Evidence of anisotropy in tropospheric microstructure

In recent years several groups using VHF radars have reported highly anisotropic echoes originating from stable regions of the troposphere and stratosphere. The echoes are most intense looking vertically, and their occurrence is well correlated with regions of hydrostatic stability. Observations also show that the echoes are usually much stronger when observed with coarse spatial resolution. There is no commonly used scattering or reflection model that can account for these features. In this paper we present a new model to explain the enhanced vertical echoes. The model pertains to scattering from the half-wavelength Fourier component of horizontally coherent free structure in the radio refractive index within a region of f'mite vertical extent. We refer to this echoing mechanism as 'Fresnel scattering.' The new model is applied to simulate the magnitude of VHF radar echoes by using radiosonde temperature soundings as input. Good agreement is found between these simulations and observations at the Sunset VHF radar in Colorado.

Section: The Fresnel Scattering Modelmentioning

confidence: 99%

Fresnel scattering model for the specular echoes observed by VHF radar

Gage¹,

Balsley²,

Green³

1981

“…Table 1 All experiments contain examples of averaged response patterns. See also Bolgiano [1963] and Gjessing [1960,1963,1964,1966 Experiments reported by Bolgiano [1963], Gjessing [1960,1962,1963,1964] beam-swinging experiment at 15.7 GHz over a 500km path has also been reported by Altschuler et al [1968]. )…”

Section: Experimental Progressmentioning

confidence: 76%

Review of Techniques in Transhorizon Propagation Research and their Relationship to Parameters of the Troposphere

Cox¹

1969

Transhorizon propagation techniques and their usefulness in determining atmospheric parameters are discussed. It is concluded that the state or structure of the atmosphere cannot be inferred unambiguously from individual measurement techniques used in experiments reported to date. However, because of the sensitivity of the transhorizon radio signals to changes in the parameters of the troposphere, it is suggested that combinations of available techniques be pursued to determine the parameters uniquely.tional Academy of Sciences, Committee on Atmospheric Sciences, and was presented to the Panel in March 1968. Drift (wind) and internal motions as theyaffect the Doppler spectrum or fading of the radio signals; 2. Statistical characteristics or properties of the random fluctuations o.r variations in the atmosphere as they are related to frequency and angular dependence of the mean scattered power; 3. Nonuniformities in the statistical characteristics of the atmosphere such as inhomogeneity or anisotropy (including stratification tendencies) that exist to a scale resolvable by the measuring devices (sometimes antennas) or the technique. Much of the early work in transhorizon radio propagation was concerned with communications system applications, and the parameters measured in the experiments were usually averaged over long periods of time (many hours or even days) and over large regions of space (sometimes from just above the earth's surface to the top of the sensible atmosphere). These measurements were not very useful for determining atmospheric characteristics at any given time or location in space. For an experiment to be useful in determining atmospheric parameters, all information necessary for their determination should be contained in the radio measurements or in other measurements made simultaneously; that is, it should not be necessary to include in the interpretation any assumptions that are not supportable by the measurements themselves. Experiments that deal entirely with parameters of use only to communications system designers are not included in this review. Thus this review concerns itself with the subject of transhorizon propaga-905 906 DONALD C. COX tion only as it relates to the parameters of the atmosphere. BRIEF REVIEW OF THE THEORY Theories that attempt to describe transhorizon propagation phenomena can be classified into two groups. The models used to describe the variations or fluctuations in the refractive index of the asmosphere are the major distinguishing features of the groups. Turbulence theories. By far the greatest effort expended in describing transhorizon propagation phenomena has been based on a statistical description of turbulence in the atmosphere. A rather extensive review of this subject can be found in Staras and Wheelon, [1959]; it has been treated by many authors [e.g., ]. In this approach, the refractive index (or dielectricpermittivity) fluctuations are described as a small random change about mean level. Early work [e.g., Booker and Gordon, 1950] dealt with the time fluc...

“…Each of these Fourier modes has a wave number that is larger than k,and that is uniquely determined by the direction of the Fourier mode such that this wave number, when projected onto the direction a, will appear to be of wave number k, Of the Fourier modes directed perpendicular to the direction a, only those of infinite wave number contribute to S•(ka). Bolgiano [1963]…”

Section: For a Given Direction A The Value Of The Onedimensional Reamentioning

confidence: 99%

Radar Backscattering from the Turbulent Clear Atmosphere

Óttersten¹

1969

Radar backscattering from the turbulent clear atmosphere is caused by irregular small‐scale fluctuations in the radio refractive index produced by turbulent mixing. This note considers the theoretical relationships between the refractive‐index microstructure and the back‐scattering at radar wavelengths from meters to centimeters. Spectral relationships are clarified and the basic difference between radar sampling and refractometer sampling of refractivity spectra is explained. Theoretical expressions are given for the relationship between the radar reflectivity and spectral representations of the refractive‐index variability for the general case, for the isotropic case, and for the inertial subrange.