Relationship between temperature and apparent shape of pristine ice crystals derived from polarimetric cloud radar observations during the ACCEPT campaign

Abstract

Abstract. This paper presents first quantitative estimations of apparent ice particle shape at the top of liquid-topped clouds. Analyzed ice particles were formed under mixed-phase conditions in the presence of supercooled water and in the temperature range from −20 to −3 °C. The estimation is based on polarizability ratios of ice particles measured by a Ka-band cloud radar MIRA-35 with hybrid polarimetric configuration. Polarizability ratio is a function of the geometrical axis ratio and the dielectric properties of the observed hydrometeors. For this study, 22 cases observed during the ACCEPT (Analysis of the Composition of Clouds with Extended Polarization Techniques) field campaign were used. Polarizability ratios retrieved for cloud layers with the cloud-top temperatures of ∼ −5, ∼ −8, ∼ −15, and ∼ −20 °C were 1.6, 0.9, 0.6, and 0.9, respectively. Such values correspond to prolate, quasi-isotropic, oblate, and quasi-isotropic particles, respectively. Data from a free-fall chamber were used for the comparison. A good agreement of detected apparent shapes with well-known shape–temperature dependencies observed in laboratories was found. Polarizability ratios used for the analysis were estimated for areas located close to the cloud top, where aggregation and riming processes do not strongly affect ice particles. We concluded that, in microwave scattering models, ice particles detected in these areas can be assumed to have pristine shapes. It was also found that even slight variations of ambient conditions at the cloud top with temperatures warmer than ∼ −5 °C can lead to rapid changes of ice crystal shape.

The HD(CP)<sup>2</sup> Observational Prototype Experiment (HOPE) – an overview

“…Values of around −20 dBZ (−10 • C < T < −5 • C), −30 dBZ 4904 A. Macke et al: The HD(CP) 2 Observational Prototype Experiment (HOPE) (−20 • C < T < −10 • C), and −25 dBZ (T < −20 • C) indicate columnar, dendritic, and bullet-rosette-like shapes, respectively Myagkov et al, 2016). Knowing about the relationship between ice water content, liquid water content, temperature, and shape of freshly formed ice crystals is an important step towards new approaches for the evaluation of ice formation schemes in numerical weather forecast models.…”

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

“…On the one hand, radiosondes can retrieve regions of super-cooled liquid water within precipitating cloud systems when other methods or sensors are limited. On the other hand, the temperature profiles in super-cooled liquid layer and the polarimetric radar signatures can help identifying the dominant ice particle shape affecting the growth process in the 5 mixed-phase area (Bader et al, 1987;Pruppacher and Klett, 1996;Fukuta and Takahashi, 1999;Myagkov et al, 2016).…”

“…An instrumental synergy was used during ACCEPT to detect the different phases, the variety of sizes and shapes of the involved hydrometeors. For this purpose the TARA radar (Heijnen et al, 2000) at the site was extended by several other sensors: the Leipzig Aerosol and Cloud Remote Observation System (LACROS) (Bühl et al, 2013), a second vertically pointing K a -band cloud radar Mira (Görsdorf et al, 2015), and the Raman Lidars Polly XT (Engelmann et al, 2016;Baars et al, 2016) and CAELI (CESAR Water Vapour, Aerosol and Cloud Lidar) (Apituley et al,30 2009). A picture of the setup is provided in Figure 1.…”

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

“…Data from the same measurement campaign are used by Kneifel et al (2015), where they apply a radar triple frequency method (W-, K a -, and X-band) to distinguish between different ice hydrometeor types (aggregates, rimed particles) and their particle size distribution characteristics during snowfall events. Myagkov et al (2016) uses polarimetric radar measurements (K a -band) to derive the shape and orientation of mainly pristine ice particles generated at liquid topped mixed phase clouds.…”

“…Myagkov et al (2016) developed a retrieval to obtain the shapes of new generated ice crystals at liquid topped single layer mixed-phase clouds and compared their results to laboratory studies. Good agreement was obtained.…”