Signals of Nonturbulent Motions Caused by Stable Stratification in Near-Surface Sonic Anemometer Data

Abstract Understanding the interactions between turbulent and nonturbulent motions has been a persistent challenge faced by the community studying stably stratified turbulent flows. For flows with high Reynolds number, high Rossby number, and stable stratifications, nonturbulent motions involve physical mechanisms acting against instability development. Because turbulent motions are generated through an energy cascade via instability development, the presence of nonturbulent motions is expected to modify the energy distribution across scales compared to that of solely turbulent motions. The objective of this work is to identify in field data statistical signals of nonturbulent motions caused by stable stratification. The need to resolve energy-containing motions in both space and time requires high-frequency time series of velocity fluctuations collected using arrays of sonic anemometers. The analysis is performed using data from the Canopy Horizontal Array Turbulence Study (CHATS), during which a total of 31 sonic anemometers were deployed on a horizontal array and on a 30-m tower. Compared to other field campaigns which were also equipped with arrays of sonic anemometers, CHATS took an important advantage of already published nighttime canopy-scale waves derived from aerosol backscatter lidar images. After precluding complexities caused by nonstationarity and horizontal heterogeneity, signals of nonturbulent motions caused by stable stratification are identified from spatial autocorrelations of time-block-averaged velocity fluctuations. These signals are interpreted using existing understanding of turbulent canopy flows and two-dimensional Kelvin–Helmholtz instability development. The associated estimates of critical wavelengths and buoyancy periods agree well with the overall properties of nighttime canopy-scale waves derived from lidar images. Significance Statement This work investigates statistical signals of nonturbulent motions caused by stable stratification in sonic anemometer measurements of near-surface atmospheric flows. The detected signals of nonturbulent motions agree with theoretical predictions of the impacts of stable stratification on turbulent canopy flows. This agreement suggests potential advantages for understanding stably stratified near-surface flows using canopy-resolving simulations. The automatic, objective, statistical detection procedures, as well as the intermediate products of the periods of statistically stationary, horizontally homogeneous, approximately two-dimensional mean flows, are useful for improving the understanding of canopy flows for various stability conditions.