Medium scale gravity waves in the ionospheric F-region and their possible origin in weather disturbances

Planet.

Space

Sci. 1975, Vol.

23, pp. 493 to 507.

Persamon

Press.

Printed

in Northern

Imhnd

MEDIUM SCALE GRAVITY WAVES IN THE IONOSPHERIC F-REGION AND THEIR POSSIBLE ORIGIN IN WEATHER DISTURBANCES F. BERTIN and J. TESTUD Centre National D’Etudes des Tel~ommu~~tio~, 38-40 Rue du General Leclerc, 92 ~y-l~-~ou~n~ux, France and

L. KERSLEY Department of Physics, University College of Wales, Aberystwyth, Wales (Received injkaiform

1 J&y 1974)

Abs&ae&--Thisstudy results Eiom a coordinated experiment involvingionosphericobservations of Faraday rotation between a geostationary satellite and three ground basedreceivers at Aberystwythand Bournemouth in the U.K. and Lannion, France, together with incoherent scatter observationsat St. Santin-Nancay,France. Quasi-periodicvariations of electron content observedsimultaneouslyat the three stations are interpreted in terms of medium scale gravity wavestravellingin the ionosphericF-region. Characteristicsof these wavesare derived by means of a cross-correlationtechnique. A reverse ray tracing computation, using data on the neutral atmosphere and neutral wind stratification from the incoherent scatter observations, has been used m an attempt to locate the sources of these waves. The results show that some of the waves are almost certainly generated above 100 km altitude, probably by aurora1 phenomena, while the others could be produced near ground level by mekorological sources. The reverse ray tracing indicates that the latter sources are

in general located in a geographicarea in the vicinity of a weather disturbance. A production rn~~~~ for these waves is proposed involving ag~s~op~c ~rt~bat~o~ of the neutral wind in a jet stream. 1. INTRODUCTION Observations of travel@ ionospheric disturbances made by many workers using a variety of different techniques have often been interpreted in terms of atmospheric waves but identification of the sources of these waves has always been a difficult problem. Georges (1967) has proposed a cl~sification of these quasi-periodic dis~rban~s into three types: (a) Disturbances of very short period, of the order of several minutes, like those observed by Rao et al. (1969), correspond to acoustic waves. In certain instances, thunderstorms under the observing region have been identified as the source of the waves (Baker and Davies, 1969). (b) Perturbations in the middle category show the characteristics of gravity waves with periods of the order of 20 min. Their speeds and directions of propagation show a seasonal variation (Mum-o, 1958) and their occurrence is independent of magnetic activity. It has been suggested that these waves originate in meteorological phenomena with a seasonal variation, like the jet stream (Munro, 1958). In practice the precise determination of the sources of waves in this class has proved difficult and no decisive results appear to have been reported. Cowling et al. (1971) have shown that the propagation of these waves from their source location in the troposphere to .their point of observation at #+-region heights is strongly influenced by the neutral winds in the high atmosphere which act as a directional filter on the waves. (c) The large scale perturbations, with periods greater than 90 min and horizontal velocities of several hundred metres per second, likewise show characteristics of gravity 493

