Geophys. J. R. ustr. SOC.(1971) 26, 111-133.
Natural Infrasound as an Atmospheric Probe
William L. Donn and David Rind
(Received 1971 July 1)
Summary
For four years continuous recording of infrasonic signals in the frequency
range 0.1 to 1 Hz, known as microbaroms, has been conducted at Palisades,
New York. The microbaroms we recorded are radiated into the atmosphere
by interfering ocean waves in the North Atlantic as far as 2000 km away.
A characteristic diurnal variation in the amplitude of the received signal
has been noted, independent of any variation in the source. We conclude
that the variation is due to variations of the factors affecting atmospheric
sound propagation, namely wind and temperature.
In winter a semidiurnal variation in signal amplitude is observed, with
maximum reception around 11 : 00 and 22 : 00 local time. Reference to
wind and temperature observations in the literature shows that at these
times the lowest level of reflection of the vertically propagating signal occurs
between 100 and llOkm due to the presence of strong east winds. At
18 : 00, time of minimum amplitudes, the reflection level rises to about
115 km because of a change in tidal wind phase. Viscous dissipation
associated with the changed reflection height can account for the observed
signal weakening. A third maximum, a less regular effect, is found to be
related to more variable winds between 95 and 105km.
In summer, reflection is found to occur from about 50 km due to the
presence of stratospheric easterlies. The summer diurnal variation,
different from that of the winter, exhibits only a weak minimum about
20 : 00. This appears to result from a diurnal temperature variation superimposed on a diurnal wind variation. Abnormally high microbaroms were
recorded at times that can be related to an atmospheric event known as a
stratospheric warming. Microbaroms thus provide a continuously
available natural mechanism for probing the upper atmosphere. We
conclude that the establishment of microbarom observation systems
could give a comprehensive technique for monitoring several upper
atmospheric parameters.
1. Introduction
For the past four years a tripartite array of capacitor microphones has been
used at the Lamont-Doherty Geological Observatory, Palisades, N.Y., (41" N,
73" W) to record infrasonic waves with periods of 4-7 s (Donn & Posmentier 1968).
111
H
William L. Donn and David Rind
112
These oscillations, with amplitudes of a few microbars, are known as microbaroms.
Their origin has been explained by generation by interfering ocean waves in marine
storm areas (Posmentier 1968). Posmentier & Donn (1969) suggested that temporal
variations of the amplitude of this continuously generated infrasound could provide
information about wind variations in the E-layer, including atmospheric tides. In
this paper we attempt to provide a more complete explanation of the relationship
between microbaroms and upper atmospheric winds in order to show that infrasound
can be used to monitor changing conditions in the upper atmosphere.
The pressure sensors used in this study are Globe capacitor microphones whose
characteristics have been discussed earlier by Kaschak, Donn & Fehr (1970). Each
transducer is installed in the centre of a 1000-ft Daniels acoustic line filter described
in detail by Donn & Posmentier (1967). Azimuths to the sources of microbaroms
were determined by a tripartite array of sensors. Signals are recorded on seismic-type
drums for visual monitoring and analogue magnetic tape for more complete data
processing. Each drum record gives a 24-hr recording.
2. Microbarom observations
In general, microbarom amplitudes are highest in winter and lowest in summer,
with those of fall and early spring being about the same. This effect exists because
ocean storms in winter are more numerous and intense than in summer.
Since microbaroms are associated with marine storms, an irregular variation in
signal strength is to be expected as storms wax and wane, but the hourly means of
microbarom amplitudes would not be expected to show a systematic diurnal pattern
related to the source. Nevertheless characteristic diurnal patterns do exist in our
data. Figs 1-5 summarize these by giving several years’ average hourly amplitude
variations for autumn (October), winter (January), early spring (April), late
spring (May a, b), and summer (July), respectively. The seasonal amplitude effect,
due to changing storm conditions, is quite evident in a comparison of these graphs.
More important to problems of propagation of microbaroms are the diurnal variations
quite evident in Figs 1-5, whose pattern and range varies seasonally.
(a) Winter-type diurnal variations
In fall, winter, and early spring, a semidiurnal oscillation (called ‘ winter-type
variations ’) described briefly by Donn (1967) is common with maximum pressure
amplitude occurring around 11 : 00 and 22 : 00 (Fig. 6(a)) (all times in this report
FIG.1. Average hourly amplitudes of microbaroms for October (3 years data).
The same arbitrary amplitude scale is used in Figs 1-5.
