Mixing in weakly turbulent stably stratified flows

We analyse mixing in three weakly turbulent stably stratified flows, namely low amplitude breaking internal gravity waves, stably stratified homogeneo us decaying turbulence and a stably stratified unstable shear layer. We use the method proposed by Winter et al. [J. Fluid Mech. 289 (1995) 115], whic h provides an exact expression of the diffusive flux of density responsible for mixing. When the three flows have stabilized and organized into quasi- horizontal layers, we show that the diffusion coefficient behaves linearly as a function of a dynamical parameter that characterizes the largest scale s of the flow (a Froude number squared or the inverse of a Richardson numbe r) and we provide a simple expression of this linear law. (C) 2001 Elsevier Science B.V. All rights reserved.