F2-region acoustic waves from severe weather

Journttl of Atmospheric andTerrestrial Physics, 1969.Vol.31,pp.1346-1363.Per3amon Press.Printed inNorthern Ireland

F2-region

aconstic waves

from

severe

weather

D. M. BAKER and K. DAVIES Space Disturbances Laboratory, Research Laboratories, ESSA, Boulder, Colo. 80302, U.S.A. (Received 4 April 1969) Ah&a&-A certain type of wavelike ionospheric disturbance, characterizedby periods in the range 2-5 min, is frequently observed during Summer evenings over Kansas and Nebraska. These disturbances are apparently closely associated with severe local storms in the troposphere and are interpreted as manifestations of acoustic waves generated by the storms. 1. INTRODUCTION THE PURPOSE of this peper is to present evidence

for the existence of atmospheric waves in the P®ion, with periods in the range 2-5 min, which are closely associated with the occurrence of severe local storms in the troposphere. Evidence for relationships between meteorological and ionospheric phenomena have been advanced at intervals over the past 30 yr. Several of these, s,s for example BEYNON and BROWN (1951), have been based on temporal variations of ground-level pressure and the critical frequencies of the ionospheric layers. Others have been based on the passage of certain well-defined meteorological phenomena and ionospheric parameters. For example: RASTOCJI (1962), among others, has related ionospheric sporadic-E with the occurrence of thunderstorms, while BAUER (1958) has related variations of the F2-layer critical frequency with the passage of hurricanes. Still other relationships have been based on the type of ground-level air mass and ionospheric layer height. In China, for example, GHERZI (1950) utilized ionospheric observations to predict the large scale weather pattern on the ground. More direct evidence for coupling of energy from the troposphere into the ionosphere has recently come from observations of the temporal variations in the phase and frequency of ionospherically reflected radio waves. Some events have originated from man-made explosions (BARRY, GRIFPITHS and TAENZER, 1966; BAKER and DAVIES, 1968) and others from natural sources, such as the 1964 Alaskan earthquake (DAVIES and BAKER, 1965). GEORGES (1968a, b) has recently pointed out an apparent relationship between certain ionospheric disturbances and the occurrence of severe weather in the troposphere. The troposphereionosphere coupling we wish to discuss is similar to that observed by Georges. 2. THE OBSERVATIONS The ionospheric disturbances were observed by measuring the frequency fluctuations of ionospherically reflected CW radio signals (DAVIES and BAKER, 1966). Since April 1966 we have continuously monitored the frequency fluctuations of several radio signals propagated over a 1300 km path from Havana, 1345

f

23bOUT

0600

OlbO

OlbO UT

0200

O?lilO

0300 UT

0400

0500

9.9 MHz, HAVANA, ILL. TO BOULDER, 13-14 June 1967

COLO.

Fig. 1. Doppler records of a wavelike disturbance in the ionosphere over the Kansas-Nebraska area. Such disturbances are frequently observed in Summer.

1346

Illinois, to Boulder, Colorado. The midpoint of this path is locetcd approximately above Concordia, Kansas (39*58’X, 97*65”\1’). Prequently in t.hc late afternoon and evening during May through September n-c observed oscillatory frequency variations with periods of about 3 min. An example of such an event observed on 13-14 June 1967 is given in Fig. 1, which shows the variation of the received frequency with time. From 23.00 to 23.30 U.T. (C.T. : 10~1 menn time at path midpoint + 6 hr 31 min) on 13 June the frequency variations wcw t,ypical of those observed during undisturbed conditions in the late afternoon in Summer. Near 23.50 the records began to show definite signs of disturbanc~c~. and b)- 00.25 on 14 June an oscillatory disturbance, with periods ranging from 2’ t,o ;5 min, had bccomc well established. This disturbanccl 1)crsistcd until about, 04.20 U.1’. The simplest interpretation of this record is that the height of reflection of the radio wave was oscillating up and down with the same period as the frequency variations. Some of the frequency variations, however. could have been caused t)y oscillations in the electron density below the hcG,ght of rclflcctio1l. Forty-two such ionospheric disturbances ha.vc been observc~l during t,Jic~ ‘l’hc seasonal distribution of thcst: period May 1966 through December 1968. disturbances is shown in Fig. 2! from which WC!see that t,he disturbances have been observed only during the months May through September. Moreover, they always occur between about 00.00 and 06.00 C.T. (17.30-23.30 local mean time at the that is. a disturbance observed over the path midpoint), and they arc localized: Kansas-Nebraska area is not observed above Boulder. where we make similar measurements. The disturbances h:lvc> been observed to last, from less than 1 hr to as long as 9 hr; most events having durations from 2 to 4 hr. The period varies slightly from event to event and during a single evrllt. but, it, is always around 3 min. Observations made on several radio freyucncies (s.!). !I-‘J. 1 I. I, 12.1 :mtl 13.0 lMHz) indicate that the disturbances travel upward. but. as ,vct. we have not data along been able to accurately determine the speed. ‘l’hc lack of ionospheric the path (routine ionograms are available only at Boulder) make it impossible to determine the exact reflection heights which arc gcanerally iti the 206 300 km region.

