摘要:

The models for worst‐month analysis are reconsidered. A new procedure is proposed for the estimation of the expected distribution of annual worst‐month statistics from a series of monthly exceedances or from climate zone based rain attenuation models. The new procedure is easy to apply, and the results are consistent with available observati

ISSN:0048-6604    

DOI:10.1029/91RS00985

年代:1991

数据来源: WILEY

摘要:

The problem of double edge diffraction in the near‐field region of a coplanar skewed edge geometry, illuminated by a plane wave, is studied asymptotically via an extended spectral theory of diffraction approach. The resulting uniform dyadic double edge diffraction coefficient is expressed in terms of a universal integral (the generalized Fresnel integral) and remains valid when any one of the edges is within the transition region of a singly diffracted wave, while it asymptotically reduces to the ordinary geometrical theory of diffraction double diffraction coefficient elsewhere. Comparisons with method of moments computations for radiation patterns of sources in the vicinity of flat plate structures demonstrate the validity of the asymptotic approximatio

ISSN:0048-6604    

DOI:10.1029/91RS00988

年代:1991

数据来源: WILEY

摘要:

Experimental output data from a multifrequency, continuous wave,Cband system have been investigated and discussed with reference to physical principles involved in modulation of small‐scale centimetric water waves by larger‐scale wave trains. Attention payed at field and laboratory measurements described in the literature show that the propagation velocity of certain wave components below the spectral peak occasionally differs from predictions of conventional linear theory. A one‐dimensional theoretical analysis of the dual‐frequency microwave response, including Doppler features and modulational effects induced by modulated wave trains, has been developed to throw light on this phenomenon. The theoretical expressions derived predict that wave train modulation should manifest itself as two narrow, Doppler‐shifted peaks in the cross‐product power spectrum generated from two distinct microwave returns. The wave field involved in the modulational process, is modeled as not only consisting of independently propagating Fourier components obeying the linear dispersion relation but also being composed of wave groups of Stokeslike waves, forming independent wave packets. Furthermore, each isolated packet is constructed to be a perturbation solution of the wave equations representing the sea surface boundary conditions. Experimentally, pronounced spectral components were observed with variations in magnitude and Doppler behavior which are consistent with proposed theory, surface truth, and results presen

ISSN:0048-6604    

DOI:10.1029/91RS01005

年代:1991

数据来源: WILEY

摘要:

A method to estimate cloud particle orientation and shape parameters using a polarization diversity radar is presented. The method is demonstrated for low‐reflectivity ice clouds containing crystals that are modeled as prolate and oblate spheroids. The parameters to be estimated are the particle mean canting angle and the axis ratio in the incident wave polarization plane. The scattering matrix concept is used to obtain a relationship between two orthogonally polarized received powers and the parameters of interest. It is shown that the use of an elliptically polarized radar signal diminishes the expected difference between received powers in the two receiving polarization channels. This makes polarization studies of ice clouds with relatively low reflectivities possible; these clouds often are “invisible” in one of the receiving channels when conventional linear or circular polarization is used. An estimation of errors in the retrieved mean canting angle value caused by the spread in particle orientation is also

ISSN:0048-6604    

DOI:10.1029/91RS00965

年代:1991

数据来源: WILEY

摘要:

We conducted meteor wind observations with the middle and upper atmosphere (MU) radar at Shigaraki, Japan (35°N, 136°E), utilizing an interferometer to determine the arrival angle of a meteor echo. We found that meteor echoes are widely distributed in zenith angles as large as 50° and that the narrow main lobe of a transmitting antenna cannot effectively detect meteor trails but that the sidelobes of the antenna illuminate most of meteor trails. Wind velocity profiles determined using meteor echoes are consistent with MU radar observations determined using turbulence echoes. The amplitudes and phases of both diurnal and semidiurnal tides were analyzed for a 96‐hour continuous run in September 1989, which agreed reasonably well with the characteristics of tides determined from Kyoto meteor radar observat

ISSN:0048-6604    

DOI:10.1029/91RS01164

年代:1991

数据来源: WILEY

摘要:

