The influence of topography on single-tower-based carbon flux measurements under unstable conditions: a modeling perspective

Abstract

Based on high-resolution numerical simulations, the influence of topography on the one-dimensional (1-D) tower-based measurements of the net ecosystem–atmosphere exchange rate (NEE) of CO2 was analyzed under unstable conditions. Airflow and transport of a passive scalar were simulated over undulating surfaces covered by tall trees. Compared to their flat surface counterparts, the wind and scalar mixing ratio fields are more disturbed over a steeper surface and/or under a weaker background wind condition, resulting in larger errors in the 1-D NEE estimation. The magnitude of the error is generally larger on the windward side than on the lee side when topography-induced circulation (TIC) does not occur. Applying the ensemble streamline coordinate system to estimating NEE could result in larger errors than applying the local earth coordinate system or terrain coordinate system when the atmospheric flow is significantly distorted. This is especially true when TIC occurs. NEE estimated from the 1-D framework under convective conditions with calm or weak background winds may incur significant errors even over gentle topography. Preference in the selection of a flux tower location is given to the crest area.