case the slope of the line of

best ®t for the magnitudes is not quite 1.0, due to the e�ect scattereranisotropy. This e�ect is usually corrected for, but we have chosen notto do so in this case. See the text for a more extensive discussion ofscatterer anisotropy

W. K. Hocking: Strengths and limitations of MST radar measurements of middle-atmosphere winds 1113

Indeed (and for similar reasons), comparisons betweenany pairs of methods (radar or otherwise) often becomeworse at these higher altitudes. It is this level ofagreement of di�erent techniques, especially with regardto radar methods, which forms a major thrust of thisarticle. However, in order to set things into perspective,we will begin not by looking at radar weaknesses, butrather at their strengths.

2 Strengths of radar techniques

The greatest strengths of radar techniques are almostcertainly their ability to provide wind measurementsduring conditions of both clear air and cloud, and in allsorts of di�erent geographic and atmospheric environ-ments. The data acquired by radar techniques aregenerally among the most continuous of any procedure.The ability to run radars unattended for long periods oftime, and often during both day and night, is also a keystrength of the technique.

Another important advantage of radar systems istheir resolution ability. Because a radar can determinethe range to a scattering region, it has importantadvantages over other techniques in this regard. Forexample, passive optical techniques which rely on localatmospheric generation of the source radiation whichthey utilize often su�er from the fact that the exactheight of the source region is unknown. In addition, itcan be true that the source region can be spread over awide band in height, producing a signi®cant smearing ofwind measurements.

The spatial accuracy of a radar system is often de®nedby the radar volume, which is the volume de®ned by thebeam width and e�ective pulse length of the radars used.(In this context, ``e�ective pulse length'' refers to theradar resolution, i.e. the true transmitted pulse lengthdivided by 2, to allow for the fact that the transmittedpulse travels up but the re¯ected pulse down). To ®rstorder, this volume is usually a cylindrical region, with theaxis of circular symmetry directed along the radar beam(see Fig. 1a). The length of the cylinder is the pulselength, and the radius is taken as the half-power half-width of the radar beam multiplied by the range understudy. The angular beam width of the radar will notchange, so that as one increases in altitude the physicalwidth of the beam in the direction transverse to itspointing direction will increase, resulting in a worseningresolution at higher altitudes. For example, a radar with abeam full width of perhaps 5° has a diameter for its radarvolume at 10 km range of 1 km, whilst the transverseextent covered by the beam at an altitude of 90 km can betypically 10 km across. Broader beams can correspond tohorizontal extents of several tens of kilometers at thesehigher heights, and we will see later that these largerhorizontal beam-width scales at the higher heights can beimportant, especially when considered in connection withthe intermittency of the scattering process. With respectto pulse lengths, some radars such as VHF radars canachieve pulse lengths down to several hundred metersand better, whereas other radar systems such as MF

(medium-frequency) radars are restricted to pulse lengthsof the order of several kilometers.

It should be emphasized that the sorts of spatial andtemporal resolutions which can be obtained with radartechniques are certainly as good as, if not better, thanmany optical techniques, but it must nevertheless beborne in mind that they are not perfect. Lidars, forexample, have superior spatial and temporal resolution(e.g. Chanin and Hauchecorne, 1981; Gardner andVoelz, 1985), but have only recently developed theability to measure winds, and are limited in theircoverage temporally, being restricted to conditions ofno cloud and night-time studies. On the other hand,radars are certainly superior to many other ground-based optical techniques in their spatial resolution, andlikewise have better spatial resolution than satelliteoptical methods. Limb-scanning optical instruments, forexample, integrate along a path of several hundredkilometers in length in their velocity determinations.

Having now pointed out the good spatial resolutioncapabilities of radars, we have to also temper thesecomments with a warning; we will see later that the factthat resolution is not perfect can still lead to someproblems. For example, we will see that the issue of thevolume of the atmosphere which returns scatteredsignals to the radar combined with the possibility ofintermittent generation of scatterers, both temporallyand spatially, can lead to problems with the datainterpretation.

The prices of radars can also be an advantage, in thatit is often possible to develop small radar systemscapable of continuous unattended operation for mea-surement of atmospheric winds at costs of US$ 100 000.of course, more expensive systems also exist; a widerange of options are available.

At this point we will say little more about theadvantage of radars ± we consider that the strengths areto a large extent self-evident. We will concentrate ratheron a critical review of the limitations of radar methods,although it is important to emphasize that whilst weintend to be quite honest about these problems, mostother techniques (including both optical and satellite)su�er from problems which are at least comparable, ifnot even worse.

3 Potential limitations of radar techniques

In the following sections, we will discuss some importantaspects relating to the application of radar techniques.Perhaps the main issue of concern is the fact that thesemethods usually (with the exception of meteor radars)depend critically on the generation of small-scaleinhomogeneities within the atmosphere by turbulentand small-scale processes. It is this fact which leads tomany of the potential problems in bias which will bediscussed in this paper. Whilst the generation of theseirregularities are certainly absolutely necessary forradars to work, the fact that the irregularities can begenerated intermittently in space and time has thepotential for leading to biases. We will return to this

1114 W. K. Hocking: Strengths and limitations of MST radar measurements of middle-atmosphere winds

issue of the nature of the scatterers in due course, butbefore doing so we also need to look at some otherspecial limiting factors of radar techniques.

3.1 Vertical motions

As our ®rst major point, we begin by noting thepossibility that some of the methods used in radartechniques can be contaminated by the e�ects of verticalmotions. This e�ect is not unique to radar studies, butwe will discuss it in this context.

