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

Abstract

As 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.

Inadvertent Localized Intensification of Precipitation by Aircraft

“…This is also supported by larger copolar correlation coefficient values inside of LIP. It should be noted, that in some cases, Z dr values inside of LIP were higher than expected from aggregates (Li et al, ; see, e.g., Figure ), that is, around 2 dB, which may be attributed to a mixture of crystals and snowflake aggregates (Keat & Westbrook, ).…”

Retrievals of Riming and Snow Density From Vertically Pointing Doppler Radars

“…Hogan et al () found that this value provided a good fit to a database of aircraft measurements as well as other studies in the literature. Combining dual‐polarization weather radar and surface based snowfall measurements at Hyytiälä, Li et al () found that the aspect ratio varies with riming fraction between 0.4 and 0.9, while analysis of precipitation imaging package (PIP) images by Tiira et al () yielded a median aspect ratio of 0.72. However, the applicability of particle images to derive particle geometrical properties was questioned by Jiang et al (); hence, Tiira et al () also used a single aspect ratio value of 0.6 for density retrievals.…”

Melting Layer Attenuation at Ka‐ and W‐Bands as Derived From Multifrequency Radar Doppler Spectra Observations

“…von Lerber et al () have shown that under similar conditions, that is, rain intensity, the rimed snowflakes would lead to lower melting layer attenuation. It should also be noted that in addition to density, snowflake shape is also affected by riming (Li et al, ), which may have a significant impact on scattering properties of ice particles especially at millimeter wavelengths.…”