Footprint-weighted tile approach for a spruce forest and a nearby patchy clearing using the ACASA model

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Gatzsche, Kathrin ; Babel, Wolfgang ; Falge, Eva ; Pyles, Rex David ; Paw U, Kyaw Tha ; Raabe, Armin ; Foken, Thomas:
Footprint-weighted tile approach for a spruce forest and a nearby patchy clearing using the ACASA model.
In: Biogeosciences. Bd. 15 (2018) Heft 9 . - S. 2945-2960.
ISSN 1726-4189
DOI: https://doi.org/10.5194/bg-15-2945-2018

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Abstract

The ACASA (Advanced Canopy–Atmosphere– Soil Algorithm) model, with a higher-order closure for tall vegetation, has already been successfully tested and vali- dated for homogeneous spruce forests. The aim of this paper is to test the model using a footprint-weighted tile approach for a clearing with a heterogeneous structure of the under- lying surface. The comparison with flux data shows a good agreement with a footprint-aggregated tile approach of the model. However, the results of a comparison with a tile ap- proach on the basis of the mean land use classification of the clearing is not significantly different. It is assumed that the footprint model is not accurate enough to separate small- scale heterogeneities. All measured fluxes are corrected by forcing the energy balance closure of the test data either by maintaining the measured Bowen ratio or by the attribution of the residual depending on the fractions of sensible and latent heat flux to the buoyancy flux. The comparison with the model, in which the energy balance is closed, shows that the buoyancy correction for Bowen ratios > 1.5 better fits the measured data. For lower Bowen ratios, the correction probably lies between the two methods, but the amount of available data was too small to make a conclusion. With an assumption of similarity between water and carbon dioxide fluxes, no correction of the net ecosystem exchange is neces- sary for Bowen ratios > 1.5.