Recent numerical experiments and field measurements have established that the near-neutral planetary boundary layer (PBL) flow is frequently organized into alternating high and low speed near-surface streaks. These streaks are of much larger scale than log-layer 'horseshoe' vortices and are of much smaller scale than PBL rolls. This paper demostrates that transient 3-D perturbations which maximize their energy on short time scales (<30 min) have characteristics which closely match these observations. The solution finds the unique initial condition which attains the maximum possible increase in energy at a given time interval. We find that these optimal perturbations begin as small-scale roll-like circulations which rapidly transform into near-surface streaks with weaker associated overturning flow. Consistent with non-normal mode instability analyses in other mean flows the rapid inital energy growth occurs on an advective timescale while the streaks decay on a slower viscous timescale. If the time at which the perturbation must achieve maximum growth is increased, unstable normal modes associated with PBL rolls are energized and dominate the solution. Previous work on this problem (Foster, JFM, 333, 1997) is extended in order to cast further light on the association between the optimal perturbations and the observed near-surface streaks.
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12th Conference on Atmospheric and Oceanic Fluid Dynamics