Modeled Variations in Short Range Infrasonic Amplitudes Using High-Resolution Radiosonde Data

Numerical simulations of long range (>250km) infrasonic amplitudes sometimes indicate a shadow zone or annulus of low amplitudes at the ground surface. This "zone of silence" is the result of changes in sound speed with altitude and the presence of a generally upward refracting troposphere beneath a stratospheric duct that refracts infrasound back toward the surface of the earth. Observations of long range infrasonic amplitudes indicate significant variations in the strength and shape of this shadow zone. These variations are often attributed to fine scale (<1.0km) changes in atmospheric parameters (temperature, wind speed, and direction). Observations in the near field (<10km) indicate significant variations in the amplitudes of infrasonic arrivals can also occur as a function of both time and azimuth. Again, these variations may be attributed to fine scale changes in atmospheric parameters. In the case of the near field, however, these changes must take place in the lower troposphere and have a smaller characteristic length. Over 100 high resolution numerical simulations of near field infrasonic waves were conducted using the Parabolic Equation (PE) method and the Time Domain Parabolic Equation (TDPE) method. We used a radiosonde database to reconstruct high resolution sound speed profiles for these simulations. We examined daily, weekly, seasonal, and azimuthal variations infrasonic amplitudes computed via the PE and TDPE. The simulations indicate that an order of magnitude or greater variation in the amplitudes of infrasonic arrivals can occur as a function of azimuth and across all of the above mentioned time scales. Analysis of the radiosonde-based sound speed profiles further show these variations can often be attributed to thermal inversions, and low altitude (<2km) wind jets. A statistical analysis was performed in order to quantify the frequency, strength, and cause of these predicted amplitude variations.