Challenges to Radar Precipitation Retrieval Caused by Hydrometeor Orientation

Polarimetric radiometer signals indicate preferential orientation distribution instead of
uniformly random distribution for non-spherical hydrometeors. The preferential
distribution will undoubtedly impact uncertainties of radar precipitation retrievals. We have
thus solved for the electromagnetic (EM) scattering properties of these irregular
hydrometers at orientations aligned with the quadrature nodes of a range of degrees.
Higher degrees of quadrature yield more accurate integration and, thus, more accurate
orientation-averaged scattering properties. But a higher degree also requires a greater
number of quadrature nodes, increasing the computation cost of solving for EM scattering.
We, therefore, aim to find the quadrature of a minimal degree with a minimal number of
nodes that still yields an accurate (within a prescribed uncertainty) orientation average for
each hydrometeor. However, we find that when the orientation-averaged scattering and
extinction efficiencies, and , respectively, asymptote at a particular degree, the
orientation-averaged backscatter, , needed for radar retrievals, still exhibits a significant
uncertainty. Furthermore, the preferential orientation distribution complicates the
uncertainty estimates of hydrometeor geometric properties, e.g., maximum dimension and
aspect ratio, derived from image-based measurements, such as 2DVD and Multi-Angle
Snow Camera (MASC), which we rely on as guides for “realistic” size distributions. We are
exploring techniques for better estimating with the same number of orientations that
yield accurate and , for example, regressing the angular distribution of backscatter
efficiency against the angular distribution of the hydrometeors mass.