The major problem which arises when using Dopplermethods is that the radial velocities measured containcontributions from both the horizontal and verticalcomponents of the true wind. In order to measure thehorizontal winds, it is necessary to make some assump-tions about the vertical motions. In some cases it isassumed that the vertical wind is zero, but since beamtilts can be as little as 10° o�-zenith, this can be adangerous assumption. Even vertical winds which areless than 5% in magnitude of the horizontal winds canproduce contamination. Alternative procedures includeusing a vertical beam to make measurements of thevertical wind, and incorporating these measurementsinto the determinations of horizontal velocities. How-ever, generally the vertical winds and the o�-verticalradial components are not measured in the samephysical volume of space, and errors can arise due tothese spatial di�erences. In addition, there can be errorsin vertical velocity measurements due to the fact thatscattering entities can have small tilts which skew theregion of scatter away from directly overhead; this issuehas been discussed by, amongst others, Larsen andRoÈ ttger (1991) and Larsen et al. (1992).

As seen, Doppler radars are especially vulnerable tothe e�ects of vertical winds because of the small zenithalbeam pointing angles often employed in radar experi-ments. It may seem that the spaced-antenna method isnot so vulnerable to this e�ect, but this is not true either.Briggs (1980) has shown how the drift of a di�ractionpattern across the ground can be envisaged as the``beating'' e�ect of signals scattered (with appropriateDoppler shifts) from di�erent points in the beam, sothat vertical motions can also produce errors inhorizontal velocity determinations in this case too.However, we should emphasize that the contaminationin this case is less severe than with Doppler radars, sinceit requires rather circumstances ± namely a situation inwhich the vertical velocity is upward on one side of thebeam and downward on the other.

The idea that even the spaced-antenna method couldbe occasionally contaminated by vertical velocities was®rst seriously considered by Royrvik (1983), withsubsequent modeling work presented by a variety ofauthors. For example, Kudeki et al. (1993) proposed awave-induced ¯uctuation model and showed that sys-tematic wind estimation errors should be expected toresult from spatial variation in the vertical component ofthe perturbation wind ®eld associated with gravitywaves. These e�ects could be especially important in

waves with short periods and short horizontal wave-lengths, which have relatively large vertical velocities.The ¯uctuation biases may be non-minor in instanta-neous velocity estimates but will average to zero mean ina long-term temporal average, suggesting that estimatesof tidal and longer-period waves should be relativelyaccurate. This explanation was further substantiated andsupported by more detailed numerical simulation resultspresented by Surucu et al. (1995). Only isotropicscattering was considered by Kudeki et al. (1993), butthe numerical simulations were carried out for bothisotropic and non-isotropic scatterers with di�erent setsof gravity wave parameters by Surucu et al. (1995). (Wewill consider the results of anisotropic scattering morefully shortly.) Surucu et al. (1995) found that individualhorizontal SA wind estimates are very well predicted bythe model of Kudeki et al. (1993) as long as the gravitywave has a horizontal wavelength larger than thehorizontal dimension of theMF radar scattering volume.

Thus we see that while vertical velocities can a�ectradar measurements of horizontal winds, the e�ects aremost serious on time-scales of a few minutes, andgenerally are not of great concern for studies of motionswith temporal scales of hours and more. Even at theshorter time-scales the radars can still produce mea-surements, but greater care is needed in recognizing andremoving the e�ects of vertical motions.

3.2 Scatterer anisotropy

Another potential problem with radar measurements isthe fact that the scatterers themselves are on the averageanisotropic. In other words, the scattered signal magni-tude varies as a function of the angle at which the beamis pointed, with generally stronger scatter from overheadand increasingly weaker scatter as the angle of the beamis tilted further and further away from vertical. This cana�ect measurements of the magnitudes of the winds

Radar beam bore angle

Effective pointingangle of beam

Polar diagram of radar

Effectivepolardiagram

Fig. 3. Schematic diagram showing how anisotropic scatterers alterthe e�ective beam pattern for a Doppler radar. The bulk of the radioscatter comes back through the ``e�ective beam'', rather than the truebeam, due to the asymmetry introduced by the scatterers

W. K. Hocking: Strengths and limitations of MST radar measurements of middle-atmosphere winds 1115

because it can result in a bias of the e�ective look-angleof the radar beam. This e�ect is illustrated in Fig. 3,where it can be seen that scatter is preferentiallyproduced from angles closer to overhead than the truebore angle of the radar beam (also see RoÈ ttger, 1981).Thus when horizontal velocities are estimated via arelation like vH � vrad= sin�h�, and h is chosen to be thebore angle of the beam, then the calculated horizontalwind speed is an underestimate. Figure 2b shows anexample of this type of bias, where it can be seen that theslope of the graph of radiosonde wind magnitudes vsradar wind magnitudes is not unity. We emphasize thatis possible to correct for these e�ects (see shortly); ourpoint here is simply to recognize that the problem exists.Note that we should also emphasize that while thesescatterers are often modeled as ellipsoids (e.g. Hocking,1987), they are in reality twisted and contorted string-like structures which are simply elliptical in a statisticalsense (e.g. Hocking and Hamza, 1997).

Figure 4 illustrates the di�erent types of scatteringshapes which can be statistically representative of thescattering medium. These may involve steps, or possiblyeven peaks and dips, in refractive index, which have abroadly ellipsoidal shape on average. They are repre-sented by analytical functions such as those shown in the®gure and generally produce backscattered power as afunction of angle which follows the form

P�h� / expfÿ�h2�=�h2s �g:There are other possible models, but this is at least themost common model representation of the way in whichpower falls o�.

It is possible to measure the degree of anisotropy by avariety of techniques (Hocking, 1987; Lesicar andHocking, 1992; Lesicar et al., 1994) and extensivestudies of this parameter have been made by manyauthors. The most complete set is probably that due toLesicar et al. (1994), who performed studies at frequen-cies of around 2 MHz at polar, mid-latitude, andequatorial stations. Similar studies have been made inthe mid-latitude troposphere and stratosphere at VHF

frequencies (around 50 MHz) by Tsuda et al. (1986) andHocking et al. (1990). The e�ect of this anisotropy is toproduce underestimates of the mean wind, and (asalready noted) Fig. 2b shows an example. It is relativelyeasy to correct for the e�ects of this anisotropy todetermine horizontal winds (e.g. Hocking, 1989). Indeedspaced-antenna wind measurements are not even sensi-tive to the degree of anisotropy, and measure windspeeds without any bias due to this e�ect.