494

F. BERTIN, J. TESTUD and L. KERSLEY

waves. The occurrence of these waves correlates with magnetic activity and their direction of propagation has been found to be from high to low latitudes. These factors have given rise to suggestions that the source of these large scale waves is associated with aurora1 magnetic activity and the precipitation of energetic particles or the aurora1 electrojet (Georges, 1967). This link with aurora1 magnetic activity would now appear to have been clearly established by the statistical work of Davis and da Rosa (1969) and the individual case studies of Testud (1973). It must be appreciated that this classification is of a statistical nature and caution must be exercised in the analysis of any actually observed ionospheric disturbances. For example, it is possible that waves with a period greater than 90 min may have been produced by a source of meteorological origin which is far removed from the place of observation. The identification of possible sources of actually observed waves is difficult to treat by purely statistical analysis. The uncertainty in the actual mechanism of generation, the time delay between the formation of waves and their detection in the high atmosphere and the geographic separation between the regions of formation and observation are all factors which mitigate against statistical correlation. It is thus only in a detailed study of individual cases that positive identification of the sources is possible. The present work has been concerned with a study of ionospheric perturbations of medium scale, using two complementary techniques: (a) Faraday rotation observations using a geostationary satellite and three spaced receivers. This technique has been used to provide information on the period, horizontal phase speed and azimuth of horizontal propagation of the travelling ionospheric disturbances. (b) Incoherent scatter observations of ionic temperature and velocity have been used to determine the neutral temperature and horizontal neutral wind. Thus combining the available information from the two techniques we obtain not only the characteristics of the observed gravity wave but also a description of the neutral wind which is known to affect the propagation of the wave. From a coordinated experiment involving these two techniques, we have attempted to identify possible source locations for individual gravity waves observed during the period 22-31 January 1972. It has proved possible to differentiate between waves of aurora1 and meteorological origins and in the case of the latter certain common characteristics have been identified, allowing a suggestion to be advanced concerning the formation process of this type of wave. 2.

EXPERIMENTAL OBSERVATIONS

2.1 Faraday rotation The simultaneous recording at three receiving stations of the Faraday rotation of the 136 MHz signals from the geostationary satellite Intelsat IIF has enabled observations to be made of quasi-periodic perturbations in the ionosphere. These perturbations can be interpreted in terms of waves in the neutral atmosphere. The Faraday rotation is proportional to the total electron content so that the technique has greatest sensitivity to perturbations in electron density near the peak of the F-layer of the ionosphere. The data used in this study were obtained during daytime in winter (January 1972) when both the height of the layer peak and the thickness of the profile are at a minimum. Thus for the purposes of the present work the region of observation in which the waves have been detected by the Faraday rotation technique has been considered to be centred at an altitude of 250 km.

IONOSPHERIC

GRAVITY

WAVES

AND

WEATHER

DISTURBANCES

495

FIG. 1. GEOGRAPHICPOSITIONOFTHE250 km SUB-IONOSPHERIC POINTSFORTHETHREESTATIONS: ABERYsTwYTH, Boummtoum AND LANNION.

The three receiving stations were situated at Aberystwyth (52*4”N, 4*O”W), Bournemouth (50*7”N, 1*8’W) and Lannion (48_8”N, 3.5”W). Figure 1 shows the geographic positions of three sub-ionospheric points at the 250 km level. The observational period for the Faraday rotation measurements was chosen to correspond to the schedule of continuous operation of the incoherent scatter system at St. Santin (44~6”N, 2*2”E). The quasi-periodic oscillations observed on the continuous recordings of electron content at the three stations are isolated from the diurnal variations of this parameter by means of a numerical filter. One obtains three times series X,, X’ and XL (each with one point per minute) which represent the observed ionospheric perturbations at Aberystwyth, Bournemouth and Lannion respectively. The auto and cross correlation functions CAA, Csa, CL, and CAB, CBL, CL, respectively are calculated and their Fourier transforms give the three auto power spectra PAd, PBB, PLL and the three cross power spectra PAB, PBL and PLA. The presence of maxima corresponding to the same frequency in the three spectra shows the existence of waves or wave trains for which the horizontal phase velocity and the direction of propagation can be determined. The validity of this interpretation can be tested by examining the coefficient of coherence defined by pAB = PdB2/PggPBB for stations A and B. When the coherence reaches unity, one can be sure that the same phenomenon is observed by both the stations A and B, while a value near zero indicates that the comparison between A and B is not significant. Thus care must be taken to ensure that the three coefficients of coherence pAB, pBL, pLA are sufficiently close to magnitude unity to be confident that the same waves have been observed by the three receiving stations.

