Microbarom
Encyclopedia
In acoustics
, microbaroms, also known as the "voice of the sea",
are a class of atmospheric
infrasonic wave
s generated in marine storm
s
by a non-linear interaction of ocean surface wave
s with the atmosphere.
They typically have narrow-band, nearly sinusoidal, waveform
s with amplitudes up to a few microbars,
and wave periods near 5 seconds (0.2 hertz
).
Due to low atmospheric absorption
at these low frequencies
, microbaroms can propagate
thousands of kilometers in the atmosphere, and can be readily detected by widely separated instruments on the Earth's surface.
Microbaroms are a significant noise source that can potentially interfere with the detection of infrasound from nuclear explosion
s that is a goal of the International Monitoring System organized under the Comprehensive Nuclear-Test-Ban Treaty (which has not entered into force).
It is a particular problem for detecting low-yield tests in the one-kiloton
range because the frequency spectra overlap.
seismologists Hugo Benioff
and Beno Gutenberg
at the California Institute of Technology
at Pasadena
, based on observations from an electromagnetic microbarograph, consisting of a wooden box with a low-frequency loudspeaker mounted on top.
They noted their similarity to microseism
s observed on seismographs, and correctly hypothesized that these signals were the result of low pressure systems in the Northeast Pacific Ocean. In 1945, Swiss geoscientist L. Saxer showed the first relationship of microbaroms with wave height in ocean storms and microbarom amplitudes.
Eric S. Posmentier published his "theory of microbaroms" in 1967 based on the oscillations of the center of gravity
of the air above the Ocean surface on which the standing waves appear, which fits well with observed data, including the doubling of the ocean wave frequency in the observed microbarom frequency.
s radiate only evanescent
acoustic waves,
and don't generate microbaroms.
Microbaroms are generated by nonlinear interactions of ocean surface waves traveling in nearly opposite directions with similar frequencies in the lee of a storm,
which produce the required standing wave
conditions, also known as the clapotis
. When the ocean storm is a tropical cyclone
, the microbaroms are not produced near the eye wall
where wind speeds are greatest, but originate from the trailing edge of the storm where the storm generated waves interact with the ambient ocean swells
.
Microbaroms may also be produced by standing waves created between two storms, or when an ocean swell is reflected at the shore.
Waves with approximately 10-second periods are abundant in the open oceans, and correspond to the observed 0.2 Hz infrasonic spectral peak of microbaroms, because microbaroms exhibit frequencies twice that of the individual ocean waves. Studies have shown that the coupling produces propagating atmospheric waves only when non-linear terms are considered.
Microbaroms are a form of persistent low-level atmospheric infrasound, generally between 0.1 and 0.5 Hz, that may be detected as coherent energy bursts or as a continuous oscillation. When the plane wave
arrivals from a microbarom source are analyzed from a phased array
of closely spaced microbarographs, the source azimuth is found to point toward the low-pressure center of the originating storm.
When the waves are received at multiple distant sites from the same source, triangulation
can confirm the source is near the center of an ocean storm.
Microbaroms that propagate up to the lower thermosphere
may be carried in an atmospheric waveguide
,
refracted
back toward the surface from below 120 km and above 150 km altitudes,
or dissipated
at altitudes between 110 and 140 km.
They may also be trapped near the surface in the lower troposphere
by planetary boundary layer
effects and surface winds, or they may by ducted in the stratosphere by upper level winds and returned to the surface through refraction, diffraction
or scattering
.
These tropospheric and stratospheric ducts are only generated along the dominant wind directions,
may vary by time of day and season,
and will not return the sound rays to the ground when the upper winds are light.
The angle of incidence of the microbarom ray determines which of these propagation modes it experiences. Rays directed vertically toward the zenith are dissipated in the thermosphere, and are a significant source of heating in that layer of the upper atmosphere. At mid latitudes in typical summer conditions, rays between approximately 30 and 60 degrees from the vertical are reflected from altitudes above 125 km where the return signals are strongly attenuated first.
Rays launched at shallower angles may be reflected from the upper stratosphere at approximately 45 km above the surface in mid latitudes,
or from 60–70 km in low latitudes.
Atmospheric scientists have used these effects for inverse remote sensing
of the upper atmosphere using microbaroms.
Measuring the trace velocity of the reflected microbarom signal at the surface gives the propagation velocity at the reflection height, as long as the assumption that the speed of sound only varies along the vertical, and not over the horizontal, is valid. If the temperature at the reflection height can be estimated with sufficient precision, the speed of sound
can be determined and subtracted from the trace velocity, giving the upper level wind speed. One advantage of this method is the ability to measure continuously – other methods that can only take instantaneous measurements may have their results distorted by short-term effects.
