Nocturnal mixing in a forest subcanopy

The vertical structure of the flow in the old aspen canopy in BOREAS is exa mined 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 subca nopy in contrast to more closed canopies where strong stratification does n ot develop in the subcanopy. On clear nights with weak winds, a second weak er inversion develops at the top of the aspen canopy. On average, the subcanopy is very stable in the early evening and becomes l ess stable later in the evening. This appears to be due to a general increa se in wind speed above the canopy during the night. On some of the nights, the stability of the how in and above the canopy suddenly decreases in asso ciation with cold air advection. The characteristics of these events are ex amined. The vertical structure of the heat and momentum flux below and above the ca nopy 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 exa mined, (C) 2000 Elsevier Science B.V. All rights reserved.