Organised Motion and Radiative Perturbations in the Nocturnal Canopy Sublayer above an Even-Aged Pine Forest

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.

A case study of eddy covariance flux of N<sub>2</sub>O measured within forest ecosystems: quality control and flux error analysis

“…Using the canopy height h c instead of the measurement height z, the normalized frequency values at which the cospectra peak, occur at 0.6 for both sites, which indicates that the most energetic eddies scale with h c and that for the selected cases the ratios U hc /U are invariant between the two sites. While such a simple cospectral model described very well the sensible heat cospectra measured during daytime, it was found unsuitable for fitting the night-time measured cospectra, which were often characterized by larger uncertainty as a result of multi-scale non-stationary processes usually affecting the night-time scalar transport in the sub-canopy layer (Cava et al, 2004).…”

Modeling nighttime ecosystem respiration from measured CO2 concentration and air temperature profiles using inverse methods

“…The first reason is to eliminate nonturbulent conditions. For example, Cava et al [2004] showed how canopy waves are generated and how they can transport significant CO 2 inside to outside and outside to inside the canopy and over periods that if not properly captured by the averaging interval can lead to negative CO 2 fluxes (i.e., photosynthesis like) at night. Employing a u* threshold is primarily to ensure that runs collected under such nonturbulent conditions are removed.…”

“…Often, nocturnal conditions are dominated by vertical subsidence, lack of steadiness in mean atmospheric conditions, and intermittent turbulent transport often initiated by transients such as passage of clouds [Cava et al, 2004]. When viewed from the onedimensional vertically integrated scalar continuity equation, these factors contribute to increased ''decoupling'' between the desired R E quantity and the CO 2 flux above the canopy, the latter being the observed quantity by EC methods.…”

Variabilidade Em Escala Da Energia Cinética Turbulenta Acima De Floresta Na Amazônia Ocidental

“…Os valores médios de ECT, por escala, na estação úmida, durante a tarde, são maiores que os observados no período da manhã. Em todos os gráficos da Figura 2, entre as escalas 5 (~26,8 min) e 10 (~8,56 min), se observam mínimos relativos associados à existência da falha espectral indicando possível ocorrência de ondas de gravidade acima da copa florestal (Cava et al, 2004). Na estação seca os valores médios das ECTs, por escala temporal, durante a noite, são maiores que aqueles da madrugada.…”