Additional atmospheric information can be deduced from microbarom amplitude if the source intensity is known. Microbaroms are produced by upward directed energy transmitted from the ocean surface through the atmosphere. The downward directed energy is transmitted through the ocean to the sea floor, where it is coupled to the Earth's crust and transmitted as microseisms with the same frequency spectrum. However, unlike microbaroms, where the near vertical rays are not returned to the surface, only the near vertical rays in the ocean are coupled to the sea floor. By monitoring the amplitude of received microseisms from the same source using seismographs, information on the source amplitude can be derived. Because the solid earth provides a fixed reference frame,
the transit time of the microseisms from the source is constant, and this provides a control for the variable transit time of the microbaroms through the moving atmosphere.
Acoustics
Acoustics is the interdisciplinary science that deals with the study of all mechanical waves in gases, liquids, and solids including vibration, sound, ultrasound and infrasound. A scientist who works in the field of acoustics is an acoustician while someone working in the field of acoustics...
, microbaroms, also known as the "voice of the sea",
are a class of atmospheric
Earth's atmosphere
The atmosphere of Earth is a layer of gases surrounding the planet Earth that is retained by Earth's gravity. The atmosphere protects life on Earth by absorbing ultraviolet solar radiation, warming the surface through heat retention , and reducing temperature extremes between day and night...
infrasonic wave
Wave
In physics, a wave is a disturbance that travels through space and time, accompanied by the transfer of energy.Waves travel and the wave motion transfers energy from one point to another, often with no permanent displacement of the particles of the medium—that is, with little or no associated mass...
s generated in marine storm
Storm
A storm is any disturbed state of an astronomical body's atmosphere, especially affecting its surface, and strongly implying severe weather...
s
by a non-linear interaction of ocean surface wave
Ocean surface wave
In fluid dynamics, wind waves or, more precisely, wind-generated waves are surface waves that occur on the free surface of oceans, seas, lakes, rivers, and canals or even on small puddles and ponds. They usually result from the wind blowing over a vast enough stretch of fluid surface. Waves in the...
s with the atmosphere.
They typically have narrow-band, nearly sinusoidal, waveform
Waveform
Waveform means the shape and form of a signal such as a wave moving in a physical medium or an abstract representation.In many cases the medium in which the wave is being propagated does not permit a direct visual image of the form. In these cases, the term 'waveform' refers to the shape of a graph...
s with amplitudes up to a few microbars,
and wave periods near 5 seconds (0.2 hertz
Hertz
The hertz is the SI unit of frequency defined as the number of cycles per second of a periodic phenomenon. One of its most common uses is the description of the sine wave, particularly those used in radio and audio applications....
).
Due to low atmospheric absorption
Absorption (acoustics)
Acoustic absorption is that property of any material that changes the acoustic energy of sound waves into another form, often heat, which it to some extent retains, as opposed to that sound energy that material reflects or conducts. Acoustic absorption is represented by the symbol A in calculations...
at these low frequencies
Frequency
Frequency is the number of occurrences of a repeating event per unit time. It is also referred to as temporal frequency.The period is the duration of one cycle in a repeating event, so the period is the reciprocal of the frequency...
, microbaroms can propagate
Wave propagation
Wave propagation is any of the ways in which waves travel.With respect to the direction of the oscillation relative to the propagation direction, we can distinguish between longitudinal wave and transverse waves....
thousands of kilometers in the atmosphere, and can be readily detected by widely separated instruments on the Earth's surface.
Microbaroms are a significant noise source that can potentially interfere with the detection of infrasound from nuclear explosion
Nuclear explosion
A nuclear explosion occurs as a result of the rapid release of energy from an intentionally high-speed nuclear reaction. The driving reaction may be nuclear fission, nuclear fusion or a multistage cascading combination of the two, though to date all fusion based weapons have used a fission device...
s that is a goal of the International Monitoring System organized under the Comprehensive Nuclear-Test-Ban Treaty (which has not entered into force).
It is a particular problem for detecting low-yield tests in the one-kiloton
TNT equivalent
TNT equivalent is a method of quantifying the energy released in explosions. The ton of TNT is a unit of energy equal to 4.184 gigajoules, which is approximately the amount of energy released in the detonation of one ton of TNT...
range because the frequency spectra overlap.
History
Microbaroms were first described in 1939 by AmericanUnited States
The United States of America is a federal constitutional republic comprising fifty states and a federal district...
seismologists Hugo Benioff
Hugo Benioff
Victor Hugo Benioff was an American seismologist and a professor at the California Institute of Technology. He is best remembered for his work in charting the location of deep earthquakes in the Pacific Ocean....
and Beno Gutenberg
Beno Gutenberg
Beno Gutenberg was a German-American seismologist who made several important contributions to the science...
at the California Institute of Technology
California Institute of Technology
The California Institute of Technology is a private research university located in Pasadena, California, United States. Caltech has six academic divisions with strong emphases on science and engineering...
at Pasadena
Pasadena, California
Pasadena is a city in Los Angeles County, California, United States. Although famous for hosting the annual Rose Bowl football game and Tournament of Roses Parade, Pasadena is the home to many scientific and cultural institutions, including the California Institute of Technology , the Jet...
