Seasonal changes in atmospheric noise levels and the annual variation in pigeon homing performance.

Repeated releases of experienced homing pigeons from single sites were conducted between 1972 and 1974 near Cornell University in upstate New York and between 1982 and 1983 near the University of Pittsburgh in western Pennsylvania, USA. No annual variation in homing performance was observed at these sites in eastern North America, in contrast to results from a number of similar experiments in Europe. Assuming pigeons home using low-frequency infrasonic signals (~0.1-0.3 Hz), as has been previously proposed, the annual and geographic variability in homing performance within the northern hemisphere might be explained, to a first order, by seasonal changes in low-frequency atmospheric background noise levels related to storm activity in the North Atlantic Ocean, and by acoustic waveguides formed between the surface and seasonally reversing stratospheric winds. In addition, increased dispersion among departure bearings of test birds on some North American release days was possibly caused by infrasonic noise from severe weather events during tornado and Atlantic hurricane seasons.

Detection of regional infrasound signals using array data: Testing, tuning, and physical interpretation.

This work quantifies the physical characteristics of infrasound signal and noise, assesses their temporal variations, and determines the degree to which these effects can be predicted by time-varying atmospheric models to estimate array and network performance. An automated detector that accounts for both correlated and uncorrelated noise is applied to infrasound data from three seismo-acoustic arrays in South Korea (BRDAR, CHNAR, and KSGAR), cooperatively operated by Korea Institute of Geoscience and Mineral Resources (KIGAM) and Southern Methodist University (SMU). Arrays located on an island and near the coast have higher noise power, consistent with both higher wind speeds and seasonably variable ocean wave contributions. On the basis of the adaptive F-detector quantification of time variable environmental effects, the time-dependent scaling variable is shown to be dependent on both weather conditions and local site effects. Significant seasonal variations in infrasound detections including daily time of occurrence, detection numbers, and phase velocity/azimuth estimates are documented. These time-dependent effects are strongly correlated with atmospheric winds and temperatures and are predicted by available atmospheric specifications. This suggests that commonly available atmospheric specifications can be used to predict both station and network detection performance, and an appropriate forward model improves location capabilities as a function of time.

Comparison of ICON/MIGHTI and TIMED/TIDI Neutral Wind Measurements in the Lower Thermosphere.

This study cross-compares ICON/MIGHTI and Thermosphere, Ionosphere, Mesosphere Energetics & Dynamics (TIMED)/TIMED Doppler Interferometer (TIDI) MLT region neutral winds from middle Northern Hemisphere to low Southern Hemisphere latitudes. We utilized MIGHTI level-2.2 (v4) and TIDI level-3 (v11) neutral winds from January 2020 to November 2020 and found their conjunctions using a space-time window of LST ± 15 min, latitude ± 4°, and longitude ± 4° around each TIDI wind measurement. Due to the nature of their orbital geometry, frequent conjunctions occurred between MIGHTI and TIDI. These conjunctions are spread in longitudes and they occur at approximately fixed LSTs and latitudes, which allows us to compare their observed diurnal variability. MIGHTI and TIDI wind observations agree well (except on the TIDI coldside during forward flight) and show similar large amplitude longitudinal variations that can reach more than 100 m/s. MIGHTI and TIDI zonal and meridional winds show moderate correlations of 0.60 and 0.55, respectively. The slopes of regression fits for zonal and meridional winds are 0.92 and 0.91, respectively. The root mean square differences in zonal and meridional winds are 56 and 66 m/s, respectively. We found that TIDI coldside measurements in forward flight show a systematic bias and this behavior is repetitive as the instrument pointing direction is changed by the periodic TIMED yaw maneuver. The nature of this systematic bias suggests that the TIDI zero-wind references (at least for the coldside telescopes) need revision. This investigation can provide guidance toward improving the TIDI data analysis. In addition, the results of this study act as a validation of MIGHTI MLT winds.

Evaluation of infrasound signals from the shuttle Atlantis using a large seismic network.

Inclement weather in Florida forced the space shuttle "Atlantis" to land at Edwards Air Force Base in southern California on June 22, 2007, passing near three infrasound stations and several hundred seismic stations in northern Mexico, southern California, and Nevada. The high signal-to-noise ratio, broad receiver coverage, and Atlantis' positional information allow for the testing of infrasound propagation modeling capabilities through the atmosphere to regional distances. Shadow zones and arrival times are predicted by tracing rays that are launched at right angles to the conical shock front surrounding the shuttle through a standard climatological model as well as a global ground to space model. The predictions and observations compare favorably over much of the study area for both atmospheric specifications. To the east of the shuttle trajectory, there were no detections beyond the primary acoustic carpet. Infrasound energy was detected hundreds of kilometers to the west and northwest (NW) of the shuttle trajectory, consistent with the predictions of ducting due to the westward summer-time stratospheric jet. Both atmospheric models predict alternating regions of high and low ensonifications to the NW. However, infrasound energy was detected tens of kilometers beyond the predicted zones of ensonification, possibly due to uncertainties in stratospheric wind speeds.

A wide angle and high Mach number parabolic equation.

Various parabolic equations for advected acoustic waves have been derived based on the assumptions of small Mach number and narrow propagation angles, which are of limited validity in atmospheric acoustics. A parabolic equation solution that does not require these assumptions is derived in the weak shear limit, which is appropriate for frequencies of about 0.1 Hz and above for atmospheric acoustics. When the variables are scaled appropriately in this limit, terms involving derivatives of the sound speed, density, and wind speed are small but can have significant cumulative effects. To obtain a solution that is valid at large distances from the source, it is necessary to account for linear terms in the first derivatives of these quantities [A. D. Pierce, J. Acoust. Soc. Am. 87, 2292-2299 (1990)]. This approach is used to obtain a scalar wave equation for advected waves. Since this equation contains two depth operators that do not commute with each other, it does not readily factor into outgoing and incoming solutions. An approximate factorization is obtained that is correct to first order in the commutator of the depth operators.