Nocturnal mixing in a forest subcanopy

The vertical structure of the flow in the old aspen canopy in BOREAS is examined in terms of thermocouple profiles and sonic anemometers above, within, and below the aspen canopy. The data are composited for different periods in order to isolate seasonal changes of the canopy and sun angle. On clear nights, a strong surface inversion develops in the lower part of the subcanopy in contrast to more closed canopies where strong stratification does not develop in the subcanopy. On clear nights with weak winds, a second weaker inversion develops at the top of the aspen canopy. On average, the subcanopy is very stable in the early evening and becomes less stable later in the evening. This appears to be due to a general increase in wind speed above the canopy during the night. On some of the nights, the stability of the flow in and above the canopy suddenly decreases in association with cold air advection. The characteristics of these events are examined. The vertical structure of the heat and momentum flux below and above the canopy are examined. The drag coefficient for the subcanopy stress exhibits a maximum at neutral stability and systematically decreases with increasing subcanopy stability and also decreases slowly with increasing instability. Possible explanations for this unexpected decrease with instability are examined.