A two-fluid model, air over seawater, is used to investigate the radiation of infrasound by ocean waves. The acoustic radiation which results from the motion of the air/water interface is known to be a nonlinear effect. The second-order nonlinear contribution to the acoustic radiation is computed and the statistical properties of the received microbarom signals are related to the statistical properties of the ocean wave system. The physical mechanisms and source strengths for radiation into the atmosphere and ocean are compared. The observed ratio of atmospheric to oceanic microbarom peak pressure levels ͑approximately 1 to 1000͒ is explained.

Two electric field soundings through thunderstorm anvil clouds show similar charge structures: negatively charged screening layers on the top and bottom surfaces, a layer of positive charge in the interior, and one or two layers of zero charge. Both anvil clouds were strongly electrified: the peak magnitudes of the electric field in the two storms were 70 and 90 kV/m, respectively. The nonzero layers had charge densities comparable to those found in the cores of thunderstorms, ranging in magnitude from 0.4 to 2.7 nC/m3. Layers varied in thickness from 300 to 2000 m. The positive charge probably originated in the main positive charge region normally found at high altitudes in the core of thunderclouds. Transporting positive charge from the storm core to the anvil may influence the ratio of intracloud to cloud‐to‐ground lightning flashes and the rate of generation of charge in the core. The negatively charged layers probably were screening layers, resulting from the discontinuity in the electrical conductivity at the cloud boundaries. The lower negative screening layer appeared to be carried toward the storm core by winds below and at the lower anvil boundary.

A Green's function method is used to derive a fast, general algorithm for one-way wave propagation. The algorithm is applied to outdoor sound propagation. The general method is not limited to atmospheric sound propagation, however, and can be applied to other problems, such as sound propagation in the ocean and electromagnetic wave propagation. The new algorithm, called "GF-PE" (Green's function method for the parabolic equation), reduces to the well-known Fourier split-step algorithm for the parabolic equation (PE) when no boundary conditions are imposed (e.g., at a ground surface). With the GF-PE, range steps many wavelengths long are possible, while with a PE algorithm based on a finite-difference range step, such as the Crank-Nicolson method, the range steps are typically limited to a fraction of a wavelength. Because of its longer range step, the new algorithm is 40-450 times faster than PE algorithms that use the Crank-Nicolson method. For outdoor sound propagation over a locally reacting ground surface, the computed GF-PE field is the sum of three terms: a direct wave, a specularly reflected wave, and a surface wave. With the new method, the air-ground impedance condition is treated exactly and results in an analytic expression for the surface wave contribution. Numerical results from the GF-PE model are presented and compared to exact calculations, fast-field program (FFP) calculations, and PE results computed with the CrankNicolson method. The GF-PE algorithm is shown to be accurate and approximately two orders of magnitude faster than a PE based on the Crank-Nicolson method. Hence, the new algorithm opens the door to some useful new computational capabilities such as real-time predictions on desktop computers, fast pulse calculations, and practical three-dimensional calculations.

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The computational tools available for prediction of sound propagation through the atmosphere have increased dramatically during the past decade. The numerical techniques include analytical solutions for selected index of refraction profiles, ray trace techniques which include interaction with a complex impedance boundary, a Gaussian beam ray trace algorithm, and more sophisticated approximate solutions to the full wave equation; the fast field program (FFP) and the parabolic equation (PE) solutions. This large array of computational approaches raises questions concerning under what conditions the various approaches are reliable and concerns about possible errors in specific implementations. This paper presents comparisons of predictions from the several models assuming a complex impedance ground and four atmospheric conditions. For the cases studied, it was found that the FFP and PE algorithms agree to within numerical accuracy over the full range of conditions and agree with the analytical solutions where available. Comparisons to ray solutions define regimes where ray approaches can be used. There is no attempt to compare calculated transmission losses to measurements.