| Summary: | An 11-year dataset from a polarization lidar operated by NIWA at Lauder was used to investigate heterogeneous ice formation in clouds as a function of temperature for a site on the South Island of New Zealand. Trajectory-based tools and model data were used to relate this cloud dataset to the aerosol load and air mass sources, as aerosol conditions in the middle troposphere above Lauder are subject to contrasts. Clean, pristine airmasses from Antarctica and the Southern Ocean arrive under southerly flow conditions while elevated aerosol loads occur when air masses are advected from nearby Australia. The Lauder cloud dataset was put into context with lidar studies from contrasting regions such as Germany and southern Chile. The ice-formation efficiency found at Lauder is lower than in polluted mid-latitudes (i.e., Germany) but higher than in southern Chile. Both, Lauder and southern Chile are subject to generally low free-tropospheric aerosol loads, which suggests that the low ice-formation efficiency at these two sites is related to low ice-nucleating particle (INP) concentrations. However, Lauder sees episodes of continental aerosol, more than does southern Chile, what seems to lead to the moderately increased ice-formation efficiency. Trajectory-based tools and aerosol model reanalyses were used to relate the cloud dataset to the aerosol load and the air mass sources. Both analyses point clearly to higher ice formation efficiency for clouds which are more strongly influenced by continental aerosol, and to lower ice formation efficiency for clouds which are more influenced by Antarctic/marine aerosol and air masses.
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