9.6
Statistics of optimal perturbations in quasigeostrophic turbulence
Timothy DelSole, COLA, Calverton, MD
Optimal perturbations are initial conditions that optimize some measure of linear growth about a background flow. Many investigators, starting from at least a century ago, have suggested that optimal perturbations play an important role in turbulence. An outstanding question regarding optimal perturbations is whether they are excited in statistically steady turbulence at a rate necessary for them to play a significant role. This question does not seem to have been addressed rigorously in the published literature. Numerical experiments based on a two-layer quasigeostrophic model were undertook to resolve this question. It turns out that a certain class of optimals, called instantaneous optimals, substantially clarify the analysis. The experiments revealed that a surprisingly large fraction of energy is concentrated in the leading and trailing instantaneous optimals. However, the actual energy tendency of the nonlinear system turns out to be a small residual between the tendency of a few growing and a few decaying optimals, suggesting that the simple picture in which turbulence is maintained by random excitation of optimal perturbations is fundamentally incomplete.
Session 9, Eddy Interactions, Jets, and Turbulence
Thursday, 16 June 2005, 9:50 AM-2:05 PM, Ballroom A
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