Lidar Observations of Stratospheric Gravity Waves From 2011 to 2015 at McMurdo (77.84°S, 166.69°E), Antarctica: 2. Potential Energy Densities, Lognormal Distributions, and Seasonal Variations.

Five years of Fe Boltzmann lidar's Rayleigh temperature data from 2011 to 2015 at McMurdo are used to characterize gravity wave potential energy mass density (E pm), potential energy volume density (E pv), vertical wave number spectra, and static stability N 2 in the stratosphere 30-50 km. E pm (E pv) profiles increase (decrease) with altitude, and the scale heights of E pv indicate stronger wave dissipation in winter than in summer. Altitude mean E ¯ pm and E ¯ pv obey lognormal distributions and possess narrowly clustered small values in summer but widely spread large values in winter. E ¯ pm and E ¯ pv vary significantly from observation to observation but exhibit repeated seasonal patterns with summer minima and winter maxima. The winter maxima in 2012 and 2015 are higher than in other years, indicating interannual variations. Altitude mean N 2 ¯ varies by ~30-40% from the midwinter maxima to minima around October and exhibits a nearly bimodal distribution. Monthly mean vertical wave number power spectral density for vertical wavelengths of 5-20 km increases from summer to winter. Using Modern Era Retrospective Analysis for Research and Applications version 2 data, we find that large values of E ¯ pm during wintertime occur when McMurdo is well inside the polar vortex. Monthly mean E ¯ pm are anticorrelated with wind rotation angles but positively correlated with wind speeds at 3 and 30 km. Corresponding correlation coefficients are -0.62, +0.87, and +0.80, respectively. Results indicate that the summer-winter asymmetry of E ¯ pm is mainly caused by critical level filtering that dissipates most gravity waves in summer. E ¯ pm variations in winter are mainly due to variations of gravity wave generation in the troposphere and stratosphere and Doppler shifting by the mean stratospheric winds.

Infrasonic ray tracing applied to small-scale atmospheric structures: thermal plumes and updrafts/downdrafts.

A ray-tracing program is used to estimate the refraction of infrasound by the vertical structure of the atmosphere in thermal plumes, showing only weak effects, as well as in updrafts and downdrafts, which can act as vertical wave guides. Thermal plumes are ubiquitous features of the daytime atmospheric boundary layer. The effects of thermal plumes on lower frequency sound propagation are minor with the exception of major events, such as volcanoes, forest fires, or industrial explosions where quite strong temperature gradients are involved. On the other hand, when strong, organized vertical flows occur (e.g., in mature thunderstorms and microbursts), there are significant effects. For example, a downdraft surrounded by an updraft focuses sound as it travels upward, and defocuses sound as it travels downward. Such propagation asymmetry may help explain observations that balloonists can hear people on the ground; but conversely, people on the ground cannot hear balloonists aloft. These results are pertinent for those making surface measurements from acoustic sources aloft, as well as for measurements of surface sound sources using elevated receivers.

Infrasonic ray tracing applied to mesoscale atmospheric structures: refraction by hurricanes.

A ray-tracing program is used to estimate the refraction of infrasound by the temperature structure of the atmosphere and by hurricanes represented by a Rankine-combined vortex wind plus a temperature perturbation. Refraction by the hurricane winds is significant, giving rise to regions of focusing, defocusing, and virtual sources. The refraction of infrasound by the temperature anomaly associated with a hurricane is small, probably no larger than that from uncertainties in the wind field. The results are pertinent to interpreting ocean wave generated infrasound in the vicinities of tropical cyclones.