NONLINEAR EVOLUTION OF AN UPWARD PROPAGATING GRAVITY-WAVE - OVERTURNING, CONVECTION, TRANSIENCE AND TURBULENCE

A numerical simulation of the nonlinear evolution of an upward propagating gravity wave shows that overturning (the turning over of isopotential temperature surfaces) is the mechanism responsible for limiting the growth of the wave. Wave saturation (the state in which wave amplitude is constant with height) in the mesosphere results in turbulence (random, subkilometer-scale motions), but turbulence is not responsible for limiting wave amplitude. Therefore, parameterizations of wave drag and wave-associated eddy diffusivity that derive from the turbulence model of wave saturation have no rigorous justification and could give erroneous results if employed in studies of middle atmosphere circulation and minor constituent mixing. -from Authors