Objective threshold determination for nighttime eddy flux filtering

We recommend an automated statistical method (Moving Point Test, or MPT) to determine the friction velocity (u*) thresholds in nighttime eddy flux filtering. Our intention is to make the determination of the u* thresholds objective and reproducible and to keep flux treatment consistent over time and across sites. In developing the MPT method, we recognize that both ecosystem respiration and u* exhibit diurnal and seasonal cycles and there are potential correlative changes between them, which must be removed before u* can be used as a filter criterion. MPT uses an iterative approach to simultaneously determine a valid temperature response function, which is used to normalize nighttime flux measurements, and identify u* thresholds based on the normalized fluxes. Tests show that MPT works well for a variety of scenarios and vegetation types. We also recommend that in order to increase the reliability of nighttime flux filters, a detailed measurement of mean CO ₂ concentration profiles need to be employed to calculate canopy storage changes accurately. Preferably, multiple profiles at different locations within the nighttime flux footprint should be used so that volume-averaged storage changes can be made. In addition, efforts should be made to minimize measurement gaps in summer nights as much as possible because of the short-time duration and frequent calm conditions, which greatly limit the amount of reliable data. We emphasize that the MPT method is not meant to be a final solution to the nighttime flux issue. Continuous theoretical and experimental researches are still needed to overcome the challenges in measuring nighttime fluxes accurately.