Alignment of Surface-Atmosphere Exchange Sensors at Sloped Sites: An Integrated Strategy

Abstract:

Closure of the energy balance on the earth's surface is regarded as an indicator that the measurements of radiative, turbulent, diffusive and storage fluxes fulfill fundamental methodological assumptions. However, for sloped measurement sites the different terms contributing to the energy balance are not aligned along the axes of a single reference coordinate system. Consequently, a measurement and data processing strategy is needed that enables consistently quantifying each contributing term.

The National Ecological Observatory Network (NEON) is currently deploying surface-atmosphere exchange sensors at several dozen measurement sites, 50% of which are located on slopes up to 20 degrees. To enable unbiased observations across sites, the incident angle between a flux and its measurement should be minimized, and instrument limitations and spatial representativeness need to be considered.

Here, we present a strategy that combines site-adaptive instrument alignment with real-time attitude tracking and a set of trigonometric, radiative and source area conversions. This allows obeying the physical limitations underlying radiation, turbulence, profile and soil heat flux sensors, while providing observations in a consistent frame of reference. The strategy is evaluated against initial findings from the first months of surface-atmosphere exchange sensor deployments at NEON.