Simplified representations of ice crystals cause large errors in their scattering properties

Submitter:

Schrom, Robert — The Pennsylvania State University
Kumjian, Matthew — Pennsylvania State University

Area of research:

Cloud Processes

Journal Reference:

Schrom R and M Kumjian. 2018. "Bulk-Density Representations of Branched Planar Ice Crystals: Errors in the Polarimetric Radar Variables." Journal of Applied Meteorology and Climatology, 57(2), doi:10.1175/JAMC-D-17-0114.1.

Science

Detailed electromagnetic scattering calculations for thin, branched planar ice crystals are compared to homogeneous, bulk-density spheroids used commonly by the community to represent such crystals.

Impact

A longstanding assumption in the radar community is that homogeneous, bulk-density spheroids accurately capture the scattering properties of pristine ice crystals like plates and dendrites, especially at longer radar wavelengths. We show that this assumption can lead to large errors in the computed polarimetric radar variables at all wavelengths.

Summary

We performed detailed scattering calculations for branched planar ice crystals across a range of radar wavelengths (from W band to S band) commonly used by cloud and precipitation radars. These calculations were compared to the typical simplified treatment of such particles: as spheroids or hexagonal plates with the same aspect ratio but of uniform, bulk density reduced from that of solid ice to represent the branching. Even when particles are small compared to the radar wavelength, large errors in the scattering properties can result from this simplified treatment (Image 1). In particular, errors of up to several dB in differential reflectivity (ZDR) and up to half an order of magnitude in specific differential phase (KDP) are possible, especially for crystals with lower effective density. The errors arise because the bulk-density representation uniformly disperses mass within the bounding spheroid/hexagon, which minimizes near-field radiation interactions important for producing the observed scattering properties of pristine ice crystals. In contrast, a faithful representation of the crystal shape preserves these near-field interactions (Image 2).

Errors (in dB) in the computed differential reflectivity (ZDR) for homogeneous spheroids versus homogeneous hexagonal plates, relative to the detailed scattering calculations. Colors indicate particle effective density.

Zoomed-in example of a dendrite (left) and plate (right) showing the change in internal electric field strength owing to near-field interactions. Top row: enhancements when the polarization direction is along the particle major axis. Bottom row: reductions when the polarization direction is orthogonal to the particle major axis.