PLANETARY BOUNDARY-LAYER STRUCTURE AND AIR-MASS TRANSPORT DURING THE INTERNATIONAL ARCTIC-OCEAN EXPEDITION 1991

During the International Arctic Ocean Expedition 1991 north of latitud es 70 degrees N from August 1 to October 6 (late summer and early autu mn), the dominant air mass origin was from the seas that surround the central Arctic Ocean, while land was a much less important source. Thi s resulted in air temperatures near 0 degrees C, fogs and extensive st ratus layers. In the autumn, long residence time of the air over the p ack ice and subsidence became more common, accompanied by decreasing c loudiness, foginess and air temperature. The residence time of air ove r the pack ice and the time since land contact had medians of 52 and 9 1 h, respectively. The structure and evolution of the marine boundary layer above the pack ice is described. A stable layer, 300-1700 m high , grew in height proportionally to the square root of the distance fro m the ice edge, the fetch, up to 3000 km. After 400 km, a mixed layer formed, typically 100-250 m thick, related to the presence of low leve l jets in 60-90% of the profiles. Layers of subcritical Richardson num ber were present in 65-80% of the profiles above and below the jet cen tres. Cloud streets and roll vortices were observed above the pack ice with wavelengths of 4-12 km. The high aspect ratios of 7-9 of the few earlier roll observations above ice were confirmed. Cold air advectio n and weak convection caused the roll vortices in aged air that had be en advected usually 2000 km or more over the ice. Breaking of Kelvin-H elmholtz waves, gravity waves or other reasons for intermittent planet ary boundary layer breakdown combined with vertical gradients in conce ntration are suggested to have caused the observed sudden changes, in an hour or less, in aerosol and tracer concentrations and to have infl uenced fog formation. Roll vortices are suggested as the cause of peri odic changes in aerosol concentrations and fog formation.