113
Natural infrasound as an atmospheric probe
6
6l.J
16
1
17 I f 3
1
I
I
I
1
I
I
I
1
I
I
I
I
I
I
I
I
19 2 0 21 2 2 2 3 24 0
1 02 03 04 05 06 07 Of3 09 10 I I
I
j
I
I2
13 14
1
1
I
15
Local t i m e
FIG.2. Same as Fig, 1 for January (3 years of data).
~3
CJ
I6
~
I 7
1
1
1
1
1
I8
19 2 0 21 2 2
1
1
1
1
1
1
1
1
1
1
1
2 3 24 01 02 03 04 05 06 07 0 8 09 10 I I
L o c a l time
FIG.3. Same as Fig. 1 for April (2 years of data).
1
12 13 14
1
1
1
,
William L. Donn and David Rind
114
I OCQI t i m e
FIG.4(a) Same as Fie. 1 for May 1968. (b) Same as Fig. 1 for May 1969.
-
-
-
-
a,
u
3
0
-
30-
t 2724.21 18-
15 -
Natural infrasound as an atmospheric probe
115
refer to local standard time). A third maximum which occurs less frequently is
observed around 04 : 00. The smaller range of this oscillation evident in Figs 1 and
3 is simply a result of the averaging process. Amplitudes may be as great as those at
the other maxima. This 04: 00 maximum may occur separated from the 22: 00
maximum (Fig. 6(b)) or it may occur as a prolongation of the earlier maximum
(Fig. 6(c)).
(b) Summer-type diurnal variation
During the summer only a diurnal variation is noticeable, with a minimum
occurring a t 20 : 00 (Fig. 5). In the late spring, two types of situations seem possiblevariation more strongly reminiscent of the winter (Fig. 4(a)-May 1968) or more
closely resembling summer (Fig. 4(b)-May 1969). The return of the winter-type
variation seems to take place sometimes in September. For example, the record of
(a)
FIG.6(a) Portion of microbarom chart record for 1969 October 6-7. One fall line
represents 30 min, with a minute between two vertical divisions. Three to four
minutes in each half hour are visible here. The semidiurnal amplitude oscillation
is evident. (b) Microbarom record for 1968 January 1-2. Three separate intervals
of maximum amplitude can be seen. (c) Microbarom record for 1970 March 30-3 1.
Maximum amplitudes extend from 21 : 00 to 04 : 00. (d) Microbarom record for
1967December 26-27. Strong signal is observed continuously throughout the day.
116
\Villiam L. Donn and David Rind
(C)
FIG.6 . (c) and (d).
1969 September 7 (Fig. 7(a)) shows only very weak signals, whereas Fig. 7(b) for
September 21 shows the first semi-diurnal variation for this particular season.
Meteorological conditions in the North Atlantic Ocean were similar on both days.
(c) Abnormal microbarom intensities
Not observable in the averages of Fig. 2 is the fact that unusually high microbaroms were detected continuously for about a month during the early winter of
1967-68 (Fig. 6(d)). The signals were strong throughout the day with semidiurnal
variations superimposed on them. Following these intervals the late spring pattern,
evident in Fig. 4(b), prevailed. The remarkable intensities of microbaroms were not
the simple result of generation in any unusually intense marine storms.
(d) West coast observations
Nearly all of the observations and resulting analyses described here refer to the
north-east coast of the United States. To test conclusions derived from the east
coast study, we required microbarom data for west coast conditions. Observations of
microbaroms from Pacific Ocean storms recorded at Pullman, Washington (50" Lat.
135"Long.) have been made available (Craine & Rezvani 1970). No obvious diurnal
variation pattern is visible; again larger amplitudes occur in winter than in summer.
Natural infrasound as an atmospheric probe
117
FIG.7(a) Microbarom record for 1969 September 7. Weak signal is observed.
(b) Microbarom record for 1969 September 21. The semidiurnal oscillation is now
evident.
3. Factors controlling long-range propagation
Nearly all of the microbaroms detected in Palisades are generated by storms in
the North Atlantic Ocean, often as remote as 2000 km. In a study of microbaroms
and microseisms generated by the same storms, Donn & Posmentier (1967) showed
that the diurnal variation of microbaroms was not related to the generation mechanism
and concluded that propagation factors related to the speed of sound must control
the observed variation.
The speed of sound at any particular level is a function of temperature, as expressed
by a form of Laplace’s equation for an ideal gas: C, = (yRT/M)*where y is C,/C,
the ratio of specificheats at constant pressure to constant volume, R is the gas constant,
8.314 x lo7ergfdeg-mole, T is the absolute temperature and M is the molecular
weight of air (28.97 g/mole).
