Spatial averaging effect on local flux measurement using a displaced-beam small aperture scintillometer above the forest canopy

A scintillometer is a turbulence-measuring instrument that comprises a transmitter and a receiver. It obtains a spatially averaged flux because of its long measurement path. We used a displaced-beam small aperture scintillometer (DBSAS) 10 m above an 18 m-high deciduous forest canopy with an 86 m optical measurement path during June–November 2002 and 2003. We compared the dissipation rates of temperature fluctuations measured by DBSAS and eddy-covariance (EC) systems. Results showed that the difference in dissipation rates using the different sensors changed according to the normalized vertical wind deviation, which is linked to convective and mechanical mixing of the atmosphere, and the wind direction. The energy imbalance of the EC measurements, which is the validity criteria of the flux measurement, changed according to the difference of dissipation rates between the DBSAS and the EC. The contribution of the local vertical advection, which caused the energy imbalance, was small for the DBSAS even in conditions in which the mechanical mixing of the atmosphere was poor. The wind direction dependency of the difference of dissipation rates was inferred to result from the source area differences between the DBSAS and EC system. Considering its larger source area of flux measurement, the DBSAS applied to a mixed forest is concluded to measure a more spatially averaged turbulence signal than the EC in the flux measurement.