Stable stratification of the nocturnal lower boundary layer inhibits convective turbulence, such that tur-bulent vertical transfer of ecosystem carbon dioxide (CO2), water vapor (H2O) and energy is driven bymechanically forced turbulence, either from frictional forces near the ground or top of a plant canopy,or from shear generated aloft. The significance of this last source of turbulence on canopy flow charac-teristics in a closed and open forest canopy is addressed in this paper. We present micrometeorologicalobservations of the lower boundary layer and canopy air space collected on nearly 200 nights using acombination of atmospheric laser detection and ranging (lidar), eddy covariance (EC), and tower profilinginstrumentation. Two AmeriFlux/Fluxnet sites in mountain-valley terrain in the Western U.S. are inves-tigated: Wind River, a tall, dense conifer canopy, and Tonzi Ranch, a short, open oak canopy. On roughly40% of nights lidar detected down-valley or downslope flows above the canopy at both sites. Nights withintermittent strong bursts of “top-down” forced turbulence were also observed above both canopies. Thestrongest of these bursts increased sub-canopy turbulence and reduced canopy virtual potential tem-perature (v) gradient at Tonzi, but did not appear to change the flow characteristics within the denseWind River canopy. At Tonzi we observed other times when high turbulence (via friction velocity, u∗)was found just above the trees, yet CO2and vgradients remained large and suggested flow decoupling.These events were triggered by regional downslope flow. Lastly, a set of turbulence parameters is evalu-ated for estimating canopy turbulence mixing strength. The relationship between turbulence parametersand canopy vgradients was found to be complex, although better agreement between the canopy vgradient and turbulence was found for parameters based on the standard deviation of vertical velocity,or ratios of 3-D turbulence to mean flow, than for u∗. These findings add evidence that the relationshipbetween canopy turbulence, static stability, and canopy mixing is far from straightforward even withinan open canopy.