496

F. BERTIN, J. TESTUD and L. KBRSLEY

2.2 Incoherentscatter Four main parameters are derived from the incoherent scatter spectra measured at St. Santin-Nancay : electron density iV*, ion and electron temperatures Ti, T, and one component of ion velocity nearly parallel to the magnetic field V,,,. Analysis of these data allows the determination of several parameters of the neutral atmosphere over a wide range of altitudes (100-500 km). The neutral temperature and neutral atomic oxygen density can be obtained by means of the method of Bauer et al. (1970), and the meridional component of the neutral wind can be derived directly from Vrlr from an evaluation of the diffusion velocity based on Ti, T,, IV,data (Vasseur, 1971). Further, the two components (meridional and zonal) of the neutral wind can be determined by resolving the equations of horizontal motion for the neutral atmosphere. In this resolution, the Jacchia model of pressure is used and the T,, T,, IV, data actually observed at Santin are introduced (Amayenc and Vasseur, 1972). loo

lz,

-

/

-

1’

?’

-100

-

I

i

I

/ : FIG. 2(a). NEUTRAL WIND DETERMINATIONAT 25Okm ALTITUDE ON 26 JANUARY 1972. The meridional and zonal components as determined by the method of Amayenc and Vasseur (see text) are denoted by the dotted lines labelled Ml and Z, respectively. The full line denoted Ma corresponds to the meridional component of the neutral wind evaluated directly from the incoherent scatter data.

Figure 2(a) illustrates an example of neutral wind determination at the 250 km altitude. The diurnal evolution of the zonal and meridional components (dotted lines denoted by Z, and Ml respectively) determined following the method of Amayenc and Vasseur is compared to the meridional component directly derived from V,,, measurements (full line with error bars marked Ma). The two curves for the meridional component (M1 and M,) show a broad general agreement, particularly when it is noted that the oscillations with 6 hr period apparent in curve Ma are not represented in the model yielding the curve MP 2.3 Results Table 1 shows the results of the on three days with several different the measured phase velocities are F-region heights attain speeds of the

analysis of the Faraday rotation observations obtained data windows. Apart from the results for 30 January, all less than 200 msec-l. Since the neutral winds at order of 100 msec-l it is to be expected that the presence

IONOSPHERIC GRAVITY WAVES AND WEATHER TABLE 1. PROPA~ATIONCXLUACI-ERISTICSOPTHBOBSERVED ANDTIMEINTERVALS

Date 26.1.72

Time 0933-1433 1033-1533

29.1.72

1133-1633 1343-1643 1000-1500

30.1.72

0900-1700

DISTURBANCES

WAVESFORDIFFERRNTDAYS

Period (mm)

Speed (msec-I)

Aziiuth (de&

58

164

228 135 135 187 226 130 135 174 260 162 152 120 135 158

:: 76 59 48 20 77 22 98 57 32 190 109

497

1;; 1;: 105 125

iI

149

156 160 358 280

Relative spectral

power (%I 30 25 2 20 :: 40 :x 40 6: 10

of neutral winds is likely to affect markedly the character of waves traversing this region. It should also be noted that all the azimuths of propagation listed in Table 1 belong to the same sector, the general direction of all the waves being from north to south, and it is known that the neutral winds in the ionosphere are directed polewards during daytime so that it would appear that the waves observed were travelling in a general direction opposite to that of the neutral wind. Strong cormrmation of this conclusion is provided by considering the waves with a period of about 20 min which were observed on 26 January and whose propagation characteristics are listed in Table 2. Figure 2(b) represents again the horizontal wind deduced by TABLE 2. PROPAQATIONCHARACTER.ISICSOFTHESHORTPERIODWAVESFOR DWFERENTTIMEINTERVALSON~~JANUARY 1972

Time 1043-1343 1143-1443 1243-1543 1343-1643 1443-1743

Period (mm)

Speed (mse&)

