Objective threshold determination for nighttime eddy flux filtering

Abstract

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. #

Ecosystem CO2/H2O fluxes are explained by hydraulically limited gas exchange during tree mortality from spruce bark beetles

“…Daytime was defined when PPFD > 10 μmol m À2 s À1 . Data corresponding to u * < 0.2 m s À1 (threshold determined using the method of Gu et al [2005]) were not included. To limit outliers, g c outside the range 0-300 mmol m À2 s À1 , PET outside the range À10 to 60 mmol m À2 s À1 , or gap-filled PPFD were not included.…”

Carbon budgets for an irrigated intensively grazed dairy pasture and an unirrigated winter-grazed pasture

“…The data set was then separated into day-and nighttime runs. The CO 2 surface fluxes at night were subjected to a moving-point-threshold (MPT) test, similar to that of Gu et al (2005). The purpose of this test was to determine whether undetectable advection below the measurement height in the stable nocturnal surface layer would lead to biased flux estimates, as has been found at a majority of EC sites (Gu et al, 2005;Barr et al, 2013).…”

“…The CO 2 surface fluxes at night were subjected to a moving-point-threshold (MPT) test, similar to that of Gu et al (2005). The purpose of this test was to determine whether undetectable advection below the measurement height in the stable nocturnal surface layer would lead to biased flux estimates, as has been found at a majority of EC sites (Gu et al, 2005;Barr et al, 2013). This procedure was performed twice for each site: once with u * as the sorting parameter, as is common practice, and once with the standard deviation of vertical wind (σ w ), as suggested by Acevedo et al (2009) cause σ w is a more direct indicator of turbulence intensity and less likely to be contaminated by low-frequency motion in the boundary layer.…”

Partitioning of evapotranspiration and its relation to carbon dioxide exchange in a Chihuahuan Desert shrubland

“…Similar conditions should present problems for Bowen ratio measurements as well, especially owing to the two different sampling heights. Following a methodology similar to Gu et al (2005), we computed the average night-time CO 2 flux over increasing values of wind speed thresholds using several ten-day periods when soil moisture and temperature were fