Intermittency and the Organization of Turbulence in the Near-Neutral Marine Atmospheric Boundary Layer

Abstract

Aircraft data from the JASIN Experiment have been used to examine the role that intermittency plays in turbulent transfer in the near-neutral marine atmospheric boundary layer. Conditional sampling, using the time-varying dissipation rate as an indicator, was the technique chosen for studying the dimensions of observed bursts of dissipation and their relation to the turbulent transfer. Burst fractional area coverage, ?, showed significant height variability in the surface layer, from a value of 0.45 near the surface decreasing to a constant value of about 0.30 above Z=0.2Zi. It was shown that ? is quite sensitive in the surface layer to the height of measurement and to the surface roughness (scaling with u2*/gZ), while being independent of heat flux. The plume model of Frisch provided an estimate of the physical dimensions of the bursts. Their area varied little with height and corresponded to an average diameter of 140 m, but the number density decreased with height. The regions of high turbulence activity showed an elongation of 10% in the mean wind direction throughout the ABL. 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 fluxes were concentrated in 30% of the area over most of the ABL. The mean vertical velocity difference, ?w, between the bursts and the ambient state 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 ?, and various laboratory findings suggest that plumes may be generated by the shear properties of the flow, rather than by thermal instabilities. The turbulence kinetic energy balance showed that bursts of dissipation are also regions of enhanced turbulent transfer. In the convective case, buoyant production is concentrated in these regions. The transport of turbulence kinetic energy out of the lower ABL by the bursts actually exceeds the net transport, so that the ambient state transports turbulence kinetic energy to the surface.