Boundary-layer structure in the summertime high Arctic during the AOE2001

During the Arctic Ocean 2001 expedition we took continuous turbulence and profile measurements during 3 weeks on an 18-m mast erected on a 1.5 by 3 km large ice floe near the North Pole, at 89˚N. Together with profiling of the lower troposphere with sodar, cloud radar, wind profiler, scan-ning radiometer and regular radiosoundings, this data allows a study of the PBL structure in the very high Arctic.

The boundary layer during the entire period was very moist and unlike previously reported experience relatively well mixed. The PBL relative humidity was above 90% more than 80% of the time and the visibility was below 10 km more than half the time, while low stratus clouds dominate. As the ice is melting in August, the temperature near the surface mostly remained within the the range -2 - 0 °C. Unlike for the mid-latitude PBL, the specific humidity increased over the capping inversion about half the time, and advection of warm and moist air sometimes generated very strong inversions, more than 15K in potential temperature starting at only a few hundred meters. Low-level jets, also a dominant feature in many earlier studies, were rather rare although power spectra of wind speed components from time to time show a significant secondary peak around the inertial frequency.

In this presentation we will discuss the fluxes of momentum, heat, water and aerosols and relate these to mean profiles. The results are put into the context of boundary layer dynamics using boundary layer soundings, sodar profiles and temperature profiles from the scanning radiometer. The presentation focusses on two specif periods, when there was a very pronounced capping inversions with maximum temperatures at 1 km ~ 10-15 ˚C warmer than at the surface but when the turbulence structure was very different.