If a signal propagates with velocity C with respect to the air and the wind velocity
is W, then the signal will propagate with the sound velocity V = C+ W with respect
118
William L. Donn and David Rind
to the Earth. The magnitude of C is C , given above. In our study, W will represent
the horizontal component of the wind at each altitude. We assume that horizontal
variation of wind and temperature are negligible in the region of interest.
A sound channel is any region in the atmosphere in which a layer of smaller
sound velocity is bounded above and below by regions of higher velocity. For our
purposes, the solid earth can be regarded as the lower boundary of a channel, with
some level of equal sound velocity in the atmosphere being the upper boundary.
Thus any signal originating at ground level (e.g. microbarorns) will propagate upward
until it is reflected at the upper boundary, leading to reception at ground level sensors
after one or more reflections.
Fig. 8 gives the speed of sound versus altitude as computed in the U.S. Standard
Atmosphere Supplements (1966) as a function of temperature alone. Two possible
reflecting layers exist, one at 50 km and the second above 100 km. To determine the
sound velocities we must investigate the wind as well as any temperature fluctuations
at these levels. The varying influences of these parameters at these altitudes will be
responsible for many of the microbarom variations noted earlier.
In view of the importance of temperature and winds on sound propagation, we
surveyed the literature for systematic variations of these factors. The only repeatable
temperature variation appears to be a somewhat questionable 24-hr diurnal fluctuation
from 40 to 60 km (e.g. Beyers, Miers & Reed 1966; Theon et al. 1967; Finger & Wolf
1967; Ballard 1967). Between 60 and 125 km no agreement exists about the presence
and nature of temperature variations (for example, compare the data of Revah's
Fig. 24, 1969, and Rawer's Fig. 4, 1970). No conclusion can be drawn at this time;
in fact there may be no ' usual ' occurrence in such a complicated region where the
mean free path and relaxation time is long enough to allow radiation by CO,
molecules and thus upset thermal equilibrium, clouding the meaning of the term
temperature.
Contrary to the known variations of temperature, very strong systematic variations
of wind velocity are known to occur above the troposphere. For the most part the
diurnal variations of microbaroms are produced by variations in upper winds (as
will be described) rather than by variations in temperature.
115I 10I05 I00 -
-5
-'
c
2
9
Iu
i;l
95r
9085-
EO75706560555045 40 -
35-
30 25 -
20 15105 -S p e e d of s o u n d ( m
s-I)
FIG.8. Speed of sound versus altitude at 45"N (from U.S. Standard Atmosphere
Supplements, 1966) as a function of temperature alone, for January (dotted line),
Autumn-spring (solid line), and summer (dashed line).
119
Natural infrasound as an atmospheric probe
OL
,
J
i
F ' M
' A
M
' J
'
J
'
A
[
s 1 0 I N ' D l
FIG.9. Monthly variation of the prevailing zonal wind in middle latitudes (speed
in m s-') (after Batten 1961).
Phosc angle
0"
40
EY.. . .'
The or;
t Ic o I
AmDlitude ( m s-1)
Phase (hour o f maximum)
FIG. 10. Amplitude and phase of the diurnal variation of zonal wind component
at 30"N. Data for meridional component are also shown. (after Reed etal. 1969).
120
William L. Donn and David Rind
4. The nature of upper-level winds
The actual wind vector in the upper atmosphere is the sum of prevailing and
tidal components (mainly diurnal and semidiurnal) of varying strength and phase at
different elevations. Non-linear effects, apparently related to gravity wave propagation,
become important above 70 km. We have made a detailed survey and analysis of the
state of knowledge regarding upper winds, and give here a summary of this information.
(a) Prevailing wind
Fig. 9 summarizes the monthly variation of the prevailing wind in middle latitudes
(after Batten 1961). The outstanding feature is the well-known reversal of winds
between 30 and 80 km from summer to winter. East winds (from the east) in summer
result from the north polar high-pressure cell at these altitudes; winter westerlies
result from the polar cyclone in the same region. A transition period is evident
during May and September. Not shown are evidences that in autumn and winter
the prevailing wind above 100km is from the east, reaching maximum strength
between 105 and 110 km (Hines 1966; C. Justus, personal communication). Above
115 km the wind may return to westerly (Hines 1966).
(b) Diurnal tidaE winds
The diurnal (24 hr) atmospheric tide is largely a thermally induced oscillation.
