Horizontally oriented ice crystals observed by the synergy of zenith- and slant-pointed polarization lidar over Wuhan (30.5°N, 114.4°E), China

Highlights: • Horizontally oriented ice crystal (HOIC) layers can exist in the atmosphere without evident orientation breakup for several hours (0.3–8.5 h). • The occurrence of HOIC layers mostly accompany with warmer temperatures (−25 to −5 °C) and light horizontal wind (0–20 m/s). • Early-formed ice cloud layer can also be preferred to align horizontally when they drop to the altitudes with more favorable aerodynamic conditions. From November 2019 to June 2020, horizontally oriented ice crystal (HOIC) layers were observed on 32 occasions using the synergy of zenith- and slant-pointed polarization lidar over Wuhan (30.5°N, 114.4°E), China. The HOIC layers appeared at altitudes of 2.5–10.5 km. 20 out of 32 cases showed a specific period that HOIC layers maintained in the atmosphere without an evident sign of orientation breakup, revealing their potential non-negligible impact on the radiation budget. The duration time ranged from 0.3 to 8.5 h with a mean of 3.5 h. HOIC layers occurred mostly at temperatures ranging from -25 to -5 °C and rarely presented at temperatures lower than -30 °C. Meanwhile, they always existed accompanying with light wind of 0–20 m/s. According to the lidar-derived crystal fall streaks, the terminal velocities and corresponding crystal diameters were estimated for 12 cases to be 0.28–1.0 m/s and 464–1305 µm, respectively. Associated Reynolds numbers for the HOIC layers concentrated in a range of 4–58. The observed events can be classified into two categories: (1) stratiform HOIC layer (22 cases), associated with the pristine crystals falling out from the liquid top of mixed-phase clouds which has been widely known; (2) continuously descending HOIC layer (10 cases), which is seldomly reported to our knowledge. For the latter pattern, once the crystals drop to the altitudes with more favorable aerodynamic conditions (i.e. appropriate temperature and adequately low wind speed), these early-formed ice crystals can also be preferred to align horizontally.