Some science behind the scenes

Storms and infrasound

Storms and high winds produce infrasound.

Hurricanes generate infrasound, tornadoes generate infrasound, cyclones generate infrasound – generally of high intensity.  The Burga and Buran generate infrasound as do the Mistral and the Scirocco.

Not every wind produces infrasound, but infrasound can be produced by particularly turbulent winds or by the atmospheric turbulence generated by mountain ranges that interrupt the tropospheric wind flow.  Very turbulent winds can produce infrasound that propagates thousands of kilometers from the source region.

Infrasound produced by mountains is called ‘mountain associated infrasonic waves’ (MAW) .  They have been observed for many years by infrasonic arrays operated in places like Alaska and Antarctica, but less data is being collated elsewhere.

MAW are long period waves with very low frequency, for example they have been measured as low as  0.015 to 0.10 Hz , but their frequency is not constant because of course it depends on the turbulence patterns.  They are characterized by long period waves  in the range from 70 to 20 second periods and  long durations of quasi-sinusoidal wavetrains, of a few tenth of a Pascal amplitude,  lasting up to several days.  So they pulse.  Day and night.

Their intensity depends on how strongly the wind is blowing as well as their source, so the intensity too can vary.

Figure  I53US microphone pressure traces for MAW event  Jan. 3, 2004; source University of Fairbanks Alaska.

We can see this by the results from the following papers.  In the first papers those doing the measurements already know that wind produces infrasound and they are actually trying to get it out of the equation so that they can measure other sources of infrasound.

Detecting blast-induced infrasound in wind noise - Howard WB, Dillion KL, Shields FD; University of Mississippi National Center for Physical Acoustics,  USA.

Current efforts seek to monitor and investigate such naturally occurring events as volcanic eruptions, hurricanes, bolides entering the atmosphere, earthquakes, and tsunamis by the infrasound they generate. Often, detection of the infrasound signal is limited by the masking effect of wind noise. This paper describes the use of a distributed array to detect infrasound signals from four atmospheric detonations at White Sands Missile Range in New Mexico, USA in 2006. Three of the blasts occurred during times of low wind noise and were easily observed with array processing techniques. One blast was obscured by high wind conditions. The results of signal processing are presented that allowed localization of the blast-induced signals in the presence of wind noise in the array response.

Infrasonic wind-noise reduction by barriers and spatial filters. - Hedlin MA, Raspet R. - Institute of Geophysics and Planetary Physics, Scripps Institution of Oceanography, University of California, San Diego,  USA.

This paper reports experimental observations of wind speed and infrasonic noise reduction inside a wind barrier.

The barrier is compared with "rosette" spatial filters and with a reference site that uses no noise reduction system. The barrier is investigated for use at International Monitoring System (IMS) infrasound array sites where spatially extensive noise-reducing systems cannot be used because of a shortage of suitable land. Wind speed inside a 2-m-high 50%-porous hexagonal barrier coated with a fine wire mesh is reduced from ambient levels by 90%. If the infrasound wind-noise level reductions are all plotted versus the reduced frequency given by f*L/v, where L is the characteristic size of the array or barrier, f is the frequency, and v is the wind speed, the reductions at different wind speeds are observed to collapse into a single curve for each wind-noise reduction method. The reductions are minimal below a reduced frequency of 0.3 to 1, depending on the device, then spatial averaging over the turbulence structure leads to increased reduction.

Above the reduced corner frequency, the barrier reduces infrasonic noise by up to 20 to 25 dB. Below the corner frequency the barrier displays a small reduction of about 4 dB. The rosettes display no reduction below the corner frequency. One other advantage of the wind barrier over rosette spatial filters is that the signal recorded inside the barrier enters the microbarometer from free air and is not integrated, possibly out of phase, after propagation through a system of narrow pipes.

In this paper [from Pubmed] the researchers are trying to measure the sound.

One more example:

Infrasound induced instability by modulation of condensation process in the atmosphere.

Naugolnykh K, Rybak S. - NOAA, Earth System Research Laboratory and Zel Technologies, LLC, and CIRES, University of Colorado, Boulder, Colorado 80303-0000, USA.

A sound wave in supersaturated water vapor can modulate both the process of heat release caused by condensation, and subsequently, as a result, the resonance interaction of sound with the modulated heat release provides sound amplification.

High-intensity atmospheric perturbations such as cyclones and thunderstorms generate infrasound, which is detectable at large distances from the source. The wave-condensation instability can lead to variation in the level of infrasound radiation by a developing cyclone, and this can be as a precursor of these intense atmospheric events.