Fig. 10 shows a comparison of theory and observation for the amplitude and phase
(hour of maximum west wind) of the diurnal variation of the zonal wind component
to an elevation of 60 km at 30" N (Palisades is at 41" N). The amplitude curves agree
well, but those for phase agree only below 55 km. Above this level the observed phase
is more nearly stationary than the theoretical phase-a point we will return to in
connection with microbarom observations. At the elevations in Fig. 10, the diurnal
tide is a regular and characteristic feature.
I
I
I
I
8---l
*.....*
0-0
!
k-O-4
0--0
I
I
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I
White,, S o n d s . F e b 1964
Nov. 1964
"
July 1965
Adelalde
Garchy
I
I
1
1
40
50
60
70
I
1
I
80
90
I00
Elevation ( k m )
\
I
I10
120
I30
FIG.11. Variation with height of the zonal component of the diurnal tidal wind
(after Spizzichino 1970).
Natural infrasound as an atmospheric probe
121
Above 80km the amplitude of the zonal wind component seems to decrease,
especially at latitudes higher than 40" N (Fig. 11). Spizzichino (1969) explains this
decrease as the result of interaction between the diurnal tide and vertically propagating gravity waves, with the production of secondary gravity waves that bleed
energy from the diurnal tidal winds. The presence of these secondary gravity waves
seems evident from observations of winds at 47"N that are rich in short period
variations (Spizzichino, 1969).
(c)Semi-diurnal tidal winds
This wind is unimportant below 60 km where the amplitude is under 10 m s - l
(Miers 1965; Beyers et al. 1966). Above 80 km it is the most regular of tidal components; the amplitude grows exponentially with elevation until 105-1 10 km except
in summer when the effect is weak and variable (Spizzichino 1969). Its phase at any
elevation is relatively constant from day to day although it retards with elevation.
Above 110 km the amplitude seems to decrease. This is interpreted to be a consequence
of interaction between the semi-diurnal tidal winds and the ' secondary gravity waves '
Fig. 12 shows some of the important features of the semi-diurnal tide, especially
the wind direction change with elevation consonant with the retardation of phase with
height. A summary of the wind structure to 125 km, as now estimated, is given in
Table 1.
5. Propagation of microbaroms in winter.
Having described the non-uniform vertical temperature and wind structure of the
atmosphere, we now consider the effect of this structure on the details of diurnal
variations of microbaroms. To apply temperature-wind data to infrasound propagation, we employed a modified version of the acoustic ray-tracing program of
Pierce (1966). In regular use we have found this program to be very accurate when
sound source and reception points are known.
FIG.12. Semidiurnal tidal wind as a function of hour and elevation (95-115 km)
for Yuma, Arizona (32' N, 112" W) 1967 November 18-19. Numbers indicate
wind speed (m s-l); vectors represent direction to which horizontal wind is going.
(after C. Justus, personal communication).
William L. Donn and David Rind
122
Table 1
Atmospheric wind features to 125 km
Feature
Comments
Range of importance
(up to 125 km)
Prevailing wind
0-125 km
For Region 30-80 km reverses from east in
summer to west in winter. Speeds up to
100 m s- l . For region around 100 km appears
to be west in summer, east otherwise.
Diurnal tidal wind
0-100 km
Of major importance between 5 6 9 0 km, with
magnitudes up to 40 m s- l (uncertain).
Magnitudes greater at lower latitudes (below
40"N)
Semi-diurnal tidal wind
60-120 km
Most regular tidal component above 80 km,
magnitudes up to 100m s-l. Weaker in
summer.
Gravity waves, non-linears
interactions
Interactions in the region of importance may
be responsible for weakening of diurnal and
semidiurnal tidal winds.
70-1 25 km
Input wind data for an average winter day (from sources described above) are
shown in Fig. 13 for 04 : 00, 1 1 : 00 (or 22 : 00) and 18 : 00. These are times of
characteristic microbarom maxima and minimum. When wind data are combined
with the January temperature structure shown in Fig. 8, the total acoustic structure
leads to the propagation patterns in Figs 14, 15 and 16, respectively. Sound rays
from a point source in the generating area were computed at vertical angle intervals
of 10 degrees between 1 and 81 degrees.
-
- _ _ _ _ _ _ - _ _ _ _ _ - -_- - - - - - - - - _------_ _ _ _ _ _ - - - _- - - _ _- -----_____
80 -
6040 20 -
-
(b)
* I__--c
-
-
(01
S
*%
c
100 -
E
N----
----___
-
60 4920 0-
y
- .._ _ _ _ _ _---_----_---- _- -_
/
I00 80 -
)
E-W
----_ _ _ -
120--
- - - - - - - - - ----- - _ - _ _ _ _ _ _ __ _
_ _ __
----.__ - - _ _ ---- - - _
0-
-100 -
~
------- _ _ _---
c
80 -
~
6 040 20 -
(C
*.
