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

The occurrence of supercooled liquid water in mixed-phase cloud (MPC) affects their cloud microphysical and radiative properties. The prevalence of MPCs in the mid- and high latitudes translates these effects to significant contributions to Earth’s radiative balance and hydrological cycle. The current study develops and assesses a radar-only, moment-based phase partition technique for the demarcation of supercooled liquid water volumes in arctic, MPC conditions. The study utilizes observations from the Ka band profiling radar, the collocated high spectral resolution lidar, and ambient temperature profiles from radio sounding deployments following a statistical analysis of 5.5 years of data (January 2014–May 2019) from the Atmospheric Radiation Measurement observatory at the North Slope of Alaska. The ice/liquid phase partition occurs via a per-pixel, neighborhood-dependent algorithm based on the premise that the partitioning can be deduced by examining the mean values of locally sampled probability distributions of radar-based observables and then compare those against the means of climatologically derived, per-phase probability distributions. Analyzed radar observables include linear depolarization ratio (LDR), spectral width, and vertical gradients of reflectivity factor and radial velocity corrected for vertical air motion. Results highlight that the optimal supercooled liquid water detection skill levels are realized for the radar variable combination of spectral width and reflectivity vertical gradient, suggesting that radar-based polarimetry, in the absence of full LDR spectra, is not as critical as Doppler capabilities. The cloud phase masking technique is proven particularly reliable when applied to cloud tops with an Equitable Threat Score (ETS) of 65%; the detection of embedded supercooled layers remains much more uncertain (ETS = 27%).

Optical design of low-cost polarimetric back-scatter sondes

The optical design of a simple low-cost polarimetric balloon-borne backscatter sonde for detecting cloud thermodynamic phase is described. Simulations are performed to determine the best choice of polarization as well as the layout of the detectors with respect to the emitter. The variety of scattering angles of the detected photons, due to finite size detectors, has at least as great an effect as multiple scattering in determining the ratio of cross-polarized to co-polarized signal. The problem with aligned flat-plate ice crystals giving similar depolarization to spherical scatterers, noted in the context of cloud lidar, also occurs here even with a cloud of randomly oriented large plates. Nevertheless, we find that configurations of the instrument that can distinguish liquid droplets from a wide variety of ice crystal habits, including plates, are possible.