Temporal and spatial variability of mean flow and turbulence characteristics over a deciduous forest

A critical examination of turbulence characteristics over deciduous forest is necessary to evaluate the validity of long-term eddy-covariance measurements of ecosystem-atmosphere exchange of carbon, water and energy. It is also important to assess the quality of turbulence statistics and their proper use in diffusion models to quantify the source area and footprints of such measured fluxes.

In this study, integral turbulent statistics obtained in the roughness sublayer over a Midwestern deciduous forest (canopy height ~ 22 m) are examined for deviations from the Monin-Obukhov similarity. Correlation- and structure functions are used to estimate integral turbulent time and length scales, and to identify periods of weak and non-stationary turbulence when eddy-covariance fluxes are unlikely to represent net ecosystem sinks or sources. In addition to the typical diurnal course, we examine the effects of seasonal changes of forest morphology (LAI). The influences of local landscape (small topography, lakes, forest heterogeneity) are inferred from the variations in turbulence characteristics from different upwind directions. The effects of commonly applied coordinate rotations on covariance and correlations of "vertical" velocity and scalars are also discussed.