Estimates are made for received power due to partial reflection and turbulence scattering of VHF radar echoes from the clear atmosphere. We show that, under rather general conditions, partial reflections from stratified layers in the atmosphere can contribute significantly to the received echo power and therefore should be taken into account in data interpretation. Possible experimental techniques to investigate the partial reflection are discussed. Some results from tropospheric observations will be presented indicating high echo power from thin layers having a long correlation time.
It is proposed to describe the temporal characteristics of a wave propagating in a random medium in terms of its temporal moments. The first two moments are related to the mean arrival time and the mean pulse width. It is shown that the one-position two-frequency mutual coherence function enters in the formulation naturally. The form of the expression suggests expanding the mutual coherence function in a narrow-band expansion whose coefficients can be solved exactly from the parabolic equation that takes into account all multiple scattering effects except the backscattering. A brief survey of the literature shows that the irregularity spectrum, under various conditions, has a power-law dependence. In order to conform to this observation a Bessel function spectrum proposed by Shkarofsky is found convenient to use since it not only reduces to the desired power-law form in the proper range of wavenumber space, but also has all the finite moments. Exact expressions for the mean arrival time and mean square pulse width are obtained; some numerical examples are given. Finally, the effect of noise on these moments is discussed.
The excitation of acoustic‐gravity waves in an isothermal atmosphere is considered in this paper. It is shown that the excitation due to mass production, momentum production and heat production can be discussed by examining the same differential equation. The sources are assumed to be extended and vary both in time and in space. Asymptotic methods are used to obtain analytic expressions for the radiation field for all times, from the arrival of precursors to any time thereafter. It is found that the transient response results from contributions from one, two or all three modes depending on the times from the arrival of precursors. The three modes are the high frequency acoustic mode, the intermediate frequency buoyancy mode and the low frequency gravity mode. Additional features of the transient behavior depend on the temporal as well as spatial variation of the sources. An example is given for which numerical computations are made. Possible applications of the results to geophysical problems are duscussed and certain extensions of the results are proposed.
The excitation of acoustic-gravity waves in an isothermal atmosphere is considered in this paper. It is shown that the excitation due to mass production, momentum production and heat production can be discussed by examining the same differential equation. The sources are assumed to be extended and vary both in time and in space. Asymptotic methods are used to obtain analytic expressions for the radiation field for all times, from the arrival of precursors to any time thereafter. It is found that the transient response results from contributions from one, two or all three modes depending on the times from the arrival of precursors. The three modes are the high frequency acoustic mode, the intermediate frequency buoyancy mode and the low frequency gravity mode. Additional features of the transient behavior depend on the temporal as well as spatial variation of the sources. An example is given for which numerical computations are made. Possible applications of the results to geophysical problems are duscussed and certain extensions of the results are proposed.
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