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.