Turbulence Statistics Inside and Over Forest: Influence on Footprint Prediction |
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| Authors: | Ü Rannik T Markkanen J Raittila P Hari T Vesala |
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| Institution: | (1) Departments of Physical Sciences, University of Helsinki, Helsinki, Finland;(2) Departments of Forest Ecology, University of Helsinki, Helsinki, Finland;(3) Physical Sciences, University of Helsinki, Helsinki, Finland |
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| Abstract: | Observations of wind statistics within and above a Scots pine forest are comparedwith those predicted from an analytical second-order closure model. The roughnesssublayer (RSL) effects, and the influence of stability on similarity functions, arestudied using observations. The commonly accepted forms of similarity functionsdescribe the influence of diabatic effects above the RSL well. According to earlierstudies they are expected also to apply within the RSL. As an exception, the averagewind speed normalised with friction velocity was found to be invariant with stabilityclose to the canopy top under unstable conditions. Lagrangian stochastic trajectorysimulations were used to evaluate the influence of canopy turbulence profiles onfootprint prediction. The main uncertainty was found to arise from parameterisationof the random forcing term in the Lagrangian velocity equation. The influence ofdiabatic conditions was studied, and it was found that thermal stability affectssignificantly the footprint function above the forest canopy, but significantuncertainty exists because of uncertainties in the formulation of stability functions. |
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| Keywords: | Flux footprints Forest canopy Lagrangian trajectory simulation Turbulence statistics |
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