How Does Riming Affect Dual‐Polarization Radar Observations and Snowflake Shape?

Abstract

AbstractAs an ice particle grows by riming its shape is expected to change, resulting in a more spherical particle at the later stages of riming. This conceptual model is at the core of the current ice microphysical schemes and used for dual‐polarization radar observation based classification of hydrometeors. A quantitative relation between riming and shapes of snowflake aggregates, however, has not been established yet. This study aims to derive this relation by using surface‐based precipitation and coinciding dual‐polarization radar observations. The observations were collected during four winter seasons, 49 snowstorms, at University of Helsinki measurement station in Hyytiälä, Finland. Results show that relation between the differential reflectivity and reflectivity‐weighted rime mass fraction is not monotonic and depends on reflectivity‐weighted mean diameter. This behavior can be explained by the opposing effects of riming on dual‐polarization radar observations. Riming increases particle bulk density, which leads to more pronounced dual‐polarization radar signatures. As riming progresses the aspect ratio of snowflake increases slowly until the rime mass fraction value reaches a certainty value after which the aspect ratio increases more rapidly. Finally, coutilization of Ze, Zdr, and Kdp for inferring riming fraction is analyzed.