A study of the influence of riming of ice crystals on snow chemistry during different seasons in precipitating continental clouds

Abstract

During three field campaigns at Mt Sonnblick—SBO—(3106 m a.s.l.), Salzburg, Austria, in winter 1991, spring and summer 1992, a comprehensive study of cloud water and precipitation chemistry was performed including a microscopic study of form, size and degree of riming of precipitating ice crystals. The surface weighted average of the degree of riming of precipitating snow showed large fluctuations during all seasons studied with a range of 0.5–4.5 The average degree of riming was around 2 during the November and June campaigns and 2.6 in March. The attachment of cloud droplets to precipitating ice crystals was found to be the predominant process determining the final composition of a snowflake. This process was found to be active during all seasons studied. The strong seasonality of the sulfate concentration in precipitation at SBO with very low values during winter and high values during summer could be attributed primarily to the corresponding fluctuation of the sulfate concentration in the cloud water while the extent of riming was rather similar at least during the periods of our winter and summer campaigns. There are indications that the extent of riming is higher during the spring season as compared to winter or summer conditions which might help to explain the spring maximum of sulfate observed in continental precipitation. the amount of cloud water being attached to the ice crystals in relation to the amount of unrimed ice phase in the riming process was estimated to be around 30% during November and June and around 70% in March. The ice crystal shapes and size distributions observed during different cloud temperatures were according to the scheme of Magono and Lee ( Journal of the Faculty of Science of Hokkaido University, Series VII , 1966, 2 , 321–335).