Azimuth (de@

20 20

127 125 No correlation 92 83

116 135

22 22

the method of Amayenc and Vasseur (the resultant of the component curves Ml and 2, of Fig. 2(a)) but now plotted on a polar diagram. The two arrows show the directions of propagation of the waves with periods of about 20 min in the morning and afternoon respectively. One can see that the gravity waves are being propagated in a direction essentially opposite to that of the neutral winds. Furthermore, for the data window centred on 1400 hr for which no correlation was obtained between the data from the three stations (Table 2), it is evident that this window corresponds to a time interval when the actual wind was considerably weakened (Fig. 2(a) and curve M,) and in fact was reduced to an almost zero value at 1400 hr.

F. BERTIN, J. TBSTUD and L. KJZRSLEY

498

270'

90.

6

Fro. 2(b). HODO~RAPH OFTHENEUTRAL WINDAT250 km ALTITUDE ONTHE26 JANUARY1972. This hodogaph relates the meridional and zonal components M1 and 2, of Fig. 2(a). The two arrows show the direction of propagation of the waves with periods of about 20 min observed in the morning and afternoon of the same day.

It would thus appear that only the waves which are being propagated against the neutral wind are able to reach an altitude where they are detected. This confirms ‘the result of Cowling et al. (1971) who claimed that the neutral wind acts like a directional filter for medium scale gravity waves. In the next section we study this effect quantitatively by means of reverse ray tracing calculations. 3.

CALCULATION

OF THE REVERSE PATH

3.1 Theory of atmospheric waves The effect of a gravity wave on the ionosphere can be treated as a perturbation the total electron content Q so that we can write

(61) in

81 I N exp (iwt - ik,x - ik,z),

where o is the pulsatance of the wave, k, the horizontal wave number and k, the vertical wave number. The parameters are related in the dispersion equation (Hines, 1960) which for an isothermal atmosphere has the form: k”t=($

l)k”-;($-

l),

(1)

where wV= (r - I)l/* g/c and w, = yg/2c, with c the velocity of sound, y the ratio of specific heats, g the acceleration due to gravity and wp and W, two characteristic frequencies. 3.2 Neutral winds/and atmospheric waves In a horizontally stratified atmosphere, the horizontal neutral winds impar at Doppler shift to the frequency of atmospheric waves. The horizontal wave number remains invariant, but the vertical wave number changes as the wave propagates from layer to layer (Cowling et al., 1971).

IONOSPHERIC

GRAVITY

WAVES ANJJ WEATHER

DISTURBANCES

For a horizontal wind of velocity W, applying a transformation moving with the wind one obtains: 0

I

=o--

499

into a reference frame

k.

(2)

‘.

k’ = k

Consideration of the dispersion equation in the moving frame of reference yields expressions for the group velocity of the waves &‘k ’ I VPB=:-(3) c*(k,” + k,‘%) ---O2oP + waB’ V’

Finally, ~ansfo~g

IpI =

(%JB- d2)c2kz [ca(ke2+ k,‘*) - 2~0’~+ 0210

’

(4)

back to the fixed reference frame the group velocity becomes

vss= v’** VgpI

=

v’ga: +

I

w

(5)

.

By means of integration it is then possible to compute the group ray trajectory of the wave. In this method the neutral wind is treated as constant in each slab of the atmosphere considered and the dispersion equation used does not take into account the vertical gradients of the winds which are known to exist in the case of wind shears over short distances. However, Hines and Reddy (1967) have maintained that wind shears do not appreciably attenuate the energy of the waves when the phase speed is greater than 100 msec-l. It is possible to calculate the influence of wave shears on the characteristics of the waves traversing them. The details of the calculation will be given in a later paper by Bertin and Testud. In summary, one finds that the presence of a vertical gradient of the horizontal wind i?WjiTz introduces an additional term to the vertical wave number of the form Ak.=-&

(

k=.g

)

.

