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Observations and Modeling of Infrasound Produced by Ocean Waves. By Mark Willis Masters Student/Dept of Meteorology. Acknowledgements: Milton Garces (HIGP/ISLA), Claus Hetzer (ISLA), Steven Businger (UH Dept of Meteorology) and Paul Wittmann (FNMOC). Introduction to Infrasound.
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By Mark Willis
Masters Student/Dept of Meteorology
Acknowledgements: Milton Garces (HIGP/ISLA), Claus Hetzer (ISLA), Steven Businger (UH Dept of Meteorology) and Paul Wittmann (FNMOC)
Surf Breaking 2-5 Hz
Microbaroms 0.1-0.5 Hz
Infrasound - low frequency sound waves below the 20 Hz hearing threshold of the human ear. Infrasonic waves can propagate thousands of kilometers due to low atmospheric absorption at low frequencies.
Natural sources - severe weather, volcanoes, bolides, earthquakes, surf, mountain waves, open ocean wave-wave interactions.
* The International Monitoring System (IMS) was organized to ensure compliance of the CTBT
* IMS provides global monitoring of nuclear testing in all of the Earth’s environments: underwater, underground, and in the atmosphere
1 kiloton nuclear explosion results in infrasound in the microbarom range.
Garces (2003) developed an algorithm, based on the Arendt and Fritts model, to compute microbarom source pressure fields from ocean wave spectra provided by the NOAA Wavewatch III (WW3) model.
WW3 outputs ocean wave energy densities in 24 directional and 25 frequency bands. For thesis research, I am using WW3 output to characterize microbarom generation regions and comparing to data received at our infrasound array located near Kona, Hawaii (IS59).
IS59 – Consists of 4 Chaparral 5 microphones. 3 are organized in a triangle with 1 in the center.
Data recorded by 24-bit digitizers and sent in real time via radio telemetry to Infrasound Lab at Keahole Point.
PMCC algorithm of Cansi et al. (1995) is used to detect coherent infrasonic energy across the array. This allows us to extract speed, arrival angle, and amplitude of detected arrivals.
WW3 (Tolman, 1999) driven by NOGAPS 10m surface winds and global ice concentration values used to produce realistic wave spectra on a global 1 degree grid. WW3 outputs wave energy values (m2/Deg*Hz) in 24 directional and 25 frequency bins – this is used to calculated microbarom source fields.
* Microbarom Arrivals appear to show annual cycle associated with known weather patterns in the Pacific Basin.
Microbarom Arrival Azimuth appears to correspond with low pressure location.
Very active surface weather pattern, 7 low pressure centers evident in/near the Pacific including the intense cyclone just NNW of the Islands that created massive surf for exposed beaches on the 5th (>12m breakers observed)
Wave spectra in region of benign surface pressure pattern
Wave spectra upstream of ~950 mb storm location
(Jan. 4, 18Z 2003 Case)
Log Base 10 of Acoustic Source Pressure (Pa * m3)
Acoustic Source Pressure for 0.197 Hz shown, corresponds to ocean waves of ~ 10s interacting
Peaks in wake regions of low pressure centers, also several scattered peaks throughout the Pacific
~ The End