Distribution of wave packet sizes in microbarom wave trains observed in Alaska

Abstract

This work reports on a study of the distribution of wave packet sizes contained in intervals of continuous microbarom activity. Microbaroms are a class of atmospheric infrasound that is characterized by narrow-band, nearly sinusoidal, waveforms with periods near 5 s. They are known to be generated by marine storms, presumably through a nonlinear interaction of surface waves, however the detailed analysis of the process is still incomplete. The data analyzed were obtained using the University of Alaska infrasound array of four microphones located in central Alaska. Because of the narrow-band feature of the microbarom signals, the Hilbert transform is applicable as a method for finding phase breaks in the signal. The phase breaks are interpreted as the demarcation of the boundaries of wave packets. When applied to long sequences of microbaroms a broad distribution of packet lengths is found that diminishes monotonically with length and has a mean near 10 cycles and a variance nearly as large. The distribution function decreases exponentially with packet length. The distribution of packet sizes is influenced by the presence of multiple sources and multiple propagation paths between the sources and the sensor array. Identification of individual packets should open the way to a more detailed analysis of microbarom wave trains. After separating the wave train into individual wavelets the intermicrophone correlation is estimated as a function of microphone separation. As has been observed in earlier microbarom studies, a decrease in correlation was observed for microphone pairs orthogonal to the direction of propagation when compared to correlations between microphones spaced along the direction of wave propagation.