Optical and microphysical properties of ice crystals in Arctic clouds from lidar observations

Abstract. The vertical profiles of the optical properties, effective radius of ice crystals and ice water content (IWC) in Arctic semi-transparent stratiform clouds were assessed using quantitative ground-based lidar measurements performed from 13 to 26 May 2016 in Hammerfest (north of Norway, 70° 39′ 48″ North, 23° 41′ 00″ East). The field campaign was part of the Pollution in the ARCtic System (PARCS) project of the French Arctic Initiative. The presence of low-level semi-transparent stratus clouds was noted on 16 and 17 May, and they were sampled continuously by a ground-based Raman-N₂ lidar emitting at the wavelength of 355 nm. These clouds were located just above the atmospheric boundary layer where the 0 °C isotherm reached around 800 m above the mean sea level (a.m.s.l.). To ensure the best penetration of the laser beam into the cloud, we selected case studies with cloud optical thickness (COT). Lidar-derived multiple scattering coefficients were found to be close to 1 and ice crystal depolarization around 10 %, suggesting that ice crystals were small and had a rather spherical shape. This agrees with our Mie computations determining effective radii between ~5 and 20 µm in the clouds for ice water contents between 1 and 8 mg m^-3, respectively. Direct estimate of the microphysical parameters of ice clouds via lidar measurements is a significant asset for the study of their large-scale radiative impact, while reducing the need for experimental resources.