---
- - - - - - - - - - - _- _
j.
*.,-
<'
3
3
I
I
I
-70 -60 - 5 0
I
I
-40 -30
I
-20
I
-lC
0
I
I0
I
I
I
I
I
1
20
30
40
50
60
70
;
Natural infrasound as an atmospheric probe
Disrance f r o m source ( k m )
FIG. 14. Acoustic ray propagation in winter from a point source to the east for
04 : 00 computed from the appropriate temperature-wind structure. Vertical
angles of incidence are every 10 degrees between 1 and 81 degrees.
Distance from source ( k m )
Fro. 15. Same as Fig. 14 for 11 :00 or 22 : 00.
123
William L. Donn and David Rind
124
Fro. 16. Same as Fig. 14 for 18 :00. or 22 :00.
The strong west winds around 50 km (Fig. 13(a), (b) and (c)) prevent this level
from forming the top of a sound channel for infrasound coming from surface sources
to the east of Palisades. According to Figs 14-16, the level of possible reflections
range from 100 to 150 km. The particular reflection level varies with the angle of the
ray and the varying velocity of the wind component from the direction of the source.
This velocity variation is caused primarily by the rotating semidiurnal tidal component.
In comparing ray tracings for times of maximum and minimum microbarom
amplitudes (Figs 15 and 16 respectively) we note that (a) no difference exists for
rays with angles of 1" to 51" whose reflection levels are above 125 km; and (b) at
11 : 00 and 22 : 00 rays at 61" to 81" reflect between 102 to 108 km, while the rays are
reflected between 116 to 120 at 18 : 00 and 07 : 00. The difference in amplitudes from
maximum to minimum (11 : 00 to 18 : 00) thus seems to be related to dissipation
effects between 108 and 116 km. At 04 : 00, a time when the intervening maximum
sometimes occurs, an intermediate reflection pattern occurs. Rays at only 71" to 81"
reflect from 98 to 104 km, with other reflections being above 120 km.
To interpret the result we must consider the dissipation of acoustic energy at
about 5-s period in the elevation range of the reflections.
In a non-turbulent atmosphere acoustic amplitude will decrease by energy dissipation as exp (-m) where the dissipation coefficient cx is given by the formula
(Landau & Lifshitz 1959; Morse & Ingard 1968).
where
w = frequency (c s-')
po = density (kgmW3)
C = speed of sound (m s-')
125
Natural infrasound as an atmospheric probe
=
coefficient of viscosity related to shear (kg m-' s-')
[ = coefficient of bulk viscosity related to expansion (of the same order of
magnitude as q and thus set equal to q ; see Landau & Lifshitz 1959)
Y = C,/C,
IC = thermal
conductivity (kcal m-' s-' OK-' )
C , = specific heat at constant volume (kcal g-' OK-' )
C,
=
specific heat at constant pressure (kcal g-'
OK-')
Table 2
Dissipation coeficients (a) versus elevation for several frequencies
Elevation
1 Hz
0
20
40
60
70
75
80
85
90
95
100
105
110
115
2.0x
3 . 7 10-7
~
8 . 2 loW6
~
1 . 1 x 10-4
4.3x 1 0 - ~
9 . o 10-4
~
1 . 9 ~
4 . 2 10-3
~
9.7x
2.2x
4.9x
9.31 x lo-'
I -99 x 104.10x10-'
0 . 4 HZ
3 . 2 10-9
~
5 . 9 x 10-8
1.3x
1 . 7 ~
6.8 x
1.4~10-~
3 . 0 ~
6 . 7 ~
1 . 5 x 10-3
3 . 5 ~
7.8 x
1 . 4 8 lo-'
~
3.18 x lo-'
6 . 5 6 lo-'
~
0 . 2 5 Hz
0 . 2 Hz
1 . 2 10-9
~
2 . 3 x lo-'
5 . 1 x 10-7
6.8 x
2 . 7 x 10-5
5.6 x 10-5
1.2 x 10-4
2 . 6 10-4
~
6 . o 10-4
~
1 . 4 10-3
~
3 . 0 ~
5.80~
1 . 2 4 lo-'
~
2.55 x lo-'
7 . 9 x 10-'O
1 . 5 x lo-'
3 . 3 lo-'
~
4 - 3 x 10-6
1 . 7 ~
3 . 6 lo-'
~
7 . 5 10-5
~
1.7 x
3 . 8 10-4
~
8 . 7 ~
1 . 9 ~
3.71 x
7 . 9 10-3
~
1.63X10-'
Values were taken from the U.S. Standard Atmosphere supplements (1966). Values
of ct were calculated for different frequencies and elevations, with results shown in
Table 2. For a frequency of 0.25Hz (microbaroms) the acoustic signal suffers a
negligible amplitude decrease below 105km and about 13 per cent decrease in
travelling from 105 to 115 km. Travelling horizontally a ray at 105 km will suffer
an amplitude decrease of l/e every 170 km, while a ray at 115 km has an amplitude
decrease of l/e every 39 km. A ray travels horizontally about 25-30km in the
reflection region at these heights. The resulting viscous dissipation per reflection for
microbarom frequencies is about 60 per cent greater for a ray reflected from 115 km
compared to reflection at 105 km. Because several reflections are involved, the
difference in reflection level, accompanied by a difference in dissipation, appears to
explain the diurnal variations observed. Although the equation used is strictly
applicable only in the region where the amplitude decrease is small over the distance
of a wavelength and therefore of doubtful applicability around 115 km, the absorption
will if anything be greater here.
