The Near-Neutral Marine Atmospheric Boundary Layer with No Surface Shearing Stress: A Case Study

Data from a marine coastal experiment over the Baltic Sea, comprising airborne measurements and mast measurements, have been used to highlight the turbulence dynamics of a case with most unusual flow characteristics. The boundary layer had a depth of about 1200 m. The thermal stratification was near neutral, with small positive heat flux below 300 m and equally small negative heat flux above. The entire situation lasted about 6 hours. Turbulence levels were unexpectedly high in view of the fact that momentum flux was negligible (in fact positive) in the layers near the surface, and buoyancy flux was also small. The turbulence was found to scale with the height of the boundary layer, giving rise to velocity spectra having the shape of those characteristic of convectively mixed boundary layers. Analysis of the turbulence budget for the entire planetary boundary layer (PBL) revealed that energy was produced from shear instability in the uppermost parts of the PBL and was distributed to the lower parts of the PBL by pressure transport. Dissipation was found to be evenly distributed throughout the entire PBL. Without data on surface wave characteristics, no firm conclusions concerning air-sea interaction processes can be drawn, but there are clear indications that the dynamical decoupling observed at the surface is due to the effect of decaying sea state conditions (high wave age conditions). In any case, the process of active turbulence production in the layers close to the surface observed in "ordinary" near-neutral boundary layers has been effectively turned off here, leaving only turbulence of the "inactive" kind, imported by pressure transport from layers above. The results strongly support the findings reported in the recent literature on "laboratory turbulence" that the process of strong turbulence and shearing stress production near the wall of boundary layers of very different kinds is virtually independent of forcing from large-scale structures embedded in the flow.