Variability and vertical structure of the summer Arctic boundary layer

During the Arctic Ocean 2001 expedition to the central Arctic, in-situ and remote sensing instruments were deployed to monitor the vertical structure of the summertime Arctic boundary layer.

To analyze the vertical structure, a multi-sensor approach is necessary. Cloud radar and cloud ceiliometer data indicate a persistent cover of low clouds. At the same time, profile measurements in a mast on the ice show a strong surface control of the boundary layer; the temperatures mostly remain between the melting point of fresh water (snow on the ice) and that of salty ocean water, while the relative humidity hardly ever falls below 95%.

Vertical temperature profiles were derived with a high temporal resolution from a scanning microwave radiometer. These indicate a vertical structure that often had a weak surface inversion, followed by a near moist-adiabatic layer a few hundred meters deep and a capping inversion, sometimes very strong. Episodes of strong advection of warm air from beyond the ice edge caused large changes in the free troposphere, but hardly affected the boundary layer at all. Soundings indicate that absolute humidity often increases over the inversion and wind measurements with sodar and wind-profiler radar indicate events with low-level jets. The boundary layer is shallow but very moist and surprisingly well mixed, while the surface turbulent heat fluxes remain small.

Analysis of the turbulence measurements shows a large variability with frequent periods of intense low-frequency variability, predominantly in the horizontal wind speed components. The temperature variance analyzed from the microwave radiometer data also shows episodes of intense variability, while microbarograph measurements shows a large variability, that in episodes cascade to short period bursts.

We analyze and present the vertical structure of the Arctic boundary layer, focusing on the well-mixed conditions in spite of very small surface heat flux, and discuss possible explanations for the intense variability and for its episodic nature.