This expression is imaginary, co~espon~ng to an attenuation or an ~plifi~tion of the wave depending on the sign of the wind gradient. In a wind shear, the wind gradient can be considered to be of essentially equal magnitude but opposite sign in the lower and upper parts of the sheared region. Thus from Equation (6) one may conclude that in traversing a wind shear a wave may lose part of its energy by means of reflection, refraction, or indeed even by a transformation to turbulent motion if the increase in k, is very great, but, on emerging from the shear the propa~tion parameters of the wave will be essentially the same as on entry. That is, the propagation parameters have not been altered, only the energy may have been diminished. We can now comment further on the case of the waves with approximately 20 min periods observed on 26 January. These waves have a pulsatance cu - 5 x lO-* set-l and a horizontal wave number k, - 3 x lad m-l. In the absence of a neutral wind, using the asymptotic appro~mation k, = (~~~~) k, (Hines, 1960), the gradient of the trajectory at the observing altitude of 250 km wiIl be

F. BERTIN, J. TIZ.STUDand L. KERSIZY

500

If, by contrast, we take a horizontally stratified wind profife with veIocity zero at the ground and increasing with height to a value of 100 msec-l at 200 km then the apparent pulsatance in the frame of reference moving with the wind will be a function of altitude and can be calculated from Equation (2). Two cases are possible: (i) The wave is propagating in a direction opposite from that of the wind so that the group path steepens with height until for a wind of IO0 msec-l its gradient will be practically double the value quoted above for the case of no neutral wind. (ii) For the wave propagating in the same direction as the neutral wind the gradient decreases with altitude and tends towards zero at an altitude around 200 km where the wave becomes trapped and unable to penetrate to higher altitudes. The above approximate calculation may explain why all the observed waves with 20 min period were being propagated in a direction contrary to the neutral wind. The wind acts as a filter and the decrease in the wind velocity around 1400 hr on 26 January (Fig. 2(a), curve Mz) would appear to have resulted in the temporary removal of the filtering effect. Thus waves originating from severa different sources may have interfered in the observing region at this time and as a result no correlation is obtained in the analysis (Table 2) even though short period waves are observed in the data from individual stations. 3.3 Ap~~icaZ~on of the ray tracing method COthe observed waves The calculation of tbe trajectory of the observed waves is carried out using values of pulsatance, horizontal wave number and direction of propagation computed from the parameters in Table I. The values of o,, o, and the velocity of sound are calculated for each altitude from the parameters of the neutral atmosphere measured simultaneously by the St. Santin incoherent scatter facility. The profiles of the neutral winds are formed using data from several sources. Above 100 km, the profiles are obtained by the method already described in Section 2.2 above. At lower altitudes, the meteor radar data from I

North

1

I

-80

I

-40

0

I

I

South

40

msec-I h3.

3(a). VECRTICAL PROF~

OF THE MERIDYONAL axmm!NT OF m N~JTRAL WTND ON 26 JANUARY1972 XNTHB MCMWINO AROUNDIO U.T. The full line with the trianglea corresponda to tbc wind calculRtal by the method of Amayenc and Vasseur (above 100 km height). Thmc different interpolations arc shown between the meteorologicaldata (O-30 km) and the prosle above X00km.

IONOSPHERIC

GRAVlTY

WAVES AND

WEATHER

DISTURBANCES

501

(b)

West

,

I

I

-40

0

I

,

East

40

msec-’ Fro. 3(b). VERTICAL PROFILE OF THE ZONAL COMPONENT OF THE NEUTRAL WIND ON 26 JANUARY 1972 IN THE MORNINO. The same notation as in Fig. 3(a) is used. The black triangle corre-

sponds to the wind measured at 80 km altitude by the meteor radar of CNET at Garchy (France). (The meteor radar yields only the zonal component of the wind.)

