Influence of Ice Particle Size and Shape Biases on Radar Polarimetric Variables

As more ice particle image measurements become available, for example from the Multi-Angle Snowflake Camera (MASC), they are increasingly used to estimate ice particle shapes and sizes as a basis for estimating their scattering properties at radar wavelengths. However, the shapes and sizes of ice particles retrieved from two-dimensional images are often different from their real shapes and sizes, especially for irregular aggregates. Projected maximum dimensions are generally smaller than the real maximum dimensions, whereas projected aspect ratios are generally larger than the real aspect ratios. Oblate spheroids are usually used to represent aggregates, with an aspect ratio the same as the average projected aspect ratio from observations, which is approximately 0.6.

In this study, the radar polarimetric variables at an X-band wavelength are first calculated using a T-Matrix approach for aggregates represented by prolate spheroids with a fixed aspect ratio of 0.4 and a prescribed particle size distribution (PSD). The elevation angles between the prolate spheroids’ maximum dimension and the horizontal plane follow a Gaussian distribution of zero mean and 10° standard deviation, whereas the azimuth angles are random. Next, a projected PSD is generated from the projections of the prolate spheroids onto a plane perpendicular to a horizontal viewing direction; the average of the projected aspect ratios is 0.6. Then the radar polarimetric variables are calculated for oblate spheroids with an aspect ratio of 0.6 following the projected PSD, and compared with the calculations for the prolate spheroids. Sensitivity to different PSDs are tested.

Preliminary results show that the average size from the projected PSDs is 26% smaller than the average size from the real PSDs for the 0.4 aspect ratio prolate spheroid. The reflectivity difference is 4-8 dB between the two types of spheroids at an X-band wavelength. Z_DR and K_DP are biased by 40-55% and 60-67%, respectively, if the oblate spheroids are used to represent the prolate spheroids. In future work, a combination of different spheroid shapes will be used to represent real aggregates rather than a single prolate shape. The radar wavelength will be expanded to S- and Ka-bands. More orientation distributions will be included because aggregates tend to have a wide range of orientations from the horizontal plane.