Effects on Infrasound Propagation from a Self Consistent Spectral Gravity Wave Model

Fine scale inhomogeneities in the atmosphere are believed to have a significant impact on the propagation of infrasound. For example, the location and extent of shadow zones due to stratospheric ducting as well as the amplitude of arrivals outside of these shadow zones are both affected by the inhomogeneities. Gravity waves are identified as a dominant source of fine scale heterogeneities, but the effects of gravity waves on infrasound propagation are not fully understood. In order to quantify these effects we ran multiple numerical simulations of infrasound propagation through an atmosphere with gravity waves present. The propagation simulations were at model resolutions corresponding to the characteristic lengths of the gravity waves. To achieve this high resolution characterization of the atmosphere we used current wind and temperature specifications (i.e. HWM07 and NRL-G2S) coupled to a spectral gravity wave model that generated perturbations which were self consistent with the background flow field. The gravity waves were generated on grid resolution of 4.0 km and 0.25 km in the horizontal and vertical directions, respectively. After the wind and temperature fields were perturbed by the gravity wave model they were used as input for infrasound propagation models run at the same resolution. The propagation models included the Parabolic Equation (PE) method and the Time Domain Parabolic Equation (TDPE) method. Multiple instances of gravity wave perturbations were generated for propagation scenarios that corresponded to events such as the Ghislenghien explosion (30 July 2004), Snohomish bolide (03 June 2004), and Watusi explosion (28 Sept. 2002). Results indicated that high resolution simulations incorporating gravity waves produced arrivals inside regions that would otherwise be considered shadow zones. In addition, the amplitudes of arrivals outside of the shadow zones were affected by the presence of gravity waves.