The ability to locate explosive events with infrasound pivots on our understanding of the temporal and spatial scales of atmospheric variability that define the propagation paths that sound waves may take from the source to the receiver. Source location analyses of bolide events suggest that high-altitude sources may efficiently ensonify elevated waveguides in the stratosphere, and that this acoustic energy may leak to the ground by diffraction and scattering. There is also a precedent for ground-based explosive sources that may yield stratospheric returns even when sound is propagating against the prevailing winds. Microbaroms are infrasonic waves generated by nonlinear interactions of ocean surface waves traveling in nearly opposite directions with similar frequencies. Such interactions commonly occur between ocean waves with approximately 10-second periods, which are abundant in the open oceans and correspond to the observed 0.2 Hz infrasonic spectral peak. Although microseisms and microbaroms share a common source, microseisms in the ground and oceans are generated by the vertical component and microbaroms are generated by the near horizontal component of the source radiation pattern. In contrast to microseisms, microbarom signals are also strongly influenced by the atmospheric winds. We compared microbarom signals observed at Hawaii in 2003 with the Wavewatch 3 ocean wave model and the Naval Research Laboratory (NRL) Ground-to-Space (G2S) atmospheric specification. The arrival azimuths of coherent microbarom signals observed in Hawaii are associated with high ocean wave activity in the Pacific Basin, the dominant wind directions in the troposphere, stratosphere, and mesosphere, and the thermal structure of the atmosphere. Some of the seasonal trends in the microbarom observations can be explained by the winds in the stratosphere and lower mesosphere, while some of the daily variability can be explained by the winds in the troposphere and lower stratosphere. However, coherent energy from powerful swells may overcome the wind-carried microbarom signals and arrive to the station through thermospheric ducting. Our observations suggest that either (1) the wind speeds in the troposphere, stratosphere and lower mesosphere may be underestimated in atmospheric models or (2) leaky elevated infrasonic waveguides are persistent propagation paths that should be investigated in more detail. Although ocean swells have been previously used as a natural source for continuous measurements of atmospheric winds over long horizontal ranges, recent advances in measurement and modeling techniques can provide new insight on this complex but tractable method for continuous, passive acoustic tomography of the atmosphere. 26th Seismic Research Review Trends in Nuclear Explosion Monitoring

Severe weather in the ocean generates infrasonic signals in the 0.1-0.5 Hz frequency band that can propagate for thousands of kilometers. The source generation mechanism for these microbarom signals is attributed to the nonlinear interaction of surface ocean waves, which are predicted to radiate acoustically only if the swells are almost opposite in direction and of a near identical frequency. We study the statistics of microbarom signals detected at International Monitoring System (IMS) station I59US, Hawaii, to identify features that may be used to assess array performance in the microbarom range. For selected storm systems, we use the global ocean wave estimates produced by National Oceanic and Atmospheric Administration's (NOAA's) Wavewatch 3 (WW3) to estimate the spatial and temporal distribution of the acoustic source spectra induced by nonlinear ocean wave interactions. We then use empirical amplitude scaling relationships to predict microbarom signal levels and peak frequencies. Results are presented with a view toward applications in storm and sea-state evolution and how these affect infrasonic detection thresholds in the microbarom frequency band.

Severe weather in the ocean generates infrasonic signals in the 0.1- to 0.5-Hz frequency band that can propagate forthousands of kilometers. The source generation mechanism for microbaroms is believed to be the same as formicroseisms, and is attributed to the nonlinear interaction of surface ocean waves. We compare theoreticalpredictions with infrasonic observations of Hurricane Daniel in July of 2000. The nonlinear interaction of the oceanwave field is predicted to radiate sound waves...