Higher-order statistics of the turbulent flow in a sparse Lodgepole Pine canopy

An accurate modeling of plant-atmosphere interactions relies on an appropriate understanding and implementation of canopy turbulence. It is well known that turbulent exchange in plant canopies is significantly different from the surface layer. In the canopy sublayer we encounter very specific conditions that result in highly skewed probability density distributions, intermittency, and strong sweep-ejection cycles. The density of vegetation stands is believed to significantly control those characteristics. However a quantitative description of any relationships is further complicated by the extreme range of canopy morphologies. Interestingly, most studies done so far (field, wind-tunnel, and flume experiments, but also numerical simulations) focussed on dense canopies. Less information has been published for sparse canopies even though they form a significant part of the global land surfaces - in particular in the boreal zone.

In this contribution we analyze a data-set from a recent field experiment in a sparse Lodgepole Pine Stand in Northern British Columbia, Canada. Data was sampled using a vertical array of ultrasonic anemometers at the ‘Kennedy Siding' tower (55° 06' 43''N, 122° 50' 23''W). The stand surrounding this tower has a mean canopy height of h = 16 m, a low canopy cover of only 24.3%, and a leaf area index of 1.38. The site is located in flat terrain and the fetch in all wind directions extends to at least 1 km. Eight Campbell Scientific CSAT-3 ultrasonic anemometer thermometers were simultaneously operated at 10 Hz at different heights (z/h = 0.16, 0.44, 0.68, 0.87,1.06, 1.25, 1.56, and 1.96) over one month in August / September 2007. These measurements were complemented with a profile of fine-wire thermocouples to retrieve an accurate vertical profile of mean temperatures.

From this data-set, ensemble average profiles of different higher-order turbulence statistics and quadrant analysis measures have been calculated. The poster will feature a panel of selected vertical profiles stratified into different overall forcings based on the sensible heat and momentum flux measured at tower top (z/h = 1.96). The discussion enlightens some specific features of a sparse forest canopy by comparing the results with literature values from dense canopies.