Generation and Refraction of the Microbarom Signal by Hurricanes over the Atlantic Ocean

date February 6, 2015
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Friday, February 6, 2015 4:00pm in ETC 4.150

Dr. Roger Waxler
National Center for Physical Acoustics
The University of Mississippi
http://www.olemiss.edu/depts/physics_and_astronomy/faculty/waxler.html

It is well known that both acoustic and seismic noise spectra show an increased band of spatially coherent noise around 0.2 Hz, the so-called microbarom and microseism signals. It has been appreciated for over a half of a century that hurricanes over the open ocean are a source of microbaroms and microseisms. While the mechanism for microseism generation has been understood since the early 1960’s, a complete theory of the generation of microbaroms did not appear until 2006. It has been shown that microbaroms and microseisms are two manifestations of the same phenomenon: a radiating harmonic of the ocean surface wave field produced by the head-on collision of ocean surface waves of the same period. In 2008 it was conjectured that, for a deep ocean hurricane, the source of the colliding waves is the interaction of the hurricane generated waves with the background ocean swell. The region in which this interaction takes place, the microbarom source region, is generally several hundred kilometers from the eye of the storm. This source region’s position with respect to the eye of the storm changes slowly as the storm moves across the ocean so that it can be considered to be more or less static with respect to the storm. As the microbarom signal propagates away from the source region, propagation paths that pass through the storm are strongly refracted by the storm winds. Thus, hurricanes carry a sound source with them which might be used to probe the interior of the storm itself. To study this effect, a temporary network of infrasound stations was deployed along the US eastern seaboard during the 2010 and 2011 hurricane seasons. The underlying theory will be presented and results from the deployments will be shown.