Atmospheric boundary layer modification over sea ice leads in the Northern Fram Strait: a study based on aircraft measurements during the campaign STABLE 2013

Leads in sea ice play an important role for the heat and moisture exchange between the polar ocean and atmosphere. Large differences between the air and surface temperatures over leads induce the formation of internal atmospheric boundary layers with strong convective plumes often penetrating the lowest layers of the capping inversion. In this contribution we present results of the campaign STABLE (Spring Time Arctic Boundary Layer Experiment) that was carried out by the Alfred Wegener Institute in March 2013 using the aircraft Polar 5 for meteorological measurement over the ice pack of the Northern Fram Strait. High resolution measurements of wind and air temperature allow the derivation of mean values as well as turbulent fluxes of heat and momentum. Furthermore, the surface temperature was measured with different instrumentation including an infrared scanner. We show results concentrating on the overall effect of lead ensembles supplemented by case studies of four individual leads differing by geometry, thin-ice cover, and atmospheric forcing conditions. In the case studies, the turbulent fluxes of heat and momentum were obtained from lead parallel flight legs at different heights while the overall boundary layer structure was measured during lead orthogonal flight sections. Our measurements suggest a strong dependence of the near-surface mean atmospheric temperature on the lead concentration and downstream of individual leads near-surface temperatures increase by 0.5 to 3 K in the four observed cases. Near-surface heat fluxes have been measured in a range from 100 to 250 W/m² over leads with open water and in the order of 30 W/m² over leads covered with thin ice. Measured changes of atmospheric boundary layer temperatures and wind downstream of the leads indicate the development of internal boundary layers which depend on the lead width, the prevailing ice conditions on the leads, and the inflow boundary layer thickness.