Assessing decoupling of above and below canopy air masses at a Norway spruce stand in complex terrain

Concurrent below (0.14 • canopy height) and above canopy sonic anemometer vertical velocity (w) measurements reveal frequent decoupling events between the air masses below and above the canopy at a dense spruce forest stand in mountainous terrain. Decoupling events occurred predominantly during nighttime but not exclusively. Several single-level approaches based on steady state and integral turbulence characteristic tests as well as u_* filtering and two-level CO₂ flux filtering methods are tested. These tests aimed at evaluating the filtering schemes to address decoupling and its effect on above canopy derived eddy covariance CO₂ fluxes. In addition to the already existing two-level filtering approach based on the correlation of σ_w above and below canopy, two new filtering methods are introduced based on w raw data below and above the canopy. One is a telegraphic approximation agreement, which assumes coupling when w both above and below canopy are pointing in the same direction. Another one evaluates the cross correlation maximum between below and above canopy w data. This study suggests that none of the single-level approaches can detect decoupling when compared to two-level filtering approaches. It further suggests that the newly introduced two-level approaches based on w raw data may have advantages in comparison to the conventional σ_w approach regarding their flexibility on shorter time scales than one year. We tested the correlation of the newly introduced filtering approaches with the parameters u_*, global radiation, buoyancy forcing across the canopy and wind shear across the canopy. In any case, this correlation was not existing or weakly positive, suggesting that concurrent below and above canopy measurements are mandatory for addressing decoupling sufficiently. Sonic anemometer measurements near the forest floor and above the canopy are sufficient to apply the new procedures and can be implemented in a routine manner at any forest site globally.