Derivation of dual-scale turbulent ramp structure features and improvements to scalar surface renewal estimates

Turbulent coherent structures exhibit ramp features in scalar traces. Conventional surface renewal theory uses these ramp features from a single scale of turbulent coherent structures to estimate energy and mass exchange. However, various studies have shown some uncertainty in surface renewal exchange estimates, potentially due to multiple ramp-like scales instead of single scales. In this study, a method to account for two ramp scales is developed, using a modified structure function approach. The smaller scale is assumed to reflect non-flux bearing entrainment phenomena, while the larger scale, assumed to be the signature of flux-bearing coherent structures. Spectral analysis aids in this analysis. When turbulent data measured above bare soil and short plant canopies are analyzed with this two-scaled method, it is found that both scales seem to be dependent on mean wind shear. This may indicate that the smaller scale ramps represent entrainment phenomena driven by the larger scale, shear-driven features. The larger scale is confirmed to yield surface renewal estimates consistent with eddy-covariance, in contrast to the smaller scale features, with best accuracy under unstable conditions.