Forest wind regimes and their implications on cross-canopy coupling

An analysis of wind regimes across forests of varying density and height is presented. The hockey stick-like dependence of the friction velocity on the mean advective wind speed is used to determine the threshold between the weak-wind and the strong-wind regime for the entire canopy. Such thresholds are compared across four different sites. The height dependence of thresholds within the canopy is inverted compared to the one above grassland sites. This can be understood by extending the accepted interpretation for weak-wind thresholds from grassland sites to forested sites. Even for large fluctuations above the canopy, the fluctuations of the vertical wind velocity in the subcanopy remain small during weak-wind situations. Correspondingly, in the strong-wind regime, turbulence in the subcanopy remains strong in spite of reduced above-canopy turbulence. This fact suggests that previously used methods for determining the degree of coupling across the canopy layer based upon the ratio of the vertical wind variance between the above-canopy and subcanopy may prove erroneous. Furthermore, it emphasizes the significance of mechanisms generating subcanopy turbulence other than above-canopy turbulent transport and shear. The transport of mass and energy between the subcanopy and above-canopy layers is significantly reduced during the weak-wind regime. In particular, the vertical turbulent transport is reduced by more than one order of magnitude. This suggests a decoupling of the subcanopy layer during weak-wind situations and allows for the accumulation of carbon dioxide originating from soil respiration in the subcanopy layer during the weak-wind regime.