Scale Interactions and Flow Structures in Stable Boundary Layers

Stable boundary layer turbulence represents a challenge for numerical weather prediction due to its intermittency and unsteadiness caused by the interactions of processes on multiple scales. In weak-wind conditions, sporadic turbulence can be triggered by sub-mesoscale motions which are poorly understood and mainly not represented in models. In addition, the Reynolds stress tensor in these conditions is highly anisotropic which leads to the failure of traditional similarity scaling relationships.

Recently developed methods of nonstationary data analysis represent a promising tool to extract knowledge about turbulence triggering mechanisms from observational datasets of atmospheric turbulence. We analyse the nonstationary response of turbulent vertical velocity variance to a combination of potential triggering factors using the FEM-BV-VARX clustering method, based on the FLOSSII dataset. The influence of the mean shear is considered together with that of flow accelerations on submeso scales and buoyancy fluctuations. Several locally stationary flow regimes are identified in which the relative influence of each forcing variable on the turbulent vertical velocity variance differ. In each flow regime, we analyse multiple scale interactions and quantify the amount of turbulent variability which can be statistically explained by the individual forcing variables. We also investigate if the state of anisotropy of the Reynolds stress tensor in the different flow regimes relates to these different signatures of scale interactions.