No abstract

Using time series measurements of velocity, carbon dioxide and water vapour concentration, and temperature collected just above a 15 m tall even-aged pine forest, we quantify the role of organized motion on scalar and momentum transport within the nocturnal canopy sublayer (CSL). We propose a framework in which the nocturnal CSL has two end-members, both dominated by organised motion. These end-members represent fully developed turbulent flows at near-neutral or slightly stable stratification and no turbulence for very stable stratification. Our analysis suggests that ramps dominate scalar transport for near-neutral and slightly stable conditions, while linear canopy waves dominate the flow dynamics for very stable conditions. For intermediate stability, the turbulence is highly damped and often dominated by fine scale motions. Co-spectral analysis suggests that ramps are the most efficient net scalar mass-transporting agent while linear canopy waves contribute little to net scalar transport between the canopy and atmosphere for averaging intervals that include complete wave cycles. However, canopy waves significantly contribute to the spectral properties of the scalar time series. Ramps are the most frequently occurring organised motion in the nocturnal CSL for this site. Numerous night-time runs, however, resided between these two end-members. Our analysis suggests that when radiative perturbations are sufficient large (>20 W m −2 in net radiation), the flow can switch from being highly damped fine-scale turbulence to being organized with ramp-like properties. We also found that when ramps are already the dominant eddy motion in the nocturnal CSL, radiative perturbations have a minor impact on scalar transport. Finally, in agreement with previous studies, we found that ramps and canopy waves have comparable length scales of about 30-60 metres. Consequences to night-time flux averaging are also discussed.

In contrast to atmospheric surface-layer (ASL) turbulence, a linear relationship between turbulent heat fluxes (F T ) and vertical gradients of mean air temperature within canopies is frustrated by numerous factors, including local variation in heat sources and sinks and large-scale eddy motion whose signature is often linked with the ejection-sweep cycle. Furthermore, how atmospheric stability modifies such a relationship remains poorly understood, especially in stable canopy flows. To date, no explicit model exists for relating F T to the mean air temperature gradient, buoyancy, and the statistical properties of the ejection-sweep cycle within the canopy volume. Using third-order cumulant expansion methods (CEM) and the heat flux budget equation, a "diagnostic" analytical relationship that links ejections and sweeps and the sensible heat flux for a wide range of atmospheric stability classes is derived. Closure model assumptions that relate scalar dissipation rates with sensible heat flux, and the validity of CEM in linking ejections and sweeps with the triple scalar-velocity correlations, were tested for a mixed hardwood forest in Lavarone, Italy. We showed that when the heat sources (S T ) and F T have the same sign (i.e. the canopy is heating and sensible heat flux is positive), sweeps dominate the sensible heat flux. Conversely, if S T and F T are opposite in sign, standard gradient-diffusion closure model predict that ejections must dominate the sensible heat flux.

Within the canopy sub-layer (CSL), variability in scalar sources and sinks are known to affect flux-variance (FV) similarity relationships for water vapour (q) and carbon dioxide (C) concentrations, yet large-scale processes may continue to play a significant role. High frequency time series data for temperature (T ), q and C, collected within the CSL of an uneven-aged mixed coniferous forest in Lavarone, Italy, are used to investigate these processes within the context of FV similarity. This dataset suggests that MOST scaling describes the FV similarity function of T even though the observations are collected in the CSL, consistent with other studies. However, the measured FV similarity functions for q and C appear to have higher values than their temperature counterpart. Two hypotheses are proposed to explain the measured anomalous behaviour in the FV similarity functions for q and C when referenced to T . Respired CO 2 from the forest floor leads to large positive excursions in the C time series at the canopy top thereby contributing significantly to both C variance increase and C vertical flux decrease-both leading to an anomalous increase in the FV similarity function. For q, transport of dry air from the outer-layer significantly increases both the variance and the water vapour flux. However, the expected flux increase is much smaller than the variance increase so that the net effect remains an increase in the measured FV similarity function for water vapour above its T counterpart. The hypothesis here is that identifying these events in the temporal and/or in the frequency domain and filtering them from the C and q time series partially recovers a scalar flow field that appears to follow FV similarity theory scaling. Methods for identifying both types of events in the time and frequency domains and their subsequent effects on the FV similarity functions and corollary flow variables, such as the relative transport efficiencies, are also explored.

Oscillations in the horizontal components of the wind velocity associated with oscillations in air temperature during low–wind speed episodes are ubiquitous in the stable boundary layer and are labeled as wind meandering. The meandering structure is recognizable by a clear negative lobe in the Eulerian autocorrelation functions of the horizontal wind velocity components and of the sonic temperature and by a corresponding peak at low frequency in the velocity components and temperature spectra. These distinctive features are used to isolate meandering occurrences and to study its properties in relation to the classical description of the planetary stable boundary layer. It is shown that the ratio of the variance of the wind velocity vertical component over the variance of the composite of the wind velocity horizontal components splits the frequency distribution of meandering and nonmeandering events and divides the nocturnal boundary layer in two different regimes characterized by different turbulent properties. The data comparison with a turbulence model based on Rotta return to isotropy showed that meandering and nonmeandering cases may have similar dynamics. This suggests that meandering may not be connected to a laminarization of the flow and shows that the Rotta scheme may still describe the energetic transfer between wind velocity components in the very stable boundary layer if the Rotta similarity constant c depends on the flux Richardson number. The data confirm a c value of 2.2 for Rif = 0 compatible with its conventional value. The analysis presented refers to one year of continuous measurements on 10 levels carried out at a coastal site in southeastern Brazil.