New insights into ice multiplication using remote-sensing observations of slightly supercooled mixed-phase clouds in the Arctic

Abstract

Secondary ice production (SIP) can significantly enhance ice particle number concentrations in mixed-phase clouds, resulting in a substantial impact on ice mass flux and evolution of cold cloud systems. SIP is especially important at temperatures warmer than −10 ○C, for which primary ice nucleation lacks a significant number of efficient ice nucleating particles. However, determining the climatological significance of SIP has proved difficult using existing observational methods. Here we quantify the long-term occurrence of secondary ice events and their multiplication factors in slightly supercooled clouds using a multisensor, remote-sensing technique applied to 6 y of ground-based radar measurements in the Arctic. Further, we assess the potential contribution of the underlying mechanisms of rime splintering and freezing fragmentation. Our results show that the occurrence frequency of secondary ice events averages to <10% over the entire period. Although infrequent, the events can have a significant impact in a local region when they do occur, with up to a 1,000-fold enhancement in ice number concentration. We show that freezing fragmentation, which appears to be enhanced by updrafts, is more efficient for SIP than the better-known rime-splintering process. Our field observations are consistent with laboratory findings while shedding light on the phenomenon and its contributing factors in a natural environment. This study provides critical insights needed to advance parameterization of SIP in numerical simulations and to design future laboratory experiments.

Supercooled Liquid Water Detection Capabilities from Ka-Band Doppler Profiling Radars: Moment-Based Algorithm Formulation and Assessment

“…This statement leans on the premise that full radar-based LDR spectra may not be known, in which case moment-based methods like the one presented here can be considered as alternatives. Otherwise, radar-based polarimetry can be decisive in phase partitioning, as discussed in [31,33,34]. Moreover, derived information from the examination of the vertical gradients, particularly of reflectivity, is significant as is of the spectral width.…”

“…In principle, irregularly shaped particles produce discernibly greater LDRs than near-spherical ones; for perfect spheres, the cross-polar component is zero (z V H =0 mm 6 m −3 ), in which case LDR becomes −∞ (dB). Profiling Ka band radars with LDR capability, albeit less common (e.g., ARM SGP/NSA KaZR [37], MIRA36 [45] have been employed in cloud property and phase retrievals (see, e.g., in [31,32,[46][47][48]). The mean Doppler velocity (MDV) is the radial (along the radar line of sight) component of the reflectivity-weighted, fall speed of distributed scatterers within each sampling volume.…”

“…The fact that the NSA KaZR.MD is a dual-polarization receiver radar allows for the evaluation of radar-based LDRs at its operating frequency of 35 GHz. This variable is analyzed to ascertain whether the depolarization caused by discrepant ice/liquid geometric features and measured from profiling, mm-wavelength cloud radars is a viable option to be used in a radar-based phase partitioning, when full LDR spectra are not available (as opposed to the [31] findings). Regarding the motion variability, given the 5.5-year climatological examination of the NSA dataset, cold clouds exhibit a spectral width clustering around 0.10 m s −1 and rarely exceed 0.25 m s −1 .…”

“…However, lidar operating wavelengths (visible to near-infrared) render those sensors susceptible to signal extinction in the presence of liquid water or optically thicker ice, thereby reducing their overall potential for cloud observations as opposed to MMCRs for which atmospheric attenuation at 35-100 GHz is not as prohibitive. Hydrometeor classification in mixed-phase conditions has been conducted in terms of (i) neural networks [26], (ii) fuzzy logic implementations [27], (iii) multi-sensor applications [28,29], and (iv) radar-only [30,31] by analyzing Doppler spectra morphological features. Doppler cloud radars [32] have demonstrated potential in SLW detection capabilities by radar-only means in arctic, mixed-phase scenes in that cloud-top-condensing liquid water appears more detectable than liquid being embedded-in-ice based on statistical characteristics (radar observables' distribution parameters) of each cloud class when compared to ice cloudy volumes.…”

“…The current document demonstrates, implements, and assesses the applicability in an ice/liquid phase partition scheme of the smaller LDR yet greater σ v , ∂ z Z, and ∂ z SDV that are more likely to be encountered in condensation regimes of said mixed-phase conditions. Thereby, the need to examine either full radar-based MDV or LDR spectra (as seen in [30,31,33,34]) or any depolarization lidar-based variables, can be bypassed.…”