CNET (Spizzichino, private communication) is used around 80 km, and between ground and 30 km the measurements are from meteorological radiosondes. Between 30 and 80 km no data were available, so that it was necessary to interpolate the wind profile. This interpolation has been performed taking into account the experimental results of Labitzke (1972) which showed that the direction of the neutral wind is nearly constant between 30 and 60 km, and the maximum wind speed occurs around 50 km altitude. An example giving the components of the wind profile determined by this method is shown in Figs. 3(a) and (b). The validity of this interpolation and the possible errors introduced in the results are discussed more fully in the next section. 4.

ANALYSIS

OF RESULTS

The reverse ray tracing is started at an altitude of 250 km and continues as long as the calculation is possible to a lower limit of 10 km altitude. For the purpose of the present study the geographic location of the point at which the calculation is terminated is referred to as the ‘probable source.’ It then remains to try to identify these ‘probable sources’ with actual physical features which may be possible sources of the gravity waves. Figure 4 plots the group paths in the vertical plane of the different waves listed in Table 1. It must be noted that two types of uncertainties affect the accuracy of these ray tracings. (i) Uncertainties in the measurements of wave parameters. It can be shown that uncertainties of f10 per cent in the horizontal speed of propagation, and &6 degrees in the azimuth of propagation induce an uncertainty in the geographic location of the ‘probable source’ which amounts to & 100 km at 1000 km horizontal distance and grows linearly with increasing horizontal distance. (ii) Uncertainties in the neutral wind profile associated with the interpolation between 30 and 80 km. The three types of wind profile labelled a, a’ and a”, in Figs. 3(a) and (b) 8

F. BERTIN,

502

0

J. TESTUD

and L. KBRSLEY

2000

IO00

X,

3000

km

FIO. 4. TICAJBCTO~IM OF THE COMPUTBD RSVERSE OROUP PATHS FOR THB WAVES LISTED IN TABLB1 PLDTTRD IN TERMS OF IiEIOHT AOAINST HORIZONTAL DISTANCE, The paths II, a’ and a” for the waves of 20 min period correspond to the three interpolations shown in Figs. 3(a) and 3(b). The paths h and i become horizontal at heights of 115 and 125 km respectively. The table indicates the period and the time of transit of each wave from its starting point to the level of observation at 250 km.

have been used in the ray tracing computation. An example of the results is shown in Fig. 4 for a wave of 20 min period with the corresponding ray paths labelled a, a’ and LZ”. The resulting uncertainty in the location of the ‘probable source’ is in this case approximately & 150 km. Computations made for other cases show that this uncertainty grows roughly linearly with horizontal distance. Thus the final uncertainty in the location of the ‘probable source’ is about f250 km at 1000 km horizontal distance. Examining now the different paths of Fig. 4, we see that they tend to fall into four categories. I. The waves of short period around 20 min already mentioned (case a) would appear to follow a group path such that the ‘probable source’ is situated at a horizontal distance of less than 1000 km from the region of observation. II. A second category of group paths is found (b, c, d, e,f) where the horizontal separation of the detection zone and the ‘probable source’ is of the order of 2000 km. These correspond to waves with periods in the range from 30 to 80 min. III. The group path corresponding to case g in Fig. 4 shows that the ‘probable source’ of this wave of period 98 min is at a distance greater than 3000 km from the region of detection, nevertheless the energy associated with this wave at observation shows it to be