There has been considerable discussion over the presence of turbulence between
90 and 120 km related to gravity wave dissipation and shear (e.g. Justus & Roper
1968; Muller 1968; Bedinger et al. 1969). It would appear from microbarom reflection, concluded to occur between 100 to 115 km, that either turbulence is not a
regular feature of the atmosphere below 105 km or its scale is smaller than 1 km (the
wavelength of the infrasound) and therefore turbulent scattering of microbarom
energy does not occur at a level below that of reflection. Otherwise turbulent scattering
of infrasound of 5-s period would prevent propagation of the observed signal.
126
William L. Donn and David Rind
Before discussing the summertime situation we might verify our results by checking
the wind model used with observed wind at several locations. The ' total ' (observed)
wind for Yuma, Arizona, is shown in Fig. 17. Strong east winds are found between
100 and 110 km around 11 : 00-in agreement with interpretations from microbarom
observations. Easterly winds of slightly weaker amplitude are found between 95 and
100 km around 04 : 00-the time of occasional maximum. The same effect at 16 : 00
is not apparent to us.
Fig. 18 depicts the observed wind during several winter days at 47" N. East winds
can be seen between 100 and 110 km around noon and midnight; west winds prevail
from 05 : 00 to 09 : 00 and 14 : 00 to 21 : 00. The variability at 04 : 00 can be seen
by comparing observations for 22 February, when strong east winds are evident
between 95 and I10 km from 03 : 00 to 04 :00, with those of February 23, when the
effect was much weaker. The variability of the 04 : 00 maximum may thus be related
to the variability of winds at this time at the reflection level of about 95-105 km.
6. Propagation of microbaroms in summer
We noted previously that microbaroms recorded at Palisades undergo a summer
amplitude weakening. Lower activity in summer is primarily due to the weaker
source conditions in the North Atlantic Ocean. We noted also the absence of the
characteristic winter-type semidiurnal variation replaced by a weak diurnal minimum
about 20 : 00 (Fig. 5). Again the temperature-wind structure of the upper atmosphere
appear responsible for the summer effect.
An important change from winter to summer is the development of strong easterly
winds in the stratosphere, as is evident in Fig. 9 between 20 and 80 km. These high
winds, especially in the region of the temperature maximum around 50 km, cause
reflection of microbaroms from an easterly source. Such stratospheric reflection is
not possible in the winter. A striking difference between the summer and winter
Local time
Loco1 t i m e
Scole
C-L
100rn 5-1
Local t i m e
FIG.17. ' Total ' (observed) wind 95-115 km for Yuma, Arizona (32" N, 112"W)
1967 November 18-19. (Same wind vector notation as in Fig. 12. After Justus, C.
personal communication).
127
Natural infrasound as an atmospheric probe
-
I00
E
.Y
90
0
I
Local t i m e
-70
0
-30
East w i n d
30
70 m sWest w i n d
1
(b)
I00
A
E
Y
90
20
21
-70
22
23
-30
24
0
East w i n d
02
01
30
70
m5-I
West w i n d
FIG. 18. Observed zonal wind at Garchy, France (47"N) (after Revah 1969).
(a) 1965 December 14;(b) 1965 December 14-15; (c) 1966 February 22; (d) 1966
February 23.
