Microbarom

In acoustics, microbaroms, also known as the 'voice of the sea', are a class of atmospheric infrasonic waves generated in marine stormsby a non-linear interaction of ocean surface waves with the atmosphere. They typically have narrow-band, nearly sinusoidal waveforms 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. In acoustics, microbaroms, also known as the 'voice of the sea', are a class of atmospheric infrasonic waves generated in marine stormsby a non-linear interaction of ocean surface waves with the atmosphere. They typically have narrow-band, nearly sinusoidal waveforms 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 explosions 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. The reason for the discovery of this phenomenon was an accident: the aerologists working at the marine hydrometeorological stations and watercrafts drew attention to the strange pain that a person experiences when approaching the surface of a standard meteorological probe (a balloon filled with hydrogen). During one of the expeditions, this effect was demonstrated to the Soviet academician V. V. Shuleikin by the chief meteorologist V. A. Berezkin. This phenomenon drew genuine interest among scientists; in order to study it, special equipment was designed to record powerful but low-frequency vibrations that are not audible to human ears. As a result of several series of experiments, the physical essence of this phenomenon was clarified and in 1935 when V.V. Shuleikin published his first work entirely devoted to the infrasonic nature of the “voice of the sea”. However, a little later, N.N. Andreev published a more detailed explanation of the formation of the “voice of the sea” based on the mathematical theory of vortex formation behind streamlined bodies, and V. V. Shuleikin in his book “Physics of the Sea” recognized this approach as more accurate. Microbaroms were first described in United States in 1939 by American 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 microseisms 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. Isolated traveling ocean surface gravity waves 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.