Coherent Propagation of Polarized Millimeter Waves Through Falling Hydrometeors

Abstract

Coherent propagation of polarized millimeter waves through falling nonspherical hydrometeors is considered on the basis of Foldy's approximation. It is assumed that the distribution of hydrometeors over orientations is locally axially symmetric, the axis of symmetry being given by the local direction of air flow around the hydrometeors. An efficient rigorous method is described to compute the orientationally averaged forward-scattering amplitude matrix. This method is based on Waterman's T-matrix approach and fully exploits rotational properties of the T-matrix. First, the T-matrix is analytically averaged over hydrometeor orientations in the local coordinate system with the Z-axis along the direction of local air flow. Then, the elements of the forward-scattering amplitude matrix with respect to the local and laboratory coordinate systems are calculated via simple analytical expressions. Analytical solutions of the propagation equation for the coherent electric field are discussed. Cross polarization of the coherently transmitted linearly polarized wave is computed for canting spheroidal raindrops at 34.8 GHz, and the effects of the scatter of raindrop canting angles on the cross polarization are discussed. In particular, it is shown that the scatter of raindrop canting angles reduces the cross polarization considerably as compared with that of equioriented raindrops.