  The effects of mechanical generation of turbulent kinetic energy and buoyancy
forces on the statistics of air temperature and velocity increments are
experimentally investigated at the cross over from production to inertial range
scales. The ratio of an approximated mechanical to buoyant production (or
destruction) of turbulent kinetic energy can be used to form a dimensionless
stability parameter $\zeta$ that classifies the state of the atmosphere as
common in many atmospheric surface layer studies. Here, we assess how $\zeta$
affects the scale-wise evolution of the probability of extreme air temperature
excursions, their asymmetry and time reversibility. The analysis makes use of
high frequency velocity and air temperature time series measurements collected
at $z$=5 m above a grass surface at very large Reynolds numbers $Re=u_* z/\nu >
1\times 10^5$ ($u_*$ is the friction velocity and $\nu$ is the kinematic
viscosity of air). Using conventional higher-order structure functions,
temperature exhibits larger intermittency and wider multifractality when
compared to the longitudinal velocity component, consistent with laboratory
studies and simulations conducted at lower $Re$. Moreover, deviations from the
classical Kolmogorov scaling for the longitudinal velocity are shown to be
reasonably described by the She-Leveque vortex filament model that has no
'tunable' parameters and is independent of $\zeta$. The work demonstrates that
external boundary conditions, and in particular the magnitude and sign of the
sensible heat flux, have a significant impact on temperature
advection-diffusion dynamics within the inertial range. In particular,
atmospheric stability affects both the buildup of intermittency and the
persistent asymmetry and time irreversibility observed in the first two decades
of inertial sub-range scales.

Fluid Dynamics