Microwave attenuation by realistically distorted raindrops: Part I. Theory

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Abstract
By using a new raindrop model developed earlier and applying the first-order perturbation-expansion technique, the effects of realistic and nonaxisymmetric distortion of varying raindrop shapes on microwave attenuation are re-investigated in this paper. Under the assumption of nonaxisymmetric raindrop scatterers, a system of general equations satisfied by the scattering and transmission coefficients of the electromagnetic (EM) fields are derived under the zeroth-order and the first order approximations. Both the horizontal and the vertical polarizations are discussed. The scattered and transmitted EM fields are obtained by means of expansion of spherical vector wave functions. Under the first-order perturbation approximation, the total cross section is calculated and discussed. Compared with the previous formulas presented for the axisymmetric raindrop scattering, the new results of total cross section obtained here consist of two additional terms resulting from the nonaxisymmetric distortion. These additional terms due to the raindrop's nonaxisymmetry are represented analytically and discussed numerically. Part II of this paper will present an application of the analysis to the specific microwave attenuation due to rain in the moderate climatic region and in Singapore's tropical region