However, there is one extreme case of anisotropywhich distinguishes itself from all others because of itsunique nature, and this is the phenomenon of so-called``specular re¯ection''. In this process the backscatteredradiation acts as if it were scattered by smooth mirror-like re¯ectors (possibly with some degree of undulation)in the upper atmosphere. This has been observed onoccasions and, while a phenomenon of signi®cantintrinsic interest, it can also be important because itcan lead to biases in wind measurements. Specularre¯ectors are not by any means the main form ofscatterer, but they have had a substantial history ofdiscussion in the literature, and it would be remiss of usnot to discuss them here. They reveal themselves asscatterers with very, very small values of hs (typically lessthan about 2° or so); we will now consider these veryspecial re¯ectors in some more detail.

3.3 Specular re¯ections

The nature of specular re¯ectors has been the subject ofsubstantial debate over many years, and various modelshave been proposed to explain them. A few authors havequestioned the existence of these unusual entities (e.g.Woodman and Chu, 1989), but a substantial number ofresearchers believe that these phenomena really aresomething unique and distinct from turbulence. Recentballoon measurements (e.g. Luce et al., 1995, andreferences therein) have given extra credence to theirviability, at least in the stratosphere. Figure 5 is a fairlyconvincing example of the spectrum produced by a

Fig. 4a±c. More detailed representations of the average shapes of radio-wave scatterers (from Hocking, 1987)

1116 W. K. Hocking: Strengths and limitations of MST radar measurements of middle-atmosphere winds

specular re¯ector; it shows the simultaneous occurrenceof specular re¯ectors and turbulence scatter in theatmosphere. The diagram comes from Hocking (1987)and shows a radar experiment which demonstrates thatspecular re¯ectors and turbulence scatterers can occursimultaneously in time, although it is generally believedthat they are spatially separated. In the experiment usedto produce Fig. 5, two radar beams were used simulta-neously, one pointed vertically (producing Fig. 5a) andone pointing o�-vertically (Fig. 5b). The large spike isdue to a very strong specular re¯ector immediatelyoverhead, and although it should not have been evidenton the o�-vertical beam, the re¯ector was so strong thatits re¯ected signal was able to leak in through thesidelobes of the o�-vertical beam. The smaller cluster ofspectral points at around )0.2 Hz in Fig. 5b are genuineturbulent scatter received by the o�-vertical beam.Models for specular re¯ectors have been discussed byBolgiano (1968), Hocking et al. (1991), and referencestherein.

Specular re¯ectors have been of great interest in thehistory of radar studies, because it has been claimed thatthey can produce a number of biases in wind measure-ment, especially when the spaced-antenna method isemployed.

The procedure by which this was envisaged to applyis as follows. It was assumed that gravity waves mightperturb the specular re¯ectors to give them longsmooth oscillations, with scales matching the gravitywaves. In some cases these undulations were considereddue to gravity waves distorting already existing layers,whereas in other models it was assumed that the wavesmight have short vertical wavelengths (a few tens of

meters) and actually cause the radio-wave re¯ectionsthemselves. We shall see shortly that this second modelhas been largely refuted, but for now we will notconcern ourselves with the details. Rather, we willsimply note that these models assumed that the gravity-wave-induced oscillations were supposed to move withthe wave, and therefore with the phase velocity of thewaves. Thus, its was argued, the spaced-antennamethod would measure the phase velocities of gravitywaves, whereas say the Doppler technique wouldmeasure the true winds because an o�-vertical Dopplerbeam is not sensitive to specular re¯ectors. Hines et al.(1993) incorporated these notions, together with amodel of wave-wave critical-level interactions andwave-®ltering (which he called ``scavenging''), to showhow the interactions within a wave-®eld superposed ona mean wind could lead to preferred senses of directionof phase velocities of gravity waves at di�erent heightsunder di�erent circumstances. This model was thenused to infer predictions about the expected sorts of``winds'' which might be measured by the spaced-antenna technique if it was indeed sensitive to the phasespeeds of gravity waves.

However, for these re¯ectors to be important requiresthat the specular re¯ectors have horizontal extents ofmany kilometers and vertical depths of less than 30 or40 m at 2 MHz and less than about 1±2 m at VHF. Thelikelihood of these scatterers existing with such hugeextents, and the possible e�ects that they will have onradar measurements, have been discussed by Hockinget al. (1989) and Surucu et al. (1995). Hocking et al.(1989) have shown that if such specular re¯ectors existthey are generally intermittent, and this will destroy

Fig. 5a, b. This ®gure, from Hocking (1987),demonstrates the temporally simultaneousoccurrence of specular re¯ectors and tur-bulence scatter in the atmosphere, a verticalradar beam, b o�-vertical radar beam

W. K. Hocking: Strengths and limitations of MST radar measurements of middle-atmosphere winds 1117

their usefulness for measurement of any winds. As aresult, they will generally be rejected by traditionalspaced-antenna algorithms, and as a result will not skewthe measurements. It has also been argued that they arevery rare (e.g. Lesicar et al., 1994), and their predom-inance in the literature is simply an indication of thecuriosity value associated with them, rather than ameasure of their frequency of occurrence. However, inaddition to the arguments in Hocking et al. (1989),Surucu et al. (1995) have modeled the e�ects ofincluding such distorted re¯ectors using computer tech-niques and have concluded that even if such re¯ectorsexist they will not produce biases in long-term averagewinds. They may produce errors on short time-scales (afew minutes) if they exist, but Hocking et al. (1989) haveindicated that even this is likely to be rare.

