Examining the effects of the ground on the radiation pattern of a parabolic reflector at very low elevation angles

Accurate measurements of the tornado boundary layer and near-surface structure of the flow field within tornadoes require observations be made very near the ground. Over the past two decades, mobile radars have provided a wealth of information on the kinematic structure of tornadoes, although it is extraordinarily difficult to collect high-resolution radar measurements near enough to the ground to sample the near-surface flow owing partly to beam broadening, ground clutter, and safety considerations that limit the range between the radar and the tornado. In light of several recent datasets collected in intense tornadoes, it is important to understand how the radar beam at very low elevation angles (e.g., below 1.0 degrees) may interact with the ground. Preliminary testing indicates potential for multi-path scattering off the ground, which may appreciably modify the radiation pattern of the antenna beyond that expected from a parabolic reflector. In this paper a model based on theory of geometric optics will be used to characterize the effect of the multi-path scattering off the ground in dual-polarized antenna patterns. Ground reflections will estimated as a function of the radar height, range, elevation angle, and electrical properties of the ground. For more accurate characterization of the parabolic reflector antenna patterns, the integral equation solver that uses the method of moments (MoM) technique will be used. The results of the model will be compared with field tests from a radar with the intention of quantifying some of the issues that may arise from near-ground radar scans.