William L. Donn and David Rind
128
(C)
1 -
I
I
I
I
2
3
4
I
5
Local ti me
I
I
I
I
6
7
8
9
P
-80
-30
East wind
0
30
100 m
W e s t wind
5-1
Local time
L
-90
I
-30
I
0
E a s t wlnd
FIG.18. (c) and (d).
100 m
30
W e s t wind
5-1
Natural infrasound as an atmospheric probe
?OO(
I
I
1
i
129
I
-I
.-
-~--
1
-
.
.-
Distonce from s o u r c e ( k m )
FIG.19. Acoustic ray propagation from a point source to the east in summer, at
12 : 00. Vertical angles of incidence every 3 degrees between 52 and 85 degrees.
propagation is evident in the comparison of Fig. 19 with Figs 14-16. Reflection from
the 50 km region in Fig. 19 provides the subdued signal received from the weak summer
sources. The higher reflections, a result of the thermosphere temperature increase,
would suffer too much dissipation, as described, to provide detectable sound.
Microbaroms recorded in western United States would mostly come from Pacific
Ocean storm areas-or from the west. Hence, their propagation in winter, when
stratospheric westerlies prevail, should resemble summer propagation for Atlantic
infrasound recorded at east coast stations. Because most of the microbarom energy
would be reflected by stratospheric winds, no prominent semidiurnal wind control
in the 100-km region would be expected. As described in the observation section, no
such variation is found in the data from Pullman, Washington.
To explain the 20 : 00 minimum, we computed ray tracings on three bases:
(1) The temperature of the standard atmosphere as in Fig. 8 and a diurnal tidal
wind to 60 km with a vertically travelling phase (Fig. 10) was used. Results for this
showed no diurnal variations in propagation.
(2) In the second situation a 12 degree temperature variation reaching maximum
at noon at 50 km was added to the standard atmosphere, with the winds kept as in
(1). No significant propagation variation occurred.
(3) Theory (Lindzen 1967) and observation of meridional wind components only
(Reed, Oard & Sieminski 1969) suggest that at our latitudes the phase of the diurnal
tidal wind velocity is nearly stationary with elevation. This phase variation is a
relatively small effect and would not have been of importance in the winter calculations. In this third situation we used the same temperature data as in (2) with a
diurnal tidal wind variation of stationary phase with maximum westerly component
at 20 : 00 (a mean value of phase in Fig. 10) having the same amplitude as in the two
prior cases. Results for these data show a significant propagation difference between
12 : 00 and 20 : 00. At 12 : 00 rays with angles of 61" and higher are reflected below
130
William L. Donn and David Rind
55 km. At 20 : 00 only rays of 70" and higher are reflected from this region whereas
rays from 61" to 69" are reflected from well above 115 km (the zone of dissipation).
The loss of energy from a nine-degree sector could explain the 20 : 00 minimum.
7. Abnormal reception and stratospheric warmings
From 1967 December 22 to 1968 January 21, strong microbaroms were observed
at Palisades. The signal was often continuously high throughout the day (Fig. 6(d)),
and especially strong at 11 : 00, with 21 : 00-06 : 00 the times of normally maximum
reception. During this interval, the normal circulation of the atmosphere was disrupted, as an event known as a ' stratospheric warming' occurred (Johnson 1969). A
stratospheric ' warming ' or ' breakdown ' is characterized by the movement of a
region of warm air in the stratosphere from the Pacific Ocean or the Atlantic Ocean
towards the north pole, causing a reversal of the meridional temperature gradient
and the destruction of the characteristic westerly circulation of the winter stratosphere.
The warming noted here was the result of the movement of a stratospheric warm area
from the Atlantic Ocean.
The abnormally strong microbarom reception can be explained in terms of the
changed atmospheric structure. With west winds no longer predominating in the
middle stratosphere, the changed wind and temperature structure around 50 km may
possibly act to reflect infrasound. Furthermore the warming of this region over the
Atlantic Ocean undoubtedly affects the semidiurnal tidal wind, a thermally induced
feature. Our observation of increased amplitudes during normal hours of maximum
microbaroms leads to the speculation that either the semidiurnal wind at 100 km is
strengthened, providing for more complete reflection, or its phase is altered, leading
to reflection from below 100 km, where dissipation will be less effective.
