The budgets of turbulent kinetic energy and sensible heat flux within and above a sparse Lodgepole Pine stand

Exchange at the forest-atmosphere interface is strongly driven by the way production, relocation and destruction of turbulent kinetic energy (TKE) interacts with the vertically distributed sources and sinks of momentum, heat and mass in the canopy. Recent computational developments have promoted an increasing level of detail in large-eddy simulations, so canopy turbulence and in particular the terms of the budget of TKE within and above canopies have been modeled in several studies (e.g. Dwyer, et al., 1997, Yang et al., 2006). Nevertheless, observations from field-experiments and from scale model experiments are still a valuable source of information - not only for the validation of those models but to explore the potential success of various higher-order canopy closure schemes.

In contrast to numerical simulations only a few studies have focussed on the TKE budget in field experiments (Leclerc et al., 1990, Meyers and Baldocchi, 1991). These studies have confirmed that turbulence above and within the canopy is not in local equilibrium and that the turbulent transport of TKE is an important process throughout most forest canopies, linked to the often reported sweep-ejection cycles. On the other hand, the sensible heat budget has been shown to be helpful when characterizing non-neutral situations. For example in a recent field study, Cava et al. (2006) have related the mean air temperature gradient and buoyancy to the statistical properties of the ejection-sweep cycle under diabatic conditions. Most of those field studies however focussed on relatively dense forest canopies.

In this contribution we quantify the relevant terms of the budget equations of TKE and sensible heat flux in a moderately 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 relatively low canopy cover of 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 CSI 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. The proposed presentation will focus on the relative magnitude of the measured TKE and the sensible heat flux budget terms and discuss those in the context of higher-order closures that are used to model canopy flows.

References

Cava D. et al. (2006): ‘Buoyancy and the sensible heat flux budget within dense canopies'. Boundary-Layer Meteorol. 118, 217-240.

Dwyer M. J., Patton E. G., Shaw, R.H. (1997): ‘Turbulent kinetic energy budgets from a large-eddy simulation of airflow above and within a forest canopy'. Boundary.-Layer Meteorol. 84, 23-43.

Leclerc M. Y., et al. (1990): ‘The influence of atmospheric stability on the budgets of the Reynolds stress and turbulent kinetic energy within and above a deciduous forest'. J. Appl. Meteorol. 29, 916-933.

Meyers, T. P., Baldocchi D. D. (1991): ‘The budgets of turbulent kinetic energy and Reynolds stress within and above a deciduous forest'. Agr. Forest Meteorol. 53, 207-222.

Yang B. et al. (2006): ‘Large-eddy simulation of turbulent flow across a forest edge. Part II: Momentum and turbulent kinetic energy budgets'. Boundary-Layer Meteorol. 121, 433-457.