The most complete models for these specular re¯ec-tors are those due to Bolgiano (1968) and Hocking et al.(1991). As already noted, earlier models argued that thespecular re¯ectors could actually be due to gravitywaves themselves (e.g. Hines and Rao, 1968), butHocking et al. (1989) have argued that this wouldrequire waves which would in fact be heavily viscouslydamped and therefore, in order to produce appropriatevertical wavelengths, would have signi®cant o�-verticaltilts. This e�ect has not been observed. Thus we wouldargue that these proposed mechanisms are not impor-tant in a�ecting gravity wave measurements. There is,however, another argument which invokes gravity wavecontamination of the wind measurements produced byradars, and this proposal has much greater viability. Itrelates to the distribution of scatterers in space and time.

3.4 Temporal and spatial intermittencyof the scattering irregularities

It seems fairly well established that turbulence, espe-cially at these upper altitudes, can be and is likely to beintermittent (e.g. Hocking, 1991, 1996; Hines, 1991).These patches of turbulence have been called ``whitecaps'', and various numerical simulations of this``patchiness'' have been presented (e.g. Desaubies andSmith, 1982; Hocking, 1991; Fairall et al., 1991). Somehigh-resolution con®rmation of this ``white caps'' modelhas been provided by Sica and Thorsley (1996). As aconsequence, patches of turbulence (and thereforeradio-wave scatterers) occur at only isolated locationswithin the radar volume, thereby allowing the possibilitythat the ``wind'' measured by a radar at any instant maynot represent an average over the whole volume, butmay in fact simply represent the winds at one or twodiscrete points within the volume. The issue thenbecomes: How well do the measurements at one ortwo discrete regions in a large radar volume representthe mean wind in that region? Figure 6 illustrates thetypes of problems involved, and the answer to thisquestion now needs to be discussed.

If we begin by assuming that wind measurementsmade at any point do truly represent the wind at thatpoint in space and time, even though the measurement is

biased to that region, then we might expect that over areasonably long-term average the average of all thesemeasurements should still produce a reasonable repre-sentation of the real wind. But can we assume that anysuch measurement really does represent the wind even atthe location of the scatterer? And even if this is so, canwe assume that the averaging process is unbiased?

We shall tackle the ®rst question ®rst. Naively, onemight expect that the wind measurements really dore¯ect the real wind, but there have been argumentspresented which disagree with this supposition. It has infact been proposed that the measured wind velocitiesmay be more commensurate with the phase velocity of agenerating gravity wave than with the winds actuallygenerated in that region of space (the latter would be acombination of the external wind ®eld plus the e�ect ofthe winds due to the gravity wave itself ). This modelassumes that the radar ``sees'' the envelope of theturbulent patch and measures the drift of that envelope.Is this true? In fact it is not; in reality the radar ``sees''the scatterers inside the envelope (the eddies with scalesof the order of half a wavelength). Croft (1972) hasextensively discussed this potential con¯ict between theroles of the motion of the envelope and the motions ofthe internal scatterers, and despite some exceptionsconcluded that in general Doppler radars really domeasure the motions insider the envelope of the turbu-lence, rather than the motions of the envelope itself.Since these internal eddies are driven by the local wind,which will be due to both local winds and the gravitywave's own velocity ®eld, their motions are trulyrepresentative of the winds at this point in time. Thuswe recognize that our original ``naive'' picture of theradar measurements was in fact correct, and in mostcircumstances the radar does measure a real wind. Thisis also the generally accepted view within the radarcommunity, i.e. that the radar measures the true windbecause it measures the eddies moving inside theturbulent region. Nevertheless, we now recognize thatthis conclusion is not as trivial as we might have at ®rstthought, and some degree of caution still needs to beexercised in this regard.

Pulselength

Pulse length

a b

Fig. 6a, b. Schematic representations of the typical distribution ofscattering centers in the atmosphere. The spatial intermittency withinthe beams are especially highlighted

1118 W. K. Hocking: Strengths and limitations of MST radar measurements of middle-atmosphere winds

We now turn to the second question, viz., can weassume that the averaging process is unbiased? We canbegin by recognizing that there are occasions duringwhich the averaging process certainly is biased. Forexample Collis et al. (1992), and May et al. (1988),showed examples in which, because the turbulenceoccurs at preferred positions within the radar beam,biases of wind measurement relative to the true meancan be produced. In particular Collis et al. (1992) showhow a single turbulent layer can produce arti®cial windshears in radar data. This possibility was also recognizedby Hocking (1983). But could such biases be morewidespread? Hines (1991) and Nastrom and Van Zandt(1994, 1996) have independently proposed that this is areal possibility. Their rationale begins by recognizingthat gravity waves may preferentially generate turbu-lence at certain phases of their cycles, and so as aconsequence the wind speeds measured by the radar maybe preferentially selective. For example, suppose (as isoften likely) that a gravity wave breaks at phases of thewave during which the total wind velocity (backgroundplus wave velocities) is in the same general direction asthe phase speed of the wave. If one then averages overan ensemble of such measurements, it is possible thatone may produce a bias towards measurements whichare broadly in the direction of the wave speed simplybecause one is only measuring wind speeds at points inspace and time which have directions commensuratewith the original phase speed of the wave.

Experimental evidence of a similar e�ect has alreadyappeared with regard to vertical motions. Nastrom andVan Zandt (1994) have used experimental data to showthat preferential selection of downward velocities in theatmosphere by radars in the presence of gravity waves isa real e�ect, even in the troposphere. Hoppe and Fritts(1995) have shown similar e�ects in the mesopauseregion of the atmosphere. This downward bias, al-though slight, arises because scatterers are generatedpreferentially during downward motion rather thanother motions within the gravity wave.

Hines' (1991) original proposal was, however, interms of biases in horizontal motions, rather thanvertical velocities. One might expect that there shouldbe no such e�ect if the gravity wave ®eld were isotropic,since on average all these e�ects should cancel out.However, the e�ect can become important if the gravitywave ®eld is anisotropic; then one can produce hori-zontal biases in the mean wind due to the e�ectsdescribed by Hines (1991). Experimental evidence insupport of this model has been presented by Nastromand Van Zandt (1996) using tropospheric and strato-spheric data, although the e�ect is only slight. Hines hasproposed that the e�ect is more dramatic in the upperatmosphere, where wind variability is much greater. Ifthe root-mean-square horizontal velocities of the gravitywaves are of the order of 20±30 m s)1 at these heights,with a substantial bias of phase propagation in onedirection, then errors in the horizontal velocities of theorder of a few m s)1 can result.