IONOSPHERIC

GRAVITY

WAVES AND WEATHER DISTURBANCES

503

of some importance (Table 1). The remoteness of the source and the long transit time of the wave (-5 hr) urge caution on the interpretation of the results and the identification of the possible sources. IV. The computation of the group path in cases h and i has been stopped at altitudes of 115 and 125 km respectively, since at these heights the vertical wave number k, becomes zero. This indicates that either a reflection occurs at these points or more likely that the actual sources of the waves are located at these altitudes. In both cases the ‘probable source’ has been taken to be at the point where k, becomes zero. 4.1 The possible aurora1 origin of Category IV waves The maps of Fig. 5 show the geographic locations of the ‘probable sources’ of the waves in category IV (cases h and i). The size of the circles gives an indication of the error in the location of these ‘probable sources.’ The ‘probable source’ for case h is located to the south west of Iceland at a height of 115 km. The wave was detected in the receiving triangle during the morning and since the transit time is of the order of 2 hr one can conclude that the wave was generated between 0700 and 0900 U.T. at a time when the ‘probable source’ is in the vicinity of the aurora1 oval (Fig. 5(a)). Similarly, the wave following path i, Iirst observed around midday, has a transit time of about 2 hr. The ‘probable source’ is situated at a height of 125 km and in the geographic location of South East Greenland (Fig. 5(b)). Taking a time of origin for the waves of 1000 U.T. it is again found that the ‘probable source’ is located in the region of the lower latitude boundary of the aurora1 oval. Taken together these two waves represent the greater part of the oscillation energy recorded on 30 January (Table 1). Analysis of the magnetograms from different aurora1 stations for this day shows that the level of magnetic activity is low. However, a small perturbation is evident on several of the magnetograms around 0830 U.T. which may possibly be associated with the origins of the wave following path h. For case i, no signature has been left on the magnetograms. It is possible that this wave has the same source as the preceding case and that the transit time has been underestimated in the calculations. An alternative interpretation of case i (a)0800

UT

(b)

1000 U

T

FIG.5(a) and (b). POSITIONS OF THE AURORAL OVAL AT 08 U.T. AND 10 U.T. RESPEWLY (ApTERfkASXU, 1968). The ‘probable sources’ h and i are situated in the vicinity of the aurora1 oval.

504

F. BERTIN,

J. TESTUD

and L. RERSLEY

is given by the concept of Lamb waves as described by Lindzen and Blake (1972). Lamb waves result from a mode of oscillation of the atmosphere which is trapped vertically by a resonance between ground and 100 km altitude but with free propagation horizontally at the speed of sound. Three facts lend support to this interpretation of case i as a Lamb wave: (i) the horizontal speed in this case is close to the speed of sound in the O-100 km altitude range (around 350 msec-l), (ii) the amplitude of the wave is larger than in other cases (see Table I), (iii) the free wave trajectory starts at about 100 km level at which an escape of Lamb wave energy is to be expected. 4.2 Possible meteorological origins of waves in Categories I-III Apart from the two cases already discussed it has been possible to compute reverse paths to meteorological heights. The geographic locations of the ‘probable sources’ of the waves originating logical altitudes are presented in Fig. 6(a) for 26 January and Fig. 6(b) for Each location is represented by a circle whose diameter again corresponds

-v-

Front

at 06

-v-

Front

at ,I2 lJ T

Jet

FIG. 6(a)

and(b).

all the other at meteoro29 January. to a rough

UT

stream

METEOROLOGICALSJTUATIONAT 1200U.T. 1972 RESPECTIVELY.

ON 26 JANIJARYAND 29 JANUARY

Isobars are drawn as thin dashed lines and frontal systems at 0600 U.T. and 1200 U.T. indicated by dashed and full heavy lines respectively. The hatched region indicates the position of the jet stream. The geographic position at ground level of the extremity of each trajectory in Fig. 4 is indicated by a circle whose area corresponds to the uncertainty in the ray tracing. The circles labelled a, c, dand f are situated on or near frontal waves (developing cyclones) with circles a and d being situated (in addition) close to the path of the jet stream.

