Turbulence dependence upon winds and stability in a weak-wind canopy sub-layer over complex terrain

The daytime and nighttime turbulence structure within a weak-wind forest canopy was investigated using data collected within a mixed northern forest in northern Idaho, USA. Data were split between the daytime and nighttime and stable and unstable periods to compare the effects of atmospheric stability conditions upon turbulence intensity and behavior within the forest canopy. After determining the general flow field distribution and vertical statistical profile, we investigated the existence of a wind speed threshold signifying a switch between wind-speed independent and wind-speed dependent turbulence for each combination. No clear wind speed threshold value was seen overnight but a height and wind direction dependency upon the levels of turbulence was observed due to intermittent periods of elevated nocturnal turbulence. These were coincident with a combination of a positive horizontal temperature gradient, vertical thermal instability and wind direction as influenced by the local terrain. A short case study showing the effects of the elevated overnight turbulence compared to a quiescent time will be presented in support of the more general conclusions for nocturnal turbulence within the forest sub-canopy.