Kreister (1968) has found a correlation between warmings originating over the
Atlantic Ocean and the later appearance in the stratosphere of the summer east wind
circulation. Fig. 4(a) shows that in May 1968 the winter diurnal pattern was still
visible; presumably the winter stratospheric circulation was still dominant. The
winter of 1968-1969 did not feature an extensive warm area in the stratosphere over
the Atlantic Ocean. Fig. 4(b) shows that in May 1969 microbarom amplitude variations showed the summer diurnal pattern, and thus the summer stratospheric circulation had appeared. Our observations are thus capable of detecting fine-scale changes
in the upper atmosphere.
8. Discussion and conclusions
Four years of continuous recording of infrasound in the frequency range 0.1 to
1 Hz by a tripartite array of capacitor microphones has revealed significant seasonal
and diurnal variations of microbarom amplitudes, Microbaroms appear to be
radiated by interfering ocean waves within a marine storm. The gross change from
high-amplitude winter to low-amplitude summer microbaroms is related to the
weakening of storms in the summer season.
During the winter when microbaroms are strong, a characteristic semidiurnal
variation occurs with maxima near noon and near midnight local time. Occasionally
a strong third maximum is present about 04 : 00. This may appear as a distinct
microbarom rise after an interval of decreasing activity or it may prolong the duration
of the previous 20-24 : 00 maximum with no clear intervening decrease.
An analysis of upper atmospheric semi-diurnal tidal winds indicates that reflection
of infrasound generated in the North Atlantic Ocean and recorded in Palisades, would
occur between about 95 and 105 km at the times of microbarom maxima; rotation of
the tidal winds are such that at times of microbarom minima, reflection would be by
winds above about 110 km. Viscous dissipation calculated to be about 60 per cent
Natural infrasound as an atmospheric probe
131
higher for each ray excursion to 115 km compared to that for reflection at 105 km
appears to account for the striking diurnal variation in microbarom amplitudes. The
occasional 04 : 00 maximum is explained by occasionally favourable winds which have
sometimes been observed at the effective lower reflection level. Reflections from
levels that seem to explain our observations would indicate the lack of turbulent
dissipation below 100 km.
During the summer season a major change in upper winds occurs. The strong but
somewhat irregular stratospheric westerlies (at about 50 km) reverse to quite steady
easterlies which tend to have a weak minimum about 20 : 00. Acoustic reflection of
storm microbaroms from the east thus becomes possible at the 50-km level during
the warm season. The 20 : 00 wind speed minimum, in conjunction with a possible
diurnal temperature variation, appears to explain the corresponding microbarom
minimum.
As all of these correlations become firmly established it should be possible to
reverse the procedure of using wind knowledge to explain microbarom variations by
using microbarom variations to deduce wind movements in the upper atmosphere.
Thus, the first appearance of increased microbarom activity and the change from the
weak summer minimum at 20 : 00 to characteristic semidiurnal variations indicates
almost to the day when the stratospheric easterlies of the summer give way to the
westerlies of the winter with the associated increase in vigor of the semi-diurnal tidal
winds.
The presence of an extended midnight maximum or a prominent 04 : 00 maximum
would indicate the presence of a strong, possibly diurnal component from the east at
the favourable reflection level (below 105 km).
Intervals of abnormal winter microbarom intensities may indicate significant
changes in atmospheric structure related to stratospheric warmings and may be the
first good indication at the surface that such a warming has begun.
In summary, the long-range propagation of infrasound, nearly always available
from ocean surface generation, is controlled by the temperature-wind structure in
winter to an elevation of about 115 km and in summer to about 50 km. Variations in
this structure quickly show up as changes in microbarom behaviour. Nearly all our
observations refer to single site recordings and interpretations. It appears to us that
the establishment of several such observation systems should give a fairly comprehensive monitoring technique for conditions in the upper atmosphere.
Not only do the observations and conclusions given apply to the behaviour of
microbaroms; all atmospheric infrasound will also be affected more or less (depending
on frequency range) by the atmospheric parameters discussed here.
We appreciate the help given by Dr N. K. Balachandran in the operation of the
ray-tracing program and that given by Dr D. Cotten in the determination of attenuation coefficients. The research was supported by Grant NSF-GA-1333 and 17454
from the National Science Foundation and Contracts DAAB 0250 and DAH 0037
from the U.S. Army Electronics Command, Fort Monmouth and the U.S. Army
Research Office, Durham. We are most grateful to these individuals and institutions.
William L. Donn:
Lamont-Doherty Geological Observatory of Columbia University,
Palisades, N . Y. 10964
and
City College of New York, New York, N . Y. 10031
David Rind:
Lamont-Doherty Geological Observatory of Columbia University,
Palisades, N . Y. 10964
132
William L. Donn and David Rind
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