Vertical propagation of submeso and coherent structures at the ATTO site in different stability conditions

The ATTO-IOP-I measurements, held in October and November 2015 were used to study the influence of stability on the Atmosphere dynamics and the scalar transport both above and within the forest. The aim of the analysis is to explore the possible limitations of the mixing layer analogy when stratification departs from neutrality and the dynamics and the exchange processes in the roughness sublayer may be influenced by convective plumes and submeso motions. Five stability regimes were identified, among them a ‘super stable’ regime characterized by negligible turbulent fluxes and low-wind speeds. The dominance of large scale structure on the flow increases with increasing stability. In weakly stable conditions motions characterized by a temporal scale of about 20-30 minutes are mainly observed at the two highest levels in the horizontal wind components and in temperature (presumably horizontal meandering) and remain confined above the roughness sublayer without perturbing the canopy flow. As stability increases, submeso motions tends to propagate more and more to lower levels and become the dominant flow in the super stable regime propagating up to the canopy top. Moreover, in the SS regime the low-frequency energy is observed also in the vertical wind components (mainly at the highest levels), indicating that the submeso motions in this regime may be a mix of both horizontal meandering and gravity waves. An original method to identify the coherent structures’ scale is proposed and tested. The vertical scale of coherent structures represented by the shear lenght scale at the canopy top presented a linear dependece on stability for small departures from neutral conditions and reached asymptotic values for both unstable and very stable conditions. This reflects on the streamwise separation of the coherent structures and its behaviour with stability. The Strouhal number, the ratio between the streamwise separation and the vertical scale of the vortices, increases in both stable and unstable conditions with slightly different behaviours. An analytical realtionship relating the streamwise separation and shear lenght scale with varying stability is derived. Two-point autocorrelation functions were evaluated over 5-minute subsets for the three wind components and for the scalars, considering 40 m as reference level to study of the propagation of coherent structures generated by the canopy-top inflection point instability both above and below the vegetation .