Assessing potential effects of topography on eddy-covariance measurements
Submitter
Chamecki, Marcelo — University of California, Los Angeles
Area of research
General Circulation and Single Column Models/Parameterizations
Journal Reference
Science
Towers equipped with eddy-covariance sensors have become the standard method to derive energy, water vapor, and carbon fluxes across the biosphere-atmosphere interface. Many of the towers that are currently part of the FLUXNET global network are sited over forests on gentle complex terrain, and it is generally assumed that measurements can be interpreted as if taken over flat topography. This could lead to large biases in upscaling eddy-covariance fluxes to the ecosystem scale. In practice, using measurements from a single tower, it is hard to decide if topographic effects are important at any specific location.
Impact
The work offers a detailed analysis of the budget of turbulent kinetic energy (TKE) from large-eddy simulations of flow over gentle topography covered by forests. The focus is on identifying specific characteristics of the TKE production and dissipation mechanisms that can be estimated from observations taken on a single tower. We find that within the heights at which eddy-covariance sensors are usually deployed, the TKE budget has a distinct feature that can only be caused by the presence of topography and can thus be used in practice to assess topographic effects on the flow from tower observations. While a methodology to account for topographic effects when upscaling flux measurements is still not available, our results can be used to flag sites that may be impacted by topography and consequently less representative of their surrounding ecosystem.
Summary
We propose a new index that can be calculated from single-tower measurement to characterize the degree of topographic effects on the flow at the tower location. This can be useful to flag FLUXNET towers that require more careful interpretation when interpreting energy, water vapor, and carbon fluxes at the ecosystem scale.
Figure 1. Sonic anemometer deployed above the Amazon forest during the GoAmazon field campaign in 2014-15. Image from authors.