The atmospheric boundary layer in an Arctic wintertime on-ice air flow

A warm on-ice air flow from the open water over the Arctic sea ice in the Fram Strait was, for the first time, systematically measured on 12 March 1998 by aircraft in the lowest 3 km over a 300-km long distance. The air mass modification and the processes involved are discussed. Over the water, air temperature was lower than water temperature so that a convective boundary layer (CBL) was present as initial condition. As soon as the CBL passed the ice edge, a shallow stable internal boundary layer (IBL) was formed. In the residual CBL, turbulence and pre-existing convective clouds dissolved within about 20 km. Within about the same distance, due to the transition from unstable to stable stratification, the influence of surface friction increased in the IBL and decreased above the IBL with consequent generation of a low-level jet at IBL top. The IBL was strongly stratified with respect to both temperature and wind. The wind shear was around 0.1 s(-)1 so that the Richardson number in the IBL was subcritical and turbulence was generated. The IBL top grew to about 145 m over 230 km distance. The growth of the IBL was not monotonic and was influenced by (a) inhomogeneous ice surface temperatures caused by both different ice thickness and changes in the cloud conditions, and (b) leads in the ice deck. At the front side of the on-ice flow, the air mass boundary between the warm air and the cold Arctic air was sharp (12 K over 10 km) at low levels and tilted with height. Observations suggest that the stratified IBL was lifted as a slab on top of the cold air.