AN INERTIAL-DISSIPATION METHOD FOR ESTIMATING TURBULENT FLUX IN BUOYANCY-DRIVEN, CONVECTIVE BOUNDARY-LAYERS

A method is developed for using-wavenumber spectra pf scalar contamina nts in a boundary layer dominated by buoyancy-driven convection to est imate the magnitude of vertical turbulent fluxes. The technique is ana logous to the conventional inertial-dissipation method (IDM) for obtai ning fluxes from variance spectral density levels in tie inertial subr ange but differs in the way that turbulence scales are combined to pro vide an ''eddy density diffusivity'' for relating flux magnitude to va riance dissipation rate. The method is illustrated using spectra and d irect flux covariance data from the oceanic boundary layer at the edge of a freezing lead during the 1992 Lead Experiment in the Arctic Ocea n. There, density was controlled almost exclusively by salinity, and i t is argued that the turbulent length and velocity scales governing ve rtical exchange were the inverse of the wavenumber at the peak in the weighted salinity spectrum (considerably less than the mixed layer dep th) and the cube root of the product of turbulent length scale and the buoyancy flux magnitude, respectively. The sign of the skewness of te mperature or salinity time series is shown to be a robust indicator of the (negative) direction of vertical flux. The ''free convection'' ap proach is valid (i.e., should be used instead of the conventional IDM) only if the convective turbulent scale velocity is appreciably larger than friction velocity (square root of the Reynolds stress), so that turbulent kinetic energy dissipation is approximately equal to buoyanc y production.