IONOSPHERIC GRAVITY

-v-

WAVES AND WEATHJ3R DISTURBANCES

505

Front ai 06 UT Front at 12 UT Jet stream

Fm. 6(b).

estimate of the error computed from the uncertainties described above. The synoptic chart presented in each figure represents the meteorological situation as published by the Meteorologie Nationale, France at 1200 U.T. on the day in question, whilst the hatched zone indicates the position of the jet stream at the same time. The positions of the frontal systems at 0600 U.T. are also shown. It is immediately apparent from the charts that most of the ‘probable sources’ are in the vicinity of meteorological features associated with depressions. The sources, a, c, d and e are situated on or close to warm or cold fronts whilst sourcefis on an occluded front and g close to a depression. It should also be noted that sources a, b and dare situated close to the path of a jet stream whilst the sources a, c and d are near to depressions in the early stages of development into cyclones. It is known that meteorological movements in the atmosphere are usually approximately geostrophic in nature, that is, they are directed parallel to the isobars. In the event of an ageostrophic perturbation imposing cross-isobaric winds the atmosphere will react in such a way as to diminish the divergence (Dady, 1969). The return to the geostrophic state is believed by many meteorologists to result in the production of internal gravity waves. The ageostrophic probations responsible for the cross-isobaric winds are usually created by a wave or undulation on a polar front which precedes and accompanies a strong cyclone. This is the situation to be found in the cases of ‘probable sources’ a, c and d. Further, concerning sources CIand d there is the additional feature of a jet stream bordering the polar front and an association between jet streams and gravity waves has already been

506

I? BERTIN, J. TESTUD and L. KERSLEY

reported (Cowling et al., 1971). Godske and Bergeron (1957) have shown that a shearing instability can develop on the edge of a jet stream resulting in a frontal wave and the development of a strong depression. Thus for ‘probable sources’ a and d it is possible to establish a link between the presence of a jet stream and the formation of gravity waves. It now remains to comment on the possible forms of the frequency spectrum actually emitted by the source. It is to be expected that the width of the gravity wave spectrum will be a function of the geographic extent of the ageostrophic perturbation producing it. More precisely, it seems reasonable to estimate that the ageostrophic perturbation has approximately the same size as the undulation on the polar front. We can place a minimum size of 200 km on such a system so that the minimum wavelength of the waves launched must be of the same order. Considering the dispersion diagram of gravity waves at ground level (Hines, 1960) it thus appears that the possible spectrum of the waves launched will contain periods greater than a minimum value of 15 min. However, the waves emitted by the source are greatly dispersed during their travel from ground to F-region height. To a first approximation, the slope of each path is a direct function of wave frequency, thus at a given place of observation and at a particular altitude one observes in general, a quasi-monochromatic wave with a period which is an increasing function of the horizontal distance from the source. In addition the filtering effect of the atmosphere is a function of both frequency and phase velocity so that it would appear that for the observer the division of energy in the spectral distribution (Table 1) is likely to be a more uniform function of frequency than was actually the case at emission. 5. CONCLUSIONS

This study, limited to a period of common observation of a few days, has used as its starting point data from several types of complementary techniques and has led to a detailed case by case analysis of the observed gravity waves. By means of simultaneous measurements of the principal parameters of the neutral atmosphere it has proved possible to use ray tracing to identify possible sources responsible for the emission of these waves. It has also been possible to differentiate between waves of aurora1 origin and those of meteorological origin and there are indications that the latter may be associated with undulations on polar fronts. It is necessary to appreciate the limitations of a study of this type. These limits arise mainly because of the almost total lack of wind measurements in the height range 30-90 km. It has been shown that the iufluence of the wind at these altitudes is of importance in determining the path followed by waves of periods greater than 30 min where the sources lie more than 1000 km from the region of observation. Thus the reliability of tracing the paths of medium scale gravity waves from their meteorological origins can only be improved when methods of systematic measurements of the winds between 30 and 90 km altitude have been developed. Acknowleu&rnettts-Thanks are due to Mr. K. J. Edwards and Mr. J. Voisin for assistance in the experimental aspects of the satellite observations in the U.K. and France respectively and to Mr. D. J. Sambrook for help with the analysis of the recordings.

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