Turbulent flux measurements above and below the overstory of a boreal aspen forest

Turbulent flux measurements both above and beneath the canopy of a boreal a spen forest are described. Velocity skewness showed that, beneath the aspen canopy, turbulence was dominated by intermittent, downward penetrating gus ts. Eulerian horizontal length scales calculated from integration of the au tocorrelation function or spectral peaks were 9.0 and 1.4 times the mean as pen height of 21.5 m respectively. Above-canopy power spectral slopes for a ll velocity components followed the -2/3 power law, whereas beneath-canopy slopes were closer to -1 and showed a spectral short cut in the horizontal and vertical components. Cospectral patterns were similar both above and be neath the canopy. The Monin-Obukhov similarity function for the vertical wi nd velocity variance was a well-defined function of atmospheric stability, both above and beneath the canopy. Nocturnal flux underestimation and depar tures of this similarity function from that expected from Monin-Obukhov the ory were a function of friction velocity. Energy balance closure greater th an 80% was achieved at friction velocities greater than 0.30 and 0.10 m s(- 1), above and below the aspen canopy, respectively. Recalculating the laten t heat flux using various averaging periods revealed a minimum of 15 min we re required to capture 90% of the 30-min flux. Linear detrending reduced th e Bur at shorter averaging periods compared to block averaging. Lack of ene rgy balance closure and erratic flux behaviour led to the recalculation of the latent and sensible heat fluxes using the ratio of net radiation to the sum of the energy balance terms.