This paper presents the first evidence of long‐range detection (>1000 km) of calibrated VLF signals resulting from the HF heating of the auroral electrojet, that is, signal detection at a point of direct “line of sight” of the heated patch of ionosphere. Other workers have presented calibrated data from shorter ranges (190–550 km) or claimed the detection of uncalibrated signals at ranges greater than 6000 km, but we believe that no such calibrated and clear signals, like those presented here from a range of greater than 2000 km, have previously been reported. Also, in contrast to earlier long‐range detection experiments we record the “radial” as well as the “azimuthal” magnetic component of the signals and from their ratio obtain the waveguide mode polarization. Observed absolute magnetic field strengths and waveguide polarizations are found to be in line with the predictions of simple

ISSN:0048-6604    

DOI:10.1029/91RS00777

年代:1991

数据来源: WILEY

摘要:

In this paper we present a solution of the problem of a plane electromagnetic wave traveling parallel to a constant magnetic field (k‖B0‖z/|z|) in a horizontally stratified ionosphere. We assume, in particular, that the permittivity of the medium can be decomposed as the sum of an unperturbed and a perturbed part and that the unperturbed term is a smooth function of the heightzthat can be analytically continued in the complex plane and has furthermore a single simple zero (turning point) in a simply connected regionDwhich includes the interval (α, ∞) of the real axis. The method gives approximate solutions valid in the regionDand therefore also through the turning point. These solutions are obtained by truncating certain formal (generally divergent) series solutions of the wave equation. Furthermore, a realistic error bound for the remainder term associated with the truncated series is also derived and discussed. This method is successively applied to the case of a linearly varying permittivity of a lossless ionosphere with a superimposed Gaussian perturbing term. The possibility of applying the method when an odd number of turning points are present is also dis

ISSN:0048-6604    

DOI:10.1029/91RS00934

年代:1991

数据来源: WILEY

摘要:

Cellular diversity systems of rhomboidal structure as a means to overcome rain attenuation problems for frequencies greater than 10 GHz are analyzed and compared to conventional route diversity systems. The proposed model is applicable to systems using diversity hop lengths more than 30 km. Application of the cellular system for regions characterized by different rainfall climates, similar to those observed in Canada and Europe, with ƒ = 11 GHz gives a saving in the number of required repeaters up to about a value of 13% for high‐quality repeate

ISSN:0048-6604    

DOI:10.1029/91RS00963

年代:1991

数据来源: WILEY

摘要:

The radiation field diffracted by a vertical, perfectly conducting rectangular plate on a perfectly conducting ground plane, when it is illuminated at any azimuthal angle by a vertical current element at ground level, is derived approximately in closed form. Experimental confirmation of this calculated field on a direct propagation path with different sizes of plate, aligned at different angles to the propagation path, is provided.

ISSN:0048-6604    

DOI:10.1029/91RS00512

年代:1991

数据来源: WILEY

摘要:

In this study a model predicting the electromagnetic wave propagation has been developed. The model is initialized with a known refractivity profile and an initial transmitted field. The troposphere is assumed to be inhomogeneous in height and range. The vertical refractivity profile is computed from the meteorological upper air data. Arbitrary as well as experimentally observed profiles could be used as inputs to the model. Inhomogeneity in range is invoked by injecting refractivity profiles at known locations. A transmitter with a Gaussian beam is assumed to generate the initial field based on the transmitter parameters such as vertical beam width, vertical beam elevation, transmitter frequency, antenna height, and type of polarization. A simulation for the typical site on the coast of Arabian gulf has been chosen for the study. The field strength and radar coverage for different conditions of the typical coastal site prevailing a standard and inhomogeneous surface duct demonstrates the sensitivity of the model with the inhomogeneous and homogeneous atmosphere. Radiated fields are predicted on the basis of the initial field, boundary conditions, and atmospheric conditions considering the effect of refractivity in the case of inhomogeneous surface duct and elevated duct. Results reported in this study demonstrate the trapping of the energy in ducting atmosphere above the minimum trapping frequency with the predicted field by this model.

ISSN:0048-6604    

DOI:10.1029/91RS00092

年代:1991

数据来源: WILEY