Extremes, intermittency, and time directionality of atmospheric turbulence at the crossover from production to inertial scales

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 crossover 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 ζ that classifies the state of the atmosphere as common in many atmospheric surface layer studies. We assess how ζ affects the scalewise evolution of the probability of extreme air temperature excursions, their asymmetry, and time directionality. The analysis makes use of high-frequency turbulent velocity and air temperature time-series measurements collected at z=5m above a grass surface at very large frictional Reynolds numbers Re∗=u∗z/ν>1×105 (u∗ is the friction velocity and ν is the kinematic viscosity of air). A multitime measure of the imbalance between forward and backward phase-space trajectories is employed to investigate the time-directional properties of the scalar (temperature) field. Using conventional higher-order structure functions, we find that temperature exhibits larger intermittency and wider multifractality when compared to the longitudinal velocity component, consistent with laboratory studies and simulations conducted at lower Re∗. We find that the magnitude of ζ, rather than the sign of the heat flux, impacts the distribution of scalar increments at separation scales well within the inertial subrange. Conversely, the direction of the heat flux fingerprints the observed time-directionality properties of the scalar field in the first two decades of inertial subrange scales. These combined findings demonstrate 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.

Duke Scholars

Author Andrew Bragg Civil and Environmental Engineering

Author Gabriel G. Katul Civil and Environmental Engineering