Canopy Flows: Problems and Perspective

Canopy flow occurring within and immediately above vegetation canopies plays a substantial role in regulating atmosphere-biosphere interaction. The canopy layer is an interface between land and atmosphere, in which most natural resources humans need are produced by biochemical reactions. Canopy flow influences those biochemical processes through the control of gas exchange between the vegetation and the atmosphere, heat exchanges, and momentum exchanges. Better understanding of canopy flow behavior has many practical implications in accurately determining, for instance, terrestrial carbon sinks and sources, the fate of ozone within and above forested environments, forest fire spread rate, bark beetle management, and others. However, the applicability of classic turbulence theories (K-theory, mixing-length theory, velocity-squared law) in canopy layer is extremely limited. Here, I review and discuss the problems, successful, and perspective in application of classic turbulence theories in canopy flows. The key criterion of evaluating if a model or theory is successful by its capability to explain the observational data and permit predictions. The typical patterns of forest canopy turbulent flows are characterized by an S-shaped wind profile with an exponential Reynolds stress profile rather than the widely-used logarithmic wind profile and constant Reynolds stress observed over bare ground. The features of S-shaped wind profiles imply that K-theory and mixing-length theory break down within a forest canopy layer. This is because a mixing-length must satisfy von Karman's rule (von Karman, 1930), which indicates that a mixing length is a function of velocity distribution. Particularly, the assumption of a constant mixing-length within a canopy is not consistent with the original mixing-length theory. The mixing-length concept may be limited to use in the shear-layer from the upper-part of canopy. In comparison with mixing-length theory and K-theory, the velocity-squared law is able to use to close momentum equation to successfully predict and explain observational data for Reynolds stress profiles in forest canopies that have been observed all over the world. However, the ability of the velocity-squared law to explain the characteristics of S-shaped wind profile is limited to semi-empirical.

The features of S-shaped wind profiles also dictate the existence of super-stable layers near levels where wind speed is maximum (or minimum) and temperature inversion (temperature increasing with height) exists, leading the Richardson number to be extremely large or infinity. A super-stable layer acts as a ‘lid' or ‘barrier' that separates fluid into two uncorrelated layers: (1) the lower layer between the ground and the super-stable layer, and (2) the upper layer above the super-stable layer. This canopy flow separation was verified by SF6 diffusion observations and carbon isotope experiments. The lower layer is sometimes called a ‘decoupled layer' that is shallow, usually within the trunk space of a forest. Because the super-stable layer prohibits vertical exchanges, the decoupled layer channels air in the horizontal direction. The characteristics of the channeled air are highly dependent on soil conditions, containing a high concentration of soil respired CO2 and soil evaporated water vapor, and consisting of colder air cooled by radiative cooling at the ground surface. The channeled air is sometimes termed ‘drainage flow', which limits the accuracy of tower-based estimates of ecosystem-atmosphere exchanges of carbon, water, and energy. Sensors on the tower above the canopy cannot measure the fluxes conducted by drainage flow because the layer above the canopy is decoupled from the drainage flow by the isolating super-stable layer.

The concept of a super-stable layer is useful in interpreting data associated with stratified canopy air. However, stratified canopy flows over complex terrain are far too complex to be able to characterize considering only a super-stable layer. Canopy structure (quantified by leaf area density profile), terrain slope, and thermal stratification are three key parameters in understanding the details of stratified canopy flows over complex terrain. The thermal stratification plays a leading role in the development of pure sub-canopy drainage flows: strong thermal stratification favors drainage flow development on gentle slopes, while weak or near-neutral stratification favors drainage flow development on steep slopes. We speculate that interaction between thermal stratification and terrain slopes and vegetation canopy may result in multiple super-stable layers and complicated flow patterns, causing difficulties in understanding the mechanisms and rates of exchange of mass and energy between the terrestrial biosphere and the atmosphere.

Acknowledgement: This research was supported by PSC-CUNY ENHC- 68849-00 46.