Dual‐polarization radar signatures in snowstorms: Role of snowflake aggregation

Abstract

In this article a potential role of snowflake growth by aggregation on formation of dual‐polarization radar signatures in winter storms is discussed. We advocate that the observed bands of increased values of specific differential phase (Kdp) can be linked to the onset of aggregation. These bands are caused by high number concentrations of oblate relatively dense ice particles and take place in regions where an ice phase “seeder‐feeder” is active. On the other hand, the differential reflectivity (Zdr) bands, in absence of detectable Kdp values, are observed in the areas where crystal growth is the dominating snow growth mechanism and ice particle number concentration is lower. This distinction in underlying processes explains why Kdp and Zdr bands are not always observed at the same time. Furthermore, based on surface observations of snowflakes, it is determined that early aggregates, consisting of a small number of ice crystals, are oblate. These oblate particles could contribute to the reported dual‐polarization radar signatures in snow, especially to the Kdp. This could help to explain why, where observed at the same type, Kdp and Zdr bands do not match and the altitude of the peak value of Kdp is usually lower than the Zdr one. It also means that dual‐polarization radar signatures of snowflakes may depend on a stage of aggregation.

Precipitation and microphysical processes observed by three polarimetric X-band radars and ground-based instrumentation during HOPE

“…Z DR and K DP depressions aloft associated with increases in Z H and ρ HV above the ML suggest increases of ice particle mean sizes due to riming and/or aggregation. Recently, Moisseev et al (2015) argued that the processes responsible for enhanced K DP -and Z DRbands might be different: they advocated that the K DP bands are caused by high number concentrations of oblate relatively dense ice particles (early aggregates) and are linked to the onset of aggregation processes, while Z DR bands in the absence of K DP bands are observed when crystal growth is the dominant snow growth mechanism and the number concentration is lower. Following their arguments, it can also be speculated that aggregation processes are ongoing near the end of the observation period shown in Fig.…”

Observing ice particle growth along fall streaks in mixed-phase clouds using spectral polarimetric radar data

“…Bands of enhanced Z DR are an indicator for the generation and growth of pristine ice particles, mainly dendrites or hexagonal plates (Bader et al, 1987;Kennedy and Rutledge, 2011;Moisseev et al, 2015). However, the visible polarimetric signatures in the radar measurements in Case 2 are caused most likely by newly generated ice needles or columns.…”

“…The decrease of the differential reflectivity signatures depends strongly on the aggregated pristine particle type. Moisseev et al (2015) suggested that early aggregates of dendrites align horizontally and therefore can also contribute to a high Z DR -signature. This leads to a less strong decrease of polarimetric signatures of Z DR with increasing size of the grown aggregates.…”

“…In the past, low-frequency radars were mainly used to estimate precipitation (Bringi and Chandrasekar, 2001) and to classify hydrometeor types (Straka et al, 2000;Chandrasekar et al, 2013). However, in the last few years, the polarimetric capabilities of those radars were also used to study ice particle microphysics and growth processes in cloud systems during snowfall events or in the ice part of the cloud above the melting (Kennedy and Rutledge, 2011;Bechini et al, 2013;Moisseev et al, 2015). Microphysical studies of ice crystal habits using a S-band radar were first done by Bader et al (1987).…”

“…It was found that the enhanced bands of Z DR values occur where the aircraft measurements detected defined pristine ice particle shapes. Later it was found that a combination of Z DR and specific differential propagation phase, K DP , can be used to identify regions of plate-like particle growth processes and onset of aggregation (Kennedy and Rutledge, 2011;Moisseev et al, 2015). Andrić et al (2013) modeled polarimetric signatures for a winter storm case.…”