  Regimes of interactions between motions on different time-scales are
investigated in the FLOSSII dataset for nocturnal near-surface stable boundary
layer (SBL) turbulence. The non-stationary response of turbulent vertical
velocity variance to non-turbulent, sub-mesoscale wind velocity variability is
analysed using the bounded variation, finite element, vector autoregressive
factor models (FEM-BV-VARX) clustering method. Several locally stationary flow
regimes are identified with different influences of sub-meso wind velocity on
the turbulent vertical velocity variance. In each flow regime, we analyse
multiple scale interactions and quantify the amount of turbulent variability
which can be statistically explained by external forcing by the sub-meso wind
velocity. The state of anisotropy of the Reynolds stress tensor in the
different flow regimes is shown to relate to these different signatures of
scale interactions. In flow regimes under considerable influence of the
sub-mesoscale wind variability, the Reynolds stresses show a clear preference
for strongly anisotropic, one-component states. These periods additionally show
stronger persistence in their dynamics, compared to periods of more isotropic
stresses. The analyses give insights on how the different topologies relate to
non-stationary turbulence triggering by sub-mesoscale motions.