There is evidence that thcsc ionospheric disturbances arc associated with tho occurrence of severe weather in the troposphere. The first (:ln(~ to this possible association was discovered by GE:OIWES (1968). Following his haad, we have compared the observations of ionospheric disturbances with the: occurrence of severe weather features in th(b vicinity of the path midpoint. and we have found that whenever a disturbnncc~ was obscrvcd in the ionosphrrcb severe weathcl cbonditions, as revealed by radar summary charts of the U.S. Weilthcr .Burcnu. existed within 250 km of the path midpoint. The situation a.t 02.45 1J.T. on 14 .Junc 1967 is illustrated in Fig. 3, which shows the locations of our transmitters and receivers. the midpoint of the transmission path, and the major weather features as given hy the radar summary chart. At this t,imcb. a complicated lint of intensct

FB-region acoustic waves from severe weather

0 IO

1347

JFHAMJJASOND ,

,

,

,

,

,

,

,

,

,

(

,

,

,

[

,

,

,

,

1967

0 IO

JFMARJJASOND ,

,

,

1968

0

JFMAMJJASONIJ Month

Fig. 2. Seasonal distribution of the occmence of the type of ionosphericdisturbance illustrated in Fig. 1.

echoes (or squall line), strong convective cells, and thunderstorms existed to the North and West of the path midpoint. This situation is typical of conditions in the region of the midpoint every time an ionospheric disturbance has been observed; however, an ionospheric disturbance is not observed every time similar weather conditions exist near the path midpoint. In an attempt to make the association a little more quantitative we have devised a simple ‘severe weather index’. This daily index is merely the number of severe weather ‘indicators’ present on the radar summary charts during the

D. M. BAKER

1348 Area Line 0

Strong

320

Height

TRW

of

Radar

of

Rador Cel of

Cell

or

I>AVIES

Echoes Echoes

I Echo

Thunderstorm Area

and K.

Tops,

in Hundreds

of

Feet

Activity

Line

Movement,

Movement,

IO Knots

Per

Full

Barb

in Knots

-’ ILLINOIS

-40%

A

Transmltte

COLORADO

., ,- J ?

l --_

,L_

s

RADAR

SUMMARY

__ __ ___ ._- --- -- -t IOOkm i

CHART I lOa”W

,*-

‘-

I4

JUNE

--

1967.02= 9+w

Fig. 3. Weather conditions, as revealed by a radar summary chart, ~1 the vicinity of the transmission path midpoint during the ionospheric disturbance illustrated in Fig. I.

period of interest (OO.OO-06.00 U.T.). As indicators of severe weather conditions, we have chosen the presence of extensive areas of radar echoes, lines of echoes, thunderstorms, strong convective cells, and echo heights in excess of 40,000 ft. We have assigned an equal weight to each indicator although they are undoubtedly not of equal importance or strictly independent of one another. Thus, the index veries from 0 to 5. This index is shown for each day of 1967 in Fig. 4. The severe weather index is given by the height of the bar, and a blackened b,rtrindicates that an ionospheric event was observed on that day. Here again we see the seasonal dependence, with no ionospheric disturbances being observed in Winter. The severe weather index also generally tends to be lower in the Winter. More important, however, is the association of the ionospheric events with high severe weather with only three exceptions, the ionospheric disturbances occurred on indexes;

PP-region acoustio waves from severe weather m

Ionospheric

Effect

Observed

Fig. 4. Severe weather index during the period 00.00-06.00 U.T. for 1967. The severe weather index is given by the height of tho bar, and & blackened-in bar indicates that an ionospheric disturbance of the type illustrated in Fig. 1 was observed.

