Footprint estimation for flux measurements over tall forest canopies
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Analytical footprint models are originally developed for analyzing flux measurements over smooth terrain and their application over tall forest canopies might result in erroneous estimations. A commonly used analytical footprint model is modified to incorporate enhanced turbulence in the roughness sub layer (RSL). The wind speed in the crown space of the canopy is modeled by the exponential wind equation and the integrated stability function evaluated at is incorporated in the logarithmic wind profile. The influence of the RSL is reflected as an increase in flux maximum for all stability conditions. The modified model is applied to estimate footprint climatologies at the Howland Ameriflux site using the wind and CO0 z 2 flux data for a 5-year period. The footprint envelope of the 29 m high eddy-covariance flux tower constitutes 40% of the total 1 km 2 study area in stable conditions and 20% in unstable conditions.