| Aircraft data from the JASIN Experiment were used to demonstrate the presence of vertically coherent structures in the near-neutral marine atmospheric boundary layer (ABL) and to examine their importance in the overall turbulent transfer. Conditional sampling, using the time-varying dissipation rate as an indicator, was the technique for studying such structures. Their fractional area coverage, (gamma), showed significant height variabilty in the surface layer from a value of 0.45 near the surface, decreasing to a constant value of about 0.30 above Z = 0.2Z(,i). (gamma) was shown to be quite sensitive in the surface layer to the height of measurement and to the surface roughness (scaling with u(,*)('2)/gZ) while being independent of heat flux.;Bursts of dissipation rate were generally coincident with regions of enhanced flux. Conditional statistics showed that 50--60% of the vertical velocity variance, stress, and water vapor flux was concentrated in 30% of the area over most of the ABL. The mean vertical velocity difference between the burst structures and the ambient state, (DELTA)W, was found to reflect buoyant input of energy into the ABL through a dependence on the convective scaling velocity w(,*). This observation, the roughness height dependence of (gamma), and various laboratory findings suggest that coherent structures are generated by the shear properties of the flow, rather than by thermal instabilities.;The turbulence kinetic energy balance showed that burst structures dominate overall ABL turbulent transfer. Dissipation and, in the convective case, buoyant production are concentrated in these regions. The transport of energy out of the lower ABL by these structures actually exceeds the net transport, so that the ambient state is a source of turbulent kinetic energy near the surface.;A case study has shown that the surface stress field varies significantly over the extratropical ocean in response to sea surface temperature variations and that burst structure distributions are in equilibrium with the local stress field. The study also suggests that variations in the stress field may be partially responsible for the organization of mesoscale ABL secondary flow.;The plume model of Frisch (1970) provided an estimate of physical dimensions of the structures. Their area varied little with height and corresponded to an average diameter of 140 m, but the number density decreased with height. The structures showed an elongation of 10% in the mean wind direction throughout the ABL. |