, based on observations from an electromagnetic microbarograph, consisting of a wooden box with a low-frequency loudspeaker mounted on top.
They noted their similarity to microseism
Microseism
In seismology, a microseism is defined as a faint earth tremor caused by natural phenomena. The term is most commonly used to refer to the dominant background seismic noise signal on Earth, which are mostly composed of Rayleigh waves and caused by water waves in the oceans and lakes...
s observed on seismographs, and correctly hypothesized that these signals were the result of low pressure systems in the Northeast Pacific Ocean. In 1945, Swiss geoscientist L. Saxer showed the first relationship of microbaroms with wave height in ocean storms and microbarom amplitudes.
Eric S. Posmentier published his "theory of microbaroms" in 1967 based on the oscillations of the center of gravity
Center of gravity
In physics, a center of gravity of a material body is a point that may be used for a summary description of gravitational interactions. In a uniform gravitational field, the center of mass serves as the center of gravity...
of the air above the Ocean surface on which the standing waves appear, which fits well with observed data, including the doubling of the ocean wave frequency in the observed microbarom frequency.
Theory
Isolated traveling ocean surface gravity waveGravity wave
In fluid dynamics, gravity waves are waves generated in a fluid medium or at the interface between two media which has the restoring force of gravity or buoyancy....
s radiate only evanescent
Evanescent wave
An evanescent wave is a nearfield standing wave with an intensity that exhibits exponential decay with distance from the boundary at which the wave was formed. Evanescent waves are a general property of wave-equations, and can in principle occur in any context to which a wave-equation applies...
acoustic waves,
and don't generate microbaroms.
Microbaroms are generated by nonlinear interactions of ocean surface waves traveling in nearly opposite directions with similar frequencies in the lee of a storm,
which produce the required standing wave
Standing wave
In physics, a standing wave – also known as a stationary wave – is a wave that remains in a constant position.This phenomenon can occur because the medium is moving in the opposite direction to the wave, or it can arise in a stationary medium as a result of interference between two waves traveling...
conditions, also known as the clapotis
Clapotis
In hydrodynamics, the clapotis is a non-breaking standing wave pattern, caused for example, by the reflection of a traveling surface wave train from a near vertical shoreline like a breakwater, seawall or steep cliff....
. When the ocean storm is a tropical cyclone
Tropical cyclone
A tropical cyclone is a storm system characterized by a large low-pressure center and numerous thunderstorms that produce strong winds and heavy rain. Tropical cyclones strengthen when water evaporated from the ocean is released as the saturated air rises, resulting in condensation of water vapor...
, the microbaroms are not produced near the eye wall
Eye (cyclone)
The eye is a region of mostly calm weather found at the center of strong tropical cyclones. The eye of a storm is a roughly circular area and typically 30–65 km in diameter. It is surrounded by the eyewall, a ring of towering thunderstorms where the second most severe weather of a cyclone...
where wind speeds are greatest, but originate from the trailing edge of the storm where the storm generated waves interact with the ambient ocean swells
Swell (ocean)
A swell, in the context of an ocean, sea or lake, is a series surface gravity waves that is not generated by the local wind. Swell waves often have a long wavelength but this varies with the size of the water body, e.g. rarely more than 150 m in the Mediterranean, and from event to event, with...
.
Microbaroms may also be produced by standing waves created between two storms, or when an ocean swell is reflected at the shore.
Waves with approximately 10-second periods are abundant in the open oceans, and correspond to the observed 0.2 Hz infrasonic spectral peak of microbaroms, because microbaroms exhibit frequencies twice that of the individual ocean waves. Studies have shown that the coupling produces propagating atmospheric waves only when non-linear terms are considered.
Microbaroms are a form of persistent low-level atmospheric infrasound, generally between 0.1 and 0.5 Hz, that may be detected as coherent energy bursts or as a continuous oscillation. When the plane wave
Plane wave
In the physics of wave propagation, a plane wave is a constant-frequency wave whose wavefronts are infinite parallel planes of constant peak-to-peak amplitude normal to the phase velocity vector....
arrivals from a microbarom source are analyzed from a phased array
Phased array
In wave theory, a phased array is an array of antennas in which the relative phases of the respective signals feeding the antennas are varied in such a way that the effective radiation pattern of the array is reinforced in a desired direction and suppressed in undesired directions.An antenna array...
of closely spaced microbarographs, the source azimuth is found to point toward the low-pressure center of the originating storm.
