TURBULENT EDDY MOTION AT THE FOREST-ATMOSPHERE INTERFACE

Ejection and sweep eddy motions in the atmospheric surface layer (ASL) are widely accepted as being responsible for much of land surface eva poration, sensible heat flux, and momentum flux; however, less is know n about this type of eddy motion within the canopy sublayer (CSL) of f orested systems. The present study analyzed the ejection-sweep propert ies at the canopy-atmosphere interface of a 13 m tall, uniformly aged southern loblolly pine stand and a 33 m tall, unevenly aged hardwood s tand using velocity and scalar (temperature, water vapor, and carbon d ioxide) fluctuation measurements at the canopy-atmosphere interface. I t was found that the measured sweeps and ejections time fractions for scalars and momentum are comparable and are in good agreement with oth er laboratory and field experiments. This investigation demonstrates t hat the third-order cumulant expansion method (GEM) reproduces the mea sured relative flux contribution of ejections and sweeps (Delta S-0) a nd the difference between sweep and ejection time fractions for both m omentum and scalars at the canopy-atmosphere interface in contrast to findings from a previous ASL experiment. A linkage between Delta S-0 a nd the scalar flux budget is derived and tested via the third-order CE M at the canopy-atmosphere interface for the pine and the hardwood sta nds. It is shown that Delta S-0 can be related to the dimensionless sc alar flux transport term whose gradient is central to the scalar varia nce budget. Also, the derived relationship is independent of canopy ro ughness or scalar sources and sinks. Hence this investigation establis hes an analytical linkage between second-order closure models, the eje ction-sweep cycle,and third-order CEM at the canopy-atmosphere interfa ce. Dissimilarity between the ejection-sweep cycle for scalar and mome ntum transport is considered via conditional probability distributions at both forest stands. In contrast to a laboratory heat dispersion ex periment, it is shown that while the ejection-sweep cycles for scalar and momentum transport are intimately linked, they are not identical. Therefore the results from momentum ejection-sweeps investigations can not be extrapolated to scalar transport. Comparisons with other labora tory experiments are also discussed, especially in relation to the sca lar ejection and sweep time fractions.