Intermittency of turbulence within open canopies

Eddy covariance data have been analyzed to examine intermittency of turbulence within open canopies. Intermittency consists of two aspects: one is related to the amplitude of small-scale fluctuations and the other to local frequency of oscillations (clustering). To separate the clustering tendency from amplitude effects, we first generate the telegraph approximation (TA) of turbulent signals. The binary structure of the TA removes any amplitude variability from turbulent excursions but retains their zero-crossing behaviors isolating the role of clustering in intermittency. Intermittency of canopy turbulence is examined in terms of clustering exponent, intermittency exponent and probability density distribution of inter-pulse period of TA. The used data were collected from a mature ponderosa pine site and juniper site in Central Oregon, USA. The ponderosa pine is characterized by a leaf area index of about 3 with a strong diurnal variation of stability existing in the subcanopy and has a distinct inflection point near the canopy top. The juniper tree has LAI of 1 and inflection point is not distinct. The results show that the vegetation and atmospheric stability do not alter fine-scale clustering properties significantly. The clustering exponent ranges from 0.25 to 0.35. Within the canopy, longitudinal velocity is more clustered than vertical velocity and temperature at the fine scales. Temperature shows similar clustering to vertical velocity. Intermittency exponent is higher for temperature than two velocity components for both pine and juniper canopy. When comparing intermittency for flow variables and their TA series, amplitude variation mitigates intermittency for both velocity components and temperature. However, amplitude variations play much larger role in velocity intermittency than temperature counterpart. Within the crown regions, the inter-pulse probability distribution of TA does not show ‘double regime' observed within the crown of dense canopy.

Key words: Canopy turbulence, Clustering, Intermittency, Open canopy