1348

I). M’. BAKER and K. I~AVIES

1350

days when the index was at its maximum value of 5, and in no instance was a disturbance observed on a day of index 0. These facts are summarized in Table 1. Table

1. Summary of relationship index and the occurrence

I__~___~. Days with index = 5 Days with index -2 I-4 Days wit11 index mu0

between the S+S-ere weather of ionospheric events -Ionospheric No ionospheric event ctvent ~_ .--~_. _

14

48

:i

160

0

140 .- ._.__I

A similar study for 1968 leads to the satnc conclusion: ‘I’hct index is almost always 6 on days when ionospheric disturbances are observed, and a disturbance has not yet been observed on a day when we know that the index wa.s 0. This evidence, together with the facts that the disturbances are localized and travel upward, is suggestive of a close connection between the ionospheric events and the occurrence of severe local stroms. The evidence is circumstantial, however, and any connection cannot be conclusively established until WC can isolate a specific source and explain how the energy reaches ionospheric Ileights. 4.

S~;UE POSSIBLE INTERPRETATIONS

Assuming that these events are associated with severe local storms in the troposphere, the most likely cause of the disturbances is the perturbation of the electron density in the ionosphere by acoustic waves in the neutral atmosphere which have propagated upward from below. That such waves can reach ionospheric heights has been fairly well established by observations of the effects of lowaltitude nuclear explosions (BAKER and DAVIES, 1968). C;POKL:IW (1968) has proposed that the narrow band of periods observed at ionospheric heights results from atmospheric filtering; although the source may generate a broad spectrum of acoustic waves, the longer periods are prevented from reaching the ionosphere by the maximum acoustic cutoff frequency of the atmosphere, which normally occurs at the mesopausc (about 85 km), and the shorter periods are damped out by viscous attenuation. This explanatioll. though possible, would require an atmospheric filter of very narrow bandwidth. An alternative explanation is that the narrow band of periods observed is a characteristic of the source. As far as we know, no one has identified any process connected with severe weather which has a period of 3 min. ANDERSON (1960). in a study of the vertical velocity of cumulus cloud tops, found that the velocity pulsates with a pronounced low-frequency component (period about 10 min) and higher frequency components with periods of the order of 1 min. PIERCE and CORONITI (1966) have suggested that the air movements in thunderstorms might generate acoustic-gravity waves which could reach ionospheric heights.

FZ-region acoustic waves from severe weather

1351

Several possible explanations can be found for the failure of some severe storms to generate ionospheric disturbances. The source of the aooustio waves may be absent, or if the source does have a characteristic period, this period may not be within the bandpass of the atmospheric filter either because the period of the source varies from storm to storm or because the bandpass of the filter has

(b) F& 1962 US. Standard Atmosphere, T=40 min

GroundRange, km Fig. 5. Acoustic ray p&hs in a stationary model atmosphere, wave period = 4.0 min, p. is the propagation angle with the vertical at ground level (by permission of N. J. F. Chang).

been altered by temperature variations. Another possibility is that the temperature at the mesopause has decreased sufficiently to complexly close the atmospheric filter. The localized nature of the ionospheric disturbances, which are detected at heights of 200-300 km, is consistent with the behavior of freely propagating acoustic wsves in a thermally stra;tified atmosphere. Ray tracing of acoustic waves, including the effects of frequency dispersion, anisotropy, and winds shows that, for 5tsource on the ground, only wsves which start off at propagation angles within about 30” of the vertical reach heights above 200 km; waves which start at angles to the vertical grertter than 30’ are refiected back to the ground before reaching 200 km. Sample rsy paths in the U.S. Standard Atmosphere are shown in Fig. 5 for acoustic waves with periods of 4-O min. This figure shows that waves with an initial propagation angle (& of 30” reltch a level of 300 km at a horizontal distance of around 250 km from the source. Wave with initial propagation angles much greater than 30” do not reach these heights where the radio signals are reflected. Therefore, in order that an ionospheric disturbance be detectable the source of the scoustic wsves must pass within about 250 km of the radio path midpoint.

ANDERSON

c.

E.

BAKER 11. M. and DAVIES K. BARRY G, TL.,GRIFFITH~ L. T. and TAENZERJ. C. BAUER S. .J. BEYNON l<.J. G. and BROWN t:.MM. D_~VIESfi. rand BAKERD.M. DAVIES K. and BAKER D. M. GEORGE~T. M. GEORCEN T. M.

PIERCE A. D. and CORONITI S. (1. RASTO~I R. G.

1958

1951 1965 1966 l968a 1968h