Thus we have to admit that this is a very real (evenlikely) possibility. It is potentially the most serious error

of any which we have discussed, and whilst errors of afew m s)1 may not always be of concern, they can beimportant on occasions. For example, monthly meanwinds are often only of the order of a few m s)1, so thetype of error discussed here could be important for suchdeterminations. The relative importance of this e�ect isstill yet to be properly determined, but it de®nitely needsto be investigated further.

3.5 E-region contamination

There is one other important source of wind bias whichhas not been discussed very extensively, if at all, in theliterature. This relates exclusively to MF and HF radarmeasurements above 90 km. It is well known that thefact that the E region overlies the upper middle atmo-sphere can lead to contamination of some of the dataaround 95 km due to group retarded E-region echoes(e.g. Namboothiri et al., 1993). However, there is anadditional e�ect which is not so widely known orunderstood, but which can at times a�ect wind mea-surements even down to 90 km. In this section we wishto describe this e�ect. It is an e�ect related to the ®nitepulse width of the transmitter signal and the fact that theE-region echoes can be a hundred or even a thousandtimes stronger in amplitude than the D-region echoes.

When a radar pulse transmits up into the atmosphereand returns to the receiver, it peaks some time after itreaches the receiver (Fig. 7a). The delay is only a fewtens of microseconds, but can be equivalent to typically8±10 km in range or more for a typical MF radarreceiver. When the range markers on a system arecalibrated they are set to the peak of the echo, and notthe instant of arrival of the pulse. Thus, as shown inFig. 7b, the front end of the E-region echo can co-existwith the peak of say a 95-km echo. However, the E-region echo is often much stronger than the 95-km echo;the re¯ection coe�cient for an echo in the range 92±95 km might be typically 10)3 (e.g. Hocking, 1987),whilst the E-region echo from say 100-km true altitude istotally re¯ected. Even accounting for absorption, thee�ective re¯ection coe�cient for the E-region echocould easily be 10)1 or more. If the radar pulse afterpassing through the receiver has a half-amplitude half-width of 3 km, so that its amplitude varies with the formexpfÿ�ln 2��zÿ zo�2=�32�g, then at a distance of 8 kmfrom the peak its amplitude is reduced by about 100times compared to the peak. Hence for an E-region echofrom say 103 km, the front end of the E-region echo at95 km will have an e�ective re¯ection coe�cient oftypically 10)3: comparable to the actual atmosphericsignal itself. If the peak of the E-region echo comes froman e�ective altitude delay of 100 km, its leading edgecould even dominate over a 92-km echo. Thus the frontend of the E-region echo can easily exceed the trueatmospheric echoes in the region 92±95 km. As a result,there may be times of day, especially when the E-regionecho is low in altitude and strong (as can happen aroundmidday, especially in summer), that signals from the Eregion could contaminate the real signals re¯ected from

W. K. Hocking: Strengths and limitations of MST radar measurements of middle-atmosphere winds 1119

above 92 km. As a result, ``winds'' determined at theseheights and these times by MF techniques could be amixture of E-region drifts and true winds. It has yet tobe determined just how important this e�ect could be,but preliminary experiments performed by this authorhave shown that at times there can be a signi®cant cross-correlation coe�cient between the raw fading data at 92and 100 km ± an indication that this e�ect does happenat times. The e�ect is unlikely to be important at night-time, but certainly could be important around middayon some days, thereby skewing tidal estimates and dailymean winds. We should also emphasize that ourschematic diagrams in Fig. 7 assume that the pulsereturning form the E region is a square pulse. This ishighly idealized; in reality, even the transmitted pulsewill have a Gaussian shape, and this could spread thediscussed e�ect to even lower heights (although prob-ably not below 90 km).

4 Discussion

All the preceding e�ects become ampli®ed as rangeincreases. The physical beam width becomes larger,while the degree of intermittency tends to increase and ismuch higher in the middle atmosphere than in thetroposphere. There is also the issue of the nature of thegeneration of the scatterers, which seems to be intimate-ly related to the level of gravity wave activity at thealtitude of the scatterers. In particular, in the middleatmosphere the waves reach large amplitudes and alsodevelop a level of anisotropy in preferred directionswhich can lead to some degree of bias.

We need at this point to reiterate that radar workershave been aware of these problems, or at least most ofthese problems, for many years, and, in general, havebeen able either to correct for them or recognize whenthey should not be using their data. In a sense this issomething of a self-critical review of radar methods, andif optical or rocket techniques were subject to a similardegree of inquiry equivalent errors and biases wouldalso be found in those procedures. In particular, one ofthe major weaknesses of rocket and optical techniques isthe fact that they tend to be made only intermittently intime. Rocket measurements are usually dependent oncampaigns which can last for only a week or so, andwhich occur at irregular intervals scattered throughoutthe year. Thus they have the possibility of not being atrue representative of the overall statistical situation,because each rocket pro®le represents only a singlesnapshot through the atmosphere. Optical techniquesare particularly impeded by the occurrence of sunlightwhich clearly restricts them to night-time observation,and also the occurrence of clouds which can preventmeasurements over a substantial fraction of the year.Optical techniques are also subject to the fact that theheight of scatter or the height of the source of theradiation is an unknown quantity. Thus we ask thereader to bear in mind that this attempt to present asomewhat self-critical review of the limitations of radarsshould not be perceived as a reason to rule out theviability of radar techniques. There is no question thatradars have contributed enormous amounts of informa-tion to our understanding of upper atmosphere windsand will continue to do so in the future. However, thissaid, it is certainly true that one must bear in mind thefact that there are some inherent problems which in themain become worse at higher altitudes.

