Assessment of horizontally-oriented ice crystals with a combination of multiangle polarization lidar and cloud Doppler radar

Abstract. The orientation of ice crystals plays a significant role in determining their radiative and precipitating effects, horizontally oriented ice crystals (HOICs) reflect up to ~40 % more short-wave radiation back to space than randomly oriented ice crystals (ROICs). This study for the first time introduces an automatic pixel-by-pixel algorithm for HOIC identification using a combination of ground-based zenith- and 15-degree off-zenith-pointing polarization lidars. The lidar observations provided high-resolution cloud phase information. The data were collected in Beijing over 354 days in 2022. A case study from 13 October 2022 is presented to demonstrate the effectiveness and the feasibility of the detection method. The synergy of lidars and collocated Ka-band cloud radar, radiosonde, and ERA5 data provide phenomenological insights into HOIC events. While cloud radar Doppler velocity data allowed the estimation of ice crystal size, Reynolds numbers, and turbulent eddy dissipation rates, corresponding environmental and radar-detected variables are also provided. HOICs were present accompanying with weak horizontal wind of 0–20 m s⁻¹ and relatively high temperature between -8 ℃ to -22 ℃. Compared to the ROICs, HOICs exhibited larger reflectivity, spectral width, turbulent eddy dissipation rate, and a median Doppler velocity of about 0.8 m s⁻¹. Ice crystal diameter (1029 μm to 1756 μm for 5th and 95th percentiles) and Reynolds numbers (28 to 88 for 5th and 95th percentiles) are also estimated with the help of cloud radar Doppler velocity using an aerodynamic model. One interesting finding is that the previously found switch-off region of the specular reflection in the region of cloud base shows a higher turbulence eddy dissipation rate, probably caused by the latent heat released due to the sublimation of ice crystals in cloud-base region. The newly derived properties of HOICs have the potential to aid to derive the likelihood of their occurrence in output from general circulation models (GCMs) of the atmosphere.