ORGANIZED TURBULENCE STRUCTURES IN THE NEAR-NEUTRAL ATMOSPHERIC SURFACE-LAYER

The Tiederman method, which was originally designed for unambiguous de tection of ''bursts'' in the buffer layer of smooth surface laboratory boundary-layer flow, is shown to work equally well in the neutral atm ospheric surface layer. It enables estimation of characteristic timesc ales of significant structures responsible for the momentum transport: mean duration of and mean interval between bursts (upward transport o f momentum deficit) and mean duration and interval between sweeps (dow nward transport of momentum excess). Data representing near neutral co nditions from three field experiments over hat and homogeneous areas b ut with very different roughness characteristics are used in the analy sis. For two low vegetation sites (i.e., the measuring heights are ver y much larger than the roughness sublayer height), it is found that al l above-mentioned timescales are constant for any height and identical at the two sites. Scaling the results with the friction velocity indi cates the corresponding scaling height to be proportional to the scali ng height of the neutral boundary layer. The third site is a forest si te, and the measurements were taken in the roughness sublayer. Here ca nopy inflection scaling is found to apply. The structures identified w ith the Tiederman method are shown to be responsible for more than 90% of the momentum flux, most of which is accomplished by a mean Bow com ponent within the structures. Results are discussed in light of direct numerical simulation results for fully turbulent but low Reynolds num ber how over a dynamically smooth surface and large-eddy simulation re sults for a high Reynolds number case, and these show encouraging cons istencies for the logarithmic layer. Turbulence production at the surf ace is, however, fundamentally different in the rough atmospheric case compared to the smooth case and is presumably governed by canopy wind profile inflection instability.