Variations in Snow Crystal Riming and ZDR: A Case Analysis

A case study in terms of variations in differential reflectivity Z(DR) observed at X band and snow crystal riming is presented for a light-snow event that occurred near Greeley, Colorado, on 26-27 November 2015. In the early portion of the event, Z(DR) values at near-surface levels were low (0-0.25 dB). During a second time period approximately 8 h later, Z(DR) values became distinctly positive (+2-3 dB). Digital photographs of the snow particles were obtained by a Multi-Angle Snowflake Camera (MASC) installed at a range of 13 km from the radar. Image-processing and machine-learning techniques applied to the MASC data showed that the snow particles were more heavily rimed during the low-Z(DR) time period. The aerodynamic effects of these rime deposits promoted a wider distribution of hydrometeor canting angles. The shift toward more random particle orientations underlies the observed reduction in Z(DR) during the period when more heavily rimed particles were observed in the MASC data.