Modeling Snowflakes at Microwave Wavelengths

With the advent of satellite-borne radar and radiometers, it is now possible to observe cloud processes throughout the globe with unprecedented levels of precision. However, interpreting the large amount of data generated by such instruments requires detailed understanding of how light is scattered in-cloud by ice. Unlike liquid water, ice exhibits many complex shapes and orientation-dependent effects. We examine how ice particle shape and orientation affects single-scattering behavior of hydrometeors at microwave wavelengths using the Discrete Dipole Approximation (DDA).

Generally, a particle's scattering and backscatter behavior can be considered as a function of the particle's size and shape, which can be expressed as a particle's volume, surface area and aspect ratio. Existing tables for calculating scattering by hydrometeors usually consider idealized forms, such as spheroids, dendrites, hexagonal plates, rosettes and sector snowflakes. We have constructed aggregates that match field observations with regards to particle size, aspect ratio, density and fractal dimension. Single scattering cross-sections are compared with Mie theory for solid and soft spheres and using the T-matrix approximation. Above size parameter 0.75, neither Mie theory nor the T-matrix method for spheroids accurately represent the DDA result for aggregates. Averaged relative uncertainty in backscattering due to different particle morphologies is around 15%. Two measures of particle morphology, surface area and volume, account for much of this variation.

Conversely, the internal structure of a hydrometeor is quite difficult to determine by observations. Most current and past attempts are limited to measures of particle mass and a single-angle picture. We test sensitivity to internal versus surface features by perturbing our aggregates such that they have the same surface area and volume, but have randomized interiors. The resulting relative uncertainty in backscattering is found to be around 5% for individual particles, which is much lower than the uncertainty produced by different overall particle morphologies.

Radar and radiometers are also sensitive to hydrometeor orientation effects. We examine particles and ensembles in anisotropic orientation conditions (e.g. laminar flow). We consider the effects of particle alignment and off-zenith/nadir observation angles and present a simplified model for determining bulk hydrometeor orientation.