Backscattering Gain Measurements of Spherical Ice Hydrometeors at 35.8 GHz and Comparison to Numerical Computations

Abstract

A 35.8-GHz scatterometer was tested and used for backscattering gain measurements of spherically shaped artificial hydrometeors varying in composition and size: homogeneous ice with 1.7 ≤ r ≤ 15 mm, different volume fractions of air bubbles embedded in a matrix of ice with 4.9 ≤ r ≤ 15 mm, and two-layered material (air core surrounded by a layer of ice with 1 ≤ rcore ≤ 13.5 mm and external radius rext = 15 mm; water core surrounded by a layer of ice with 0 ≤ rcore ≤ 9 mm and rext = 15 mm; ice core surrounded by a liquid water film with 1.7 ≤ rcore ≤ 15 mm; and ice core surrounded by a layer of ice containing air bubbles with 3 ≤ rcore ≤ 10 mm and rext = 15 mm). Numerical computations at the wavelength of 8.37 mm were performed for the same hydrometeors using a Mie computer code and a two-layered code in both of which the refractive index of the corresponding statistically homogeneous mixture was computed using the second-order Maxwell-Garnett and the Bruggeman theories. Laboratory measurements showed the scatterometer's marked sensitivity to such characteristic optical variations of the hydrometeors as air bubble content, hoarfrost, and liquid film formation, air and water core. A comparison to numerical computations shows good agreement for two-layered spheres, excepting ice shells containing air bubbles, and for small-sized homogeneous ice spheres. In the presence of air bubbles, the comparison indicates the need for a more accurate experimental procedure in the measurement of air volume fraction and the need to know both air bubble size distribution and gradient in the ice matrix.