R. Michael Jones

Infrasound from convective storms. III. Propagation to the ionosphere

We model mathematically the spectral features of infrasound observed in the ionosphere and believed to be radiated by severe thunderstorms. We explain the dominant 2–5‐min wave period as an effect of atmospheric filtering; shorter periods are excessively attenuated by absorption in transit to the ionosphere, and longer periods are attenuated in portions of the atmosphere where the waves are evanescent because their frequencies are below the acoustic cutoff. An observed spectral ’’fine structure’’ within the 2–5‐min band is explained in terms of resonant interactions between the waves and the atmospheric temperature structure. Accurate quantitative modeling of all these details of the storm‐to‐ionosphere transmission coefficient requires numerical integration of the acoustic‐gravity wave equation, including the effects of ground reflection, absorption, and partial reflections in the atmosphere.Subject Classification: [43]28.30.

Dissipative Waves Excited by Gravity-Wave Encounters with the Stably Stratified Planetary Boundary Layer

Abstract We suggest that the strata of strong echo returns frequently revealed by remote-sensor records of the stably stratified planetary bound layer (PBL) represent the wavefronts of dissipative waves (viscous and thermal-conduction waves) excited by gravity-wave encounters with the PBL and the earths surface. The viscous waves appear to be more strongly forced and should therefore dominate the observations. This simple picture accounts for the following observed properties of the strata: 1) their nearly ubiquitous presence within the stably stratified PBL, 2) their nearly horizontal orientation, 3) the small spacing (some tens of meters, typically) separating the strata, 4) variability in that spacing in both height and time, and 5) the high shears and temperature gradients associated with the strata. Preliminary calculations of the energy fluxes and stresses associated with the wave motions, also presented here, suggest strongly that such waves are not mere curiosities of the PBL but reveal important...

Lidar observations of stratospheric gravity waves from 2011 to 2015 at McMurdo (77.84°S, 166.69°E), Antarctica: 1. Vertical wavelengths, periods, and frequency and vertical wave number spectra

Five years of atmospheric temperature data, collected with an Fe Boltzmann lidar by the University of Colorado group from 2011 to 2015 at Arrival Heights, are used to characterize the vertical wavelengths, periods, vertical phase speeds, frequency spectra, and vertical wave number spectra of stratospheric gravity waves from 30 to 50 km altitudes. Over 1000 dominant gravity wave events are identified from the data. The seasonal spectral distributions of vertical wavelengths, periods, and vertical phase speeds in summer, winter, and spring/fall are found obeying a lognormal distribution. Both the downward and upward phase progression gravity waves are observed by the lidar, and the fractions of gravity waves with downward phase progression increase from summer ~59% to winter ~70%.

The dispersion relation for internal acoustic-gravity waves in a baroclinic fluid

The dispersion relation for internal waves in a fluid is generalized from the barotropic approximation to the baroclinic case to allow for the inclination of surfaces of constant density to surfaces of constant pressure. This generalization allows the barotropic approximation to be tested in a variety of situations. The dispersion relation applies to both acoustic waves and internal gravity waves propagating in either the ocean or the atmosphere. Imaginary terms in the dispersion relation proportional to the baroclinic vector indicate energy exchange between the wave and the mean flow, a result of buoyancy being a nonconservative force in a baroclinic fluid. A buoyancy calculation shows that the baroclinic generalization of the Brunt–Vaisala frequency N is given by N2=∇ρθ⋅∇p/ρ2, where ρ is density, ρθ is potential density, and p is pressure. The baroclinicity in a weather front or cyclonic ring can sometimes have as large an effect as the Earth’s rotation on the propagation of internal gravity waves.

On using ambient internal waves to monitor Brunt-Väisälä frequency

The practicality of obtaining significant information about profiles of Brunt-Vaisala frequency from surface observations of internal waves is investigated. The inversion method investigated uses a three-dimensional Fourier transform (two spatial and one temporal) to determine the dispersion relations of the internal-wave modes and Abel transforms to estimate those aspects of the Brunt-Vaisala frequency profile to which internal-wave dispersion relations are sensitive. If classical Fourier transforms are used to estimate the modal dispersion relations, observations over a 200-km square region for 17 hours would be necessary to obtain about a 15% error for one profile. However, if nonlinear methods, such as maximum entropy, could be extended to three dimensions, it might be practical to obtain significant information about profiles of Brunt-Vaisala frequency by observing a 40-km square region for only 3 hours. This technique cannot be used when there are no significant surface manifestations of internal waves, such as during rough seas or when a significant mixed layer is present. The ambiguity of the inversion is increased when the Brunt-Vaisala frequency profile has multiple maxima.

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.

Nonperturbative modal tomography inversion. Part I. Theory

The mathematical basis for a method to invert modal ocean acoustic tomography measurements without explicitly assuming an initial sound‐speed profile is described. The method is based on determining the group and phase travel times for each vertical slice through the tomographic region. The group travel times are determined directly as the measurements of modal pulse travel time. The phase travel times are determined by resolving the cycle ambiguities in the phase measurements with constraints connecting the group and phase travel times. Standard tomographic techniques then determine the modal group and phase speeds within the tomographic region, and Abel transforms can be used to determine the symmetric part (the difference between the upper and lower profiles) of the sound channel. As with ray tomography, modal tomographic measurements supply no information about the antisymmetric part of the sound channel (the average of the upper and lower profiles), but determining the lower part of the sound channel...

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.

NONPERTURBATIVE OCEAN ACOUSTIC TOMOGRAPHY INVERSION OF 1000-KM PULSE PROPAGATION IN THE PACIFIC OCEAN

A nonperturbative inversion was performed of acoustic tomography measurements made in the northeastern Pacific Ocean in July 1989, in which acoustic transmissions from a 250‐Hz broadband source located near the sound‐channel axis were recorded at a long vertical array of hydrophones 1000 km away. In contrast with a conventional inversion, this nonperturbative inversion does not assume that travel times are linearly related to the sound‐speed deviations from a background sound‐speed model. The inversion process involved three steps: (1) Measured pulse travel times and the source and receiver locations were used to determine the range average of the equivalent symmetric sound‐slowness profile. That part of the inversion used only curve fitting and Abel transforms, and required independent (nontomographic) information only to help identify the pulse arrivals. (2) Under the assumption that the range dependence of sound speed was small, we used the reciprocal of the range‐averaged sound‐slowness profile to app...

The perturbed structure of the neutral atmospheric boundary layer over irregular terrain

The first-order (linear) response of the planetary boundary layer is calculated for flow over periodic terrain, for variations in both surface roughness and terrain elevation. Calculations are made for horizontal wavenumbers varying from 10−4m−1 to 3 × 10−3m−1. A simple second-order closure model of the turbulence is used, and Coriolis and buoyancy forces are neglected. As expected, flow over a periodic terrain produces corresponding periodic structure in all meteorological fields above the surface. The periodic structure consists of two components. The first is very nearly evanescent with height, showing little vertical structure. It corresponds to the motion that would be observed were the atmosphere inviscid. The second component, introduced by turbulent viscosity, exhibits considerable vertical structure, with vertical wavelengths the order of 100 m, and thus could be responsible for the layering often seen on acoustic sounder observations of the atmospheric boundary layer.