Turbulence statistics inside and over forest:: Influence on footprint prediction

Observations of wind statistics within and above a Scots pine forest are compared with those predicted from an analytical second-order closure model. The roughness sublayer (RSL) effects, and the influence of stability on similarity functions, are studied using observations. The commonly accepted forms of similarity functions describe the influence of diabatic effects above the RSL well. According to earlier studies they are expected also to apply within the RSL. As an exception, the average wind speed normalised with friction velocity was found to be invariant with stability close to the canopy top under unstable conditions. Lagrangian stochastic trajectory simulations were used to evaluate the influence of canopy turbulence profiles on footprint prediction. The main uncertainty was found to arise from parameterisation of the random forcing term in the Lagrangian velocity equation. The influence of diabatic conditions was studied, and it was found that thermal stability affects significantly the footprint function above the forest canopy, but significant uncertainty exists because of uncertainties in the formulation of stability functions.