Use of Copolar correlation coefficient for probing precipitation at nearly vertical incidence

Abstract

Presents observations of the copolar correlation coefficient between horizontally and vertically polarized echoes |ρ_hν(0)|. These were made with ground-based and airborne weather radars at nearly vertical incidence. A sharp decrease of |ρ_hν(0)| occurs at the bright band bottom, and is attributed to a varying mixture of hydrometeors with diverse shape, size, and thermodynamic phase. The largest contribution to decorrelation seems to come from wet aggregates; this is substantiated by consideration of two simple models. One consists of randomly oriented wet prolate spheroids, and the other considers an ensemble of distorted spheres. Prolates with axis ratios of 3 or distorted spheres with rms roughness equal to 15% of the diameter decrease the correlation to 0.8 at S band. At Ku band and for the size range encountered in the bright band, the decrease is a function of equivalent diameter because scattering is in the Mie regime. |ρ_hν(0)| measurement at 13.8 GHz and from the aircraft are the first ever. Also, differential phase and differential reflectivity at a 10° off nadir are the first of its kind. These last two variables showed a distinct signature in the bright band. This is significant because it might lead to applications on airborne or spaceborne platforms.