Ice nucleation of cirrus clouds related to the transported dust layer observed by ground-based lidars over Wuhan (30.5°N, 114.4°E), China

Cirrus clouds related to transported dust layers were identified on 22 occasions with ground-based polarization lidar from December 2012 to February 2018 over Wuhan (30.5°N, 114.4°E), China. All the events occurred in spring and winter. Cirrus clouds were mostly located above 7.6 km on top of the aloft dust layers. In-cloud relative humidity with respect to ice (RHi) parameters derived from water vapor Raman lidar and from ERA5 reanalysis data were used as criteria to determine the possible ice nucleation regimes. Corresponding to the two typical cases shown, the observed events can be classified into two categories: (1) category A (3 cases), in-cloud peak RHi ≥ 150%, indicating competition between heterogeneous nucleation and homogeneous nucleation; and (2) category B (19 cases), in-cloud peak RHi < 150%, revealing that only heterogeneous nucleation was involved. Heterogeneous nucleation generally took place during cirrus cloud formations in the upper troposphere when advected dust particles were present. Although accompanying low cloud-top temperatures ranging from -51.9 to -30.4°C, dust-related heterogeneous nucleation could contributed to primary ice nucleation in cirrus clouds by providing ice nucleating particle concentrations of 10-3 to 102 L-1. Heterogeneous nucleation and subsequent crystal growth reduced the ambient RHi to be less than 150% by consuming water vapor and thus even completely inhibited homogeneous nucleation.