When the waves are received at multiple distant sites from the same source, triangulation
Triangulation
In trigonometry and geometry, triangulation is the process of determining the location of a point by measuring angles to it from known points at either end of a fixed baseline, rather than measuring distances to the point directly...
can confirm the source is near the center of an ocean storm.
Microbaroms that propagate up to the lower thermosphere
Thermosphere
The thermosphere is the biggest of all the layers of the Earth's atmosphere directly above the mesosphere and directly below the exosphere. Within this layer, ultraviolet radiation causes ionization. The International Space Station has a stable orbit within the middle of the thermosphere, between...
may be carried in an atmospheric waveguide
Atmospheric waveguide
An atmospheric waveguide is an atmospheric flow feature that improves the propagation of certain atmospheric waves.The effect arises because wave parameters such as group velocity or vertical wavenumber depend on mean flow direction and strength...
,
refracted
Refraction
Refraction is the change in direction of a wave due to a change in its speed. It is essentially a surface phenomenon . The phenomenon is mainly in governance to the law of conservation of energy. The proper explanation would be that due to change of medium, the phase velocity of the wave is changed...
back toward the surface from below 120 km and above 150 km altitudes,
or dissipated
Dissipation
In physics, dissipation embodies the concept of a dynamical system where important mechanical models, such as waves or oscillations, lose energy over time, typically from friction or turbulence. The lost energy converts into heat, which raises the temperature of the system. Such systems are called...
at altitudes between 110 and 140 km.
They may also be trapped near the surface in the lower troposphere
Troposphere
The troposphere is the lowest portion of Earth's atmosphere. It contains approximately 80% of the atmosphere's mass and 99% of its water vapor and aerosols....
by planetary boundary layer
Planetary boundary layer
The planetary boundary layer , also known as the atmospheric boundary layer , is the lowest part of the atmosphere and its behavior is directly influenced by its contact with a planetary surface. On Earth it usually responds to changes in surface forcing in an hour or less...
effects and surface winds, or they may by ducted in the stratosphere by upper level winds and returned to the surface through refraction, diffraction
Diffraction
Diffraction refers to various phenomena which occur when a wave encounters an obstacle. Italian scientist Francesco Maria Grimaldi coined the word "diffraction" and was the first to record accurate observations of the phenomenon in 1665...
or scattering
Scattering
Scattering is a general physical process where some forms of radiation, such as light, sound, or moving particles, are forced to deviate from a straight trajectory by one or more localized non-uniformities in the medium through which they pass. In conventional use, this also includes deviation of...
.
These tropospheric and stratospheric ducts are only generated along the dominant wind directions,
may vary by time of day and season,
and will not return the sound rays to the ground when the upper winds are light.
The angle of incidence of the microbarom ray determines which of these propagation modes it experiences. Rays directed vertically toward the zenith are dissipated in the thermosphere, and are a significant source of heating in that layer of the upper atmosphere. At mid latitudes in typical summer conditions, rays between approximately 30 and 60 degrees from the vertical are reflected from altitudes above 125 km where the return signals are strongly attenuated first.
Rays launched at shallower angles may be reflected from the upper stratosphere at approximately 45 km above the surface in mid latitudes,
or from 60–70 km in low latitudes.
Atmospheric scientists have used these effects for inverse remote sensing
Remote sensing
Remote sensing is the acquisition of information about an object or phenomenon, without making physical contact with the object. In modern usage, the term generally refers to the use of aerial sensor technologies to detect and classify objects on Earth by means of propagated signals Remote sensing...
of the upper atmosphere using microbaroms.
Measuring the trace velocity of the reflected microbarom signal at the surface gives the propagation velocity at the reflection height, as long as the assumption that the speed of sound only varies along the vertical, and not over the horizontal, is valid. If the temperature at the reflection height can be estimated with sufficient precision, the speed of sound
Speed of sound
The speed of sound is the distance travelled during a unit of time by a sound wave propagating through an elastic medium. In dry air at , the speed of sound is . This is , or about one kilometer in three seconds or approximately one mile in five seconds....
can be determined and subtracted from the trace velocity, giving the upper level wind speed. One advantage of this method is the ability to measure continuously – other methods that can only take instantaneous measurements may have their results distorted by short-term effects.
Additional atmospheric information can be deduced from microbarom amplitude if the source intensity is known. Microbaroms are produced by upward directed energy transmitted from the ocean surface through the atmosphere. The downward directed energy is transmitted through the ocean to the sea floor, where it is coupled to the Earth's crust and transmitted as microseisms with the same frequency spectrum. However, unlike microbaroms, where the near vertical rays are not returned to the surface, only the near vertical rays in the ocean are coupled to the sea floor. By monitoring the amplitude of received microseisms from the same source using seismographs, information on the source amplitude can be derived. Because the solid earth provides a fixed reference frame,
the transit time of the microseisms from the source is constant, and this provides a control for the variable transit time of the microbaroms through the moving atmosphere.