| 16 Jun 2026
Observation-based analysis of horizontally oriented ice crystals using dual-angle polarization lidar and cloud Doppler radar in Beijing
Abstract. Ice crystal orientation strongly influences cloud radiative properties and remote sensing retrievals, but long-term, high-resolution quantitative observations remain scarce. This study presents comprehensive case studies and statistical analyses of horizontally oriented ice crystals (HOICs) based on full-year (2022) synergistic observations in Beijing, China, combining a zenith-pointing micropulse lidar, a collocated 15° off-zenith polarization lidar, and a Ka-band cloud Doppler radar. Applying a novel height-resolved classification method based on dual-angle polarization lidars, HOICs are identified with high spatiotemporal resolution. HOICs are found to be common, accounting for 15.0 % of all ice-containing cloud data points annually and peaking at 24.6 % in summer, with maximum occurrence at temperatures from −20 °C to −10 °C. Macroscopically, typical HOIC layers exhibit horizontal extents of 10–100 km and durations of several hours within their optimal formation temperature ranges. Furthermore, the Euclidean-distance analysis between HOICs and past overlying cloud layers revealed a strong linkage of HOIC occurrence to supercooled liquid water clouds (SWCs), being much closer to HOIC events than randomly oriented ice crystals (ROICs). Dynamically, cloud radar observations further reveal that HOICs preferentially occur in stable environments with turbulent eddy dissipation rates below 10⁻² m² s⁻³ and exhibit lower fall velocities than ROICs. Estimates based on radar-observed vertical velocity indicate typical HOIC equivalent diameters of approximately 1200 μm with Reynolds numbers predominantly below 100. The findings provide key observational constraints for improving ice cloud microphysics and orientation parameterizations in numerical models.
This is a very solid and scientifically valuable piece of work. The authors conducted a systematic, year‑long observational analysis of horizontally oriented ice crystals (HOICs) using a unique ground‑based combination of dual‑angle polarization lidars and a cloud radar in Beijing. For the first time, the study provides long‑term statistical constraints on the occurrence frequency, macrophysical characteristics, environmental conditions, and relationships of HOICs with supercooled liquid water clouds (SWCs) and turbulence, filling a critical gap in this field. The data are of high quality, the analysis methodology is rigorous, and the results are of significant reference value for improving ice‑cloud microphysics parameterization schemes in climate models. The paper is well structured and the arguments are well supported, but some details still need improvement. Therefore, a minor revision is recommended before acceptance.
Is Figure 5(a) based on statistics over all temperatures? The fraction of water clouds in 2022 is only 5.2% – could this be double‑checked?
For Figure 6 (rigorous cloud mask) and Figure 7 (relaxed cloud mask), the numbers of statistical cases are 735 and 693 respectively, with a difference of more than 5%. Could the corresponding reasons be provided?
The paper cites previous studies multiple times, but as an independent paper for publication, it should appropriately explain those previous studies. For example, in the sentence "Moreover, HOICs produce much stronger backscatter in the zenith‑pointing lidar than in the off‑zenith‑pointing lidar, making cloud‑layer detection more easily triggered in the zenith observations (Zhao et al., 2014; Wu et al., 2025)," the physical mechanism should be articulated within this paper.
The paper states: "both confirming a statistically significant difference in their overall EDR distributions (p ≪ 0.001). Interestingly, the calculated Cliff’s Delta yields a small overall effect size (d = −0.060)." The formulas for p and d should be provided.