Spatial and temporal variability in the ice-nucleating ability of alpine snowmelt and extension to frozen cloud fraction

Abstract

Ice-nucleating particles (INPs) produce ice from supercooled water droplets through heterogeneous freezing in the atmosphere. INPs have often been collected at the Jungfraujoch research station (at 3500 m a.s.l.) in central Switzerland; yet spatially diverse data on INP occurrence in the Swiss Alps are scarce and remain uncharacterized. We address this scarcity through our Swiss alpine snow sample study which took place during the winter of 2018. We collected a total of 88 fallen snow samples across the Alps at 17 different locations and investigated the impact of altitude, terrain, time since last snowfall and depth upon freezing temperatures. The INP concentrations were measured using the home-built DRoplet Ice Nuclei Counter Zurich (DRINCZ) and were then compared to spatial, temporal and physicochemical parameters. Boxplots of the freezing temperatures showed large variability in INP occurrence, even for samples collected 10 m apart on a plain and 1 m apart in depth. Furthermore, undiluted samples had cumulative INP concentrations ranging between 1 and 200 INP mL -1 of snowmelt over a temperature range of -5 to -19 ĝ' C. From this field-collected dataset, we parameterized the cumulative INP concentrations per cubic meter of air as a function of temperature with the following equation c air ∗ ( T ) Combining double low line e - 0.7 T - 7.05 , comparing well with previously reported precipitation data presented in Petters and Wright (2015).