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 highlig ht the turbulence dynamics of a case with most unusual flow characteri stics. The boundary layer had a depth of about 1200 m. The thermal str atification was near neutral, with small positive heat flux below 300 m and equally small negative heat flux above. The entire situation las ted 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 turbule nce was found to scale with the height of the boundary layer, giving r ise to velocity spectra having the shape of those characteristic of co nvectively mixed boundary layers. Analysis of the turbulence budget fo r the entire planetary boundary layer (PBL) revealed that energy was p roduced from shear instability in the uppermost parts of the PBL and w as distributed to the lower parts of the PBL by pressure transport. Di ssipation was found to be evenly distributed throughout the entire PBL . Without data on surface wave characteristics, no firm conclusions co ncerning air-sea interaction processes can be drawn, but there are cle ar indications that the dynamical decoupling observed at the surface i s due to the effect of decaying sea state conditions (high wave age co nditions). 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 tu rbulence of the ''inactive'' kind, imported by pressure transport from layers above. The results strongly support the findings reported in t he recent literature on ''laboratory turbulence'' that the process of strong turbulence and shearing stress production near the wall of boun dary layers of very different kinds is virtually independent of forcin g from large-scale structures embedded in the flow.