Planetary boundary layer structure and air mass transport during the International Arctic Ocean Expedition 1991

Abstract

During the International Arctic Ocean Expedition 1991 north of latitudes 70° N from August 1 to October 6 (late summer and early autumn), the dominant air mass origin was from the seas that surround the central Arctic Ocean, while land was a much less important source. This resulted in air temperatures near 0°C, fogs and extensive stratus layers. In the autumn, long residence time of the air over the pack ice and subsidence became more common, accompanied by decreasing cloudiness, foginess and air temperature. The residence time of air over the pack ice and the time since land contact had medians of 52 and 91 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 from 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 level jets in 60–90% of the profiles. Layers of subcritical Richardson number were present in 65–80% of the profiles above and below the jet centres. 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 advection and weak convection caused the roll vortices in aged air that had been advected usually 2000 km or more over the ice. Breaking of Kelvin-Helmholtz waves, gravity waves or other reasons for intermittent planetary boundary layer breakdown combined with vertical gradients in concentration are suggested to have caused the observed sudden changes, in an hour or less, in aerosol and tracer concentrations and to have influenced fog formation. Roll vortices are suggested as the cause of periodic changes in aerosol concentrations and fog formation.