The role of coherent structures in subfilter-scale dissipation of turbulence measured in the atmospheric surface layer -: art. no. 040

A field experiment was designed and carried out to study the role of coherent structures on the local transfers of energy and temperature variance between resolved and subfilter (unresolved) scales relevant to large-eddy simulation of high-Reynolds-number boundary layers. In particular, 16 sonic anemometers were used in an arrangement with a 6 m high vertical array (ten anemometers) that intersected a 3 m wide horizontal array (seven anemometers). The data collected are used to calculate the subfilter-scale (SFS) stresses and fluxes, and the SFS dissipation rate (transfer rate between resolved and subfilter scales) of energy and temperature variance. With these quantities, conditional averaging is used to study the relation of strong positive (forward-scatter) and negative (backscatter) SFS dissipation events to local features of the flow. The conditionally averaged vertical and horizontal flow fields reveal vortical structures, inclined downwind at angles close to 16degrees during near-neutral atmospheric stability, and as large as 34degrees during convective conditions. These inclined vortical structures agree with the concept of a hairpin vortex (with head and trailing legs) around which sweep and ejection events are found. Localized regions of large forward-scatter are found on the upper trailing edge of these structures, whereas localized regions of large backscatter are found on the lower leading edge of the same type of structures.