Abstract
A thermally stratified wind tunnel was used to measure the probability density functions (PDFs) of scalar fluctuations and their gradients in decaying stratified turbulence. The temperature gradient PDFs exhibit very distinct exponential tails over a wide range of turbulent Reynolds number, while the corresponding PDFs of the temperature fluctuations show close to Gaussian behavior. It is found that strong buoyancy forces do not alter the exponential behavior of the PDF tails. The steepness of the tails is a decreasing function of the turbulent Reynolds number. A simple phenomenological model is proposed which is based on stirring of fluid parcels advected in the mean gradient. (O.G.)