5 Conclusions

The strengths and limitations of radar measurementshave been considered. Strengths include continuity ofdata, day and night-time operation, and the ability tosee through clouds, as well as (often) superior spatialresolution. Various limitations were also discussed, aswell as some earlier proposals which claimed that radardata could often give false winds. Some of the earlierconcepts have been largely disproved, but some prob-

Receiver delay, τ

Received pulse

Input to receiver

Input to receiver

Output from receiver

Output from receiver

Single received pulse

Pulses received from 95 km and E-region

Time

95 km pulse

E-region pulse

τ

τ

95

E

a

b Time

Fig. 7a. Schematic diagram showing the e�ect of passing a pulsethrough a receiver. Note especially the ®nal Gaussian shape anddelayed peak. b Schematic diagram showing the output of a typicalreceiver with two incident pulses, the second slightly delayed from the®rst. The ®rst pulse to arrive also has a much weaker amplitude. Seetext for details

1120 W. K. Hocking: Strengths and limitations of MST radar measurements of middle-atmosphere winds

lems have been recognized. The signi®cance of contam-ination by vertical winds when beam zenithal tilts of 10°or 15° are used, and the importance of the temporal andspatial intermittency of the scatterers, have been high-lighted, as has the fact that radar winds can be biased tosome degree in the presence of an anisotropic gravitywave ®eld. For example, if the gravity waves propagatepredominantly in the eastward direction, some bias ofwinds towards the east can occur. The level of this erroris likely to be substantially less than the RMS gravitywave amplitude, and so would not be expected to bemuch more than perhaps 5 m s)1 or so; however, futureexperiments to examine this e�ect are needed. Finally,we have shown that because the E-region echoes at MFare much stronger than D-region echoes, there is apossibility that the leading edge of the E-region echoescould contaminate MF data down to about 92 km or so.This possibility needs to be further investigated in thefuture.

However, we ®nally recognize that radar techniquesare still a powerful and important technique for upper-atmosphere wind measurements, and will continue to bea major tool in the future.

Acknowledgements. Topical Editor F. Vial thanks U.-P. Hoppe forhis help in evaluating this paper.

References

Adams, G. W., D. P. Edwards, and J. W. Brosnahan, The imagingDoppler interferometer; Data analysis, Radio Sci., 20, 1481±1492, 1985.

Astin, I., and L. Thomas, The accuracy and precision of radar windmeasurements. in STEP handbook, Proc. Fifth WorkshopTechnical and Scienti®c Aspects of MST Radar, Ed. B. Edwards,pp 15±20, 1992.

Atlas, D. (ed), Radar in meteorology: Battan memorial and 40thanniversary, radar meteorology conference, Am. Meteorol. Soc.,Boston, 1990.

Bolgiano, R., Jr. The general theory of turbulence ± turbulence inthe atmosphere, in Winds and turbulence in the stratosphere,mesosphere and ionosphere, Ed. K. Rawer, North-holland,Amsterdam, pp 371±400, 1968.

Briggs, B. H., Radar observations of atmospheric winds and tur-bulence: comparison of techniques, J. Atmos. Terr. Phys., 42,823±833, 1980.

Briggs, B. H., The analysis of spaced sensor records by correlationtechniques, Handb. MAP, 13, 166±186, 1984.

Briggs, B. H., Radar measurements of aspect sensitivity of atmo-spheric scatterers using spaced-antenna correlation techniques,J. Atmos. Terr. Phys., 54, 153±165, 1992.

Burrage, M. D., W. R. Skinner, D. A. Gell, P. B. Hays, A. R.Marshall, D. A. Ortland, A. H. Manson, S. J. Franke, D. C.Fritts, P. Ho�man, C. McLandress, R. Niciejewski, F. J.Schmidlin, G. G. Shepherd, W. Singer, T. Tsuda, and R. A.Vincent, Validation of mesosphere and lower thermospherewinds from the high-resolution Doppler imager on UARS, J.Geophys. Res., 101, 10365±10392, 1996.

Chanin, M. L., and A. Hauchecorne, Lidar observation of gravityand tidal waves in the stratosphere and mesosphere, J. Geophys.Res., 86, 9715±9721, 1981.

Collis, P. N., M. T. Rietveld, J. RoÈ ttger, and W. K. Hocking,Turbulence scattering layers in the middle-mesosphere observedby the Eiscat 224 MHz radar, Radio Sci., 27, 97±107, 1992.

Crane, R. K., Radar measurements of wind at Kwajalein, RadioSci., 15, 383±394, 1980.

Croft, T. A., Sky-wave backscatter: a means of observing ourenvironment at great distances, Rev. Geophys. Space Phys., 10,73±155, 1972.

Desaubies, Y., and W. K. Smith, Statistics of Richardson numberand instability in oceanic internal waves, J. Phys. Oceanogr., 12,1245±1259, 1982.

Fairall, C. W., A. B. White, and D. W. Thomson, A stochasticmodel of gravity-wave-induced clear-air turbulence, J. Atmos.Sci., 48, 1771±1790, 1991.

Farley, D. T., On-line data processing techniques for MST radars,Radio Sci., 20, 1177±1184, 1985.

Fukao, S., T. Sato, T. Tsuda, S. Kato, K. Wakasugi, and T.Makihira, The MU radar with an active phased array system 1.Antenna and power ampli®ers, Radio Sci., 20, 1155±1168,1985a.

Fukao, S., T. Tsuda, T. Sato, S. Kato, K. Wakasugi, and T.Makihira, The MU radar with an active phased array system 2.In-house equipment, Radio Sci., 20, 1169±1176, 1985b.

Gardner, C. S., and D. G. Voelz, Lidar measurements of gravitywave saturation e�ects in the sodium layer, Geophys. Res. Lett.,12, 765±768, 1985.

Gault, W. A., G. Thuillier, G. G. Shepherd, S. P. Zhang, R. H.Wiens, W. E. Ward, C. Tai, B. H. Solheim, Y. J. Rochon, C.McLandress, C. Lathuillere, V. Fauliot, M. Herse, C. H.Hersom, R. Gattinger, L. Bourg, M. D. Burrage, S. J. Franke,G. Hernandez, A. Manson, R. Niciejewski, and R. A. Vincent,Validation of O(S) wind measurements by WINDII: The windimaging interferometer on UARS, J. Geophys. Res., 101, 10405±10430, 1996.

Hays, P. B., V. J. Abreu, M. E. Dobbs, D. A. Gell, H. J. Grassl, andW. R. Skinner, The high-resolution Doppler imager on theupper-atmosphere research satellite, J. Geophys. Res., 98,10713±10723, 1993.

Hines, C. O., Internal atmospheric gravity waves of ionosphericheights, Can. J. Phys., 38, 1441±1481, 1960.

Hines, C. O., The saturation of gravity waves in the middleatmosphere. Part III. Formation of the turbopause and ofturbulent layers beneath it, J. Atmos. Sci., 48, 1380±1385, 1991.

Hines, C. O., and R. Rhagava Rao, Validity of three stationmethods of determining ionospheric motions, J. Atmos. Terr.Phys., 30, 979±993, 1968.

Hines, C. O., G. W. Adams, J. W. Brosnahan, F. T. Djuth, M. P.Sulzer, C. A. Tepley, and J. S. Van Baelen, Multi-instrumentobservations of mesospheric motions over Arecibo: compari-sons and interpretations, J. Atmos. Terr. Phys., 55, 241±287,1993.

Hocking, W. K., On the extraction of atmospheric turbulenceparameters from radar backscatter Doppler spectra-I: theory, J.Atmos. Terr. Phys., 45, 89±102, 1983.

Hocking, W. K., Radar studies of small-scale structure in the uppermiddle atmosphere and lower ionosphere, Adv Space Res., 7,327±338, 1987.

Hocking, W. K., Target parameter estimation, Middle AtmosphereProgram Handbook, SCOSTEP Secretariat, Dept. of Electr.Computer Eng., Univ of Illinois, Urbana, IL 61801, USA, 30,228±268, 1989.

Hocking, W. K., The e�ects of middle-atmosphere turbulence oncoupling between atmospheric regions, J. Geomag. Geoelectr.,43 (Suppl.), 621±636, 1991.

Hocking, W. K., Dynamical coupling processes between the middleatmosphere and lower ionosphere, J. Atmos. Terr. Phys., 58,735±752, 1996.

Hocking, W. K., System design, signal processing procedures andpreliminary results for CLOVAR, The University of WesternOntario VHF Atmospheric Radar, Radio Sci., in press, 1997.

Hocking, W. K., and A. M. Hamza, A quantitative measure of thedegree of anisotropy of turbulence in terms of atmosphericparameters, with particular relevance to radar studies, J. Atmos.Terr. Phys., in press, 1997.

Hocking, W. K., and T. Thayaparan, Simultaneous and co-locatedobservation of winds and tides by MF and Meteor radars over

W. K. Hocking: Strengths and limitations of MST radar measurements of middle-atmosphere winds 1121

London, Canada, (43°N, 81°W) during 1994±1996, Radio Sci.,in press, 1997.

Hocking, W. K., P. T. May, and J. RoÈ ttger, Interpretation,reliability and accuracies of parameters deduced by the spaced-antenna method in middle atmosphere applications, Pure Appl.Geophys., 130, 571±604, 1989.

Hocking, W. K., S. Fukao, T. Tsuda, M. Yamamoto, T. Sato, and S.Kato, Aspect sensitivity of stratospheric VHF radiowavescatterers, particularly above 15 km altitude, Radio Sci., 25,613±627, 1990.

Hocking, W. K., S. Fukao, M. Yamamoto, T. Tsuda, and S. Kato,Viscosity waves and thermal-conduction waves as a cause of`specular' re¯ectors in radar studies of the atmosphere, RadioSci., 26, 1281±1303, 1991.

Hoppe, U.-P., and D. C. Fritts, On the downward bias in verticalvelocity measurements by VHF radars, Geophys. Res. Lett., 22,619±622, 1995.

Kudeki, E., P. K. Rastogi, and F. Surucu, Systematic errors in radarwind estimation: implications for comparative measurements,Radio Sci., 28, 169±179, 1993.

Larsen, M. F., and J. RoÈ ttger, VHF and UHF Doppler radars astools for synoptic research, Bull. Am. Meteorol. Soc., 63, 996±1008, 1982.

Larsen, M. F., and J. RoÈ ttger, VHF radar measurements of in-beam incidence angles and associated vertical-beam radialvelocity corrections, J. Atmos Oceanic Technol., 8, 477±490,1991.

Larsen, M. F., R. D. Palmer, S. Fukao, R. F. Woodman, M.Yamamoto, T. Tsuda, and S. Kato, An analysis technique forderiving vector winds and in-beam incidence angles from radarinterferometer measurements, J. Atmos. Oceanic Technol., 9, 3±14, 1992.

Lesicar, D., and W. K. Hocking, Studies of seasonal behaviour ofthe shape of mesospheric scatterers using a 1.98-MHz radar, J.Atmos. Terr. Phys., 54, 295±309, 1992.

Lesicar, D., W. K. Hocking, and R. A. Vincent, Comparativestudies of scatterers observed by MF radars in the southernhemisphere mesosphere, J. Atmos. Terr. Phys., 56, 581±591,1994.

Luce, H., M. Crochet, F. Dalaudier, C. Sidi, Interpretation of VHFST radar vertical echoes from in situ temperature sheetobservation, Radio Sci., 30, 1003±1025, 1995.

Mathews, J. D., Incoherent-scatter radar studies of the meso-sphere, Handb. MAP, 13, 135±154, 1984.

May, P. T., S. Fukao, T. Tsuda, T. Sato, and S. Kato, The e�ect ofthin scattering layers on the determination of wind by Dopplerradars, Radio Sci., 23, 83±94, 1988.

McLandress, C., G. G. Shepherd, B. H. Solheim, M. D. Burrage, P.B. Hays, and W. R. Skinner, Combined mesosphere/thermo-sphere winds using WINDII and HRDI data from the UpperAtmosphere Research Satellite, J. Geophys. Res., 101, 10441±10454, 1996.

Namboothiri, S. P., A. H. Manson, and C. E. Meek, E-region realheights and their implications for MF radar-derived wind andtidal climatologies, Radio Sci., 28, 187±202, 1993.

Nastrom, G. D., and T. E. Van Zandt,Mean Vertical Motions seenby Radar Wind Pro®lers, J. Appl. Meterorol., 33, 984±995,1994.

Nastrom, G. D., and T. E. Van Zandt, Biases due to gravity wavesin wind pro®ler measurements of winds, J. Appl. Meteorol., 35,243±257, 1996.

Phillips, A., A. H. Manson, C. E. Meek, and E. J. Llewellyn, Along-term comparison of middle-atmosphere winds measured atSaskatoon (52°N, 107°W) by a medium-frequency radar and aFabry-Perot interferometer, J. Geophys. Res., 99, 12923±12936,1994.

RoÈ ttger, J., Investigations of lower and middle atmospheredynamics with spaced-antenna drift radars, J. Atmos. Terr.Phys., 43, 277±292, 1981.

RoÈ ttger, J., and H. M. Ierkic, Postset beam steering and interfer-ometer applications of VHF radars to study winds, waves, andturbulence in the lower and middle atmosphere, Radio Sci., 20,1461±1480, 1985.

RoÈ ttger, J., and R. A. Vincent, VHF radar studies of troposphericvelocities and irregularities using spaced-antenna techniques,Geophys. Res. Lett., 5, 917±920, 1978.

RoÈ ttger, J., J. Klostermeyer, P. Czechowsky, R. Ruester, and G.Schmidt, Remote sensing of the atmosphere by VHF radarexperiments, Naturwissenschaften, 65, 285±296, 1978.

RoÈ ttger, J., C. H. Liu, J. K. Chao, A. J. Chen, C. J. Pan, and I. J.Fu, Spatial interferometer measurements with the Chung-LiVHF radar, Radio Sci., 25, 503±515, 1990.

Royrvik, O., Spaced-antenna drift at Jicamarca, mesosphericmeasurements, Radio Sci., 18, 461±476, 1983.

Shepherd, G. G., G. Thuillir, W. A. Gault, B. H. Solheim, C.Hersom, J. M. Alunni, J.-F. Brun, S. Brune, P. Charlot, L. L.Cogger, D.-L. Desaulniers, W. F. J. Evans, R. L. Gattinger, F.Girod, D. Harvie, R. H. Hum, D. J. W. Kendall, E. J. Llewellyn,R. P. Lower, J. Ohrt, F. Pasternak, O. Peillet, I. Powell, Y.Rochon, W. E. Ward, R. H. Wiens, and J. Wimperis, WINDII,the wind imaging interferometer on the upper- atmosphereresearch satellite, J. Geophys. Res., 98, 10725±10750, 1993.

Sica, R. J., and M. D. Thorsley, Measurements of superadiabaticlapse rates in the middle atmosphere, Geophys. Res. Lett., 23,2797±2800, 1996.

Stubbs, T. J., The measurement of winds in the D-region of theionosphere by the use of partially re¯ected radio waves, J.Atmos. Terr. Phys., 35, 909±919, 1973.

Stubbs, T. J., and R. A. Vincent, Studies of D-region drifts duringthe winters of 1970±72, Aust. J. Phys., 26, 645±660, 1973.

Surucu, F., S. J. Franke, and E. Kudeki, On the in¯uence ofspecular re¯ections in MF radar wind measurements, RadioSci., 30, 1229±1244, 1995.

Tsuda, T., T. Sato, K. Hirose, S. Fukao, and S. Kato, MU radarobservations of the aspect sensitivity of backscattered VHFecho power in the troposphere and lower stratosphere, RadioSci., 21, 971±980, 1986.

Turek, R. S., K. L. Miller, R. G. Roper., and J. W. Brosnathan,Mesospheric wind studies during AIDA Act 89: morphologyand comparison of various technique, J. Atmos. Terr. Phys., 57,1321±1343, 1995.

Vincent, R. A., and T. J. Stubbs, A study of motions in the wintermesosphere using the partial re¯ection drift technique, Planet.Space Sci., 25, 441±455, 1977.

Vincent, R. A., T. J. Stubbs, R. H. O. Pearson, K. H. Lloyd, and C.H. Low, A comparison of partial re¯ection drifts with windsdetermined by rocket techniques ± I, J. Atmos. Terr. Phys., 39,813±821, 1977.

Vincent, R. A., P. May, W. K. Hocking, W. G. Elford, B. Candy,and B. H. Briggs, ``First results with the Adelaide VHF radar:spaced-antenna studies of tropospheric winds, J. Atmos. Terr.Phys., 49, 353±366, 1987.

Weber, B. L., and D. B. Wuertz, Comparisons of rawinsonde andwindpro®ler radar measurements, J. Atmos. Oceanic Technol.,7, 157±174, 1990.

Woodman, R. F., Spectral moment estimation in MST radars,Radio Sci., 20, 1185±1195, 1985.

Woodman, R. F., and Y-H Chu, Aspect sensitivity measurements ofVHF backscatter made with the Chung-LI radar: plausiblemechanisms, Radio Sci., 24, 113±125, 1989.

Woodman, R. F., and A. Guillen, Radar observations of winds andturbulence in the stratosphere and mesosphere, J. Atmos. Sci.,31, 493±505, 1974.

Zrnic, D. S., Estimation of spectral moments for weather echoes,IEEE Trans. Geosci. Electron., GE-17, 113±128, 1979.

1122 W. K. Hocking: Strengths and limitations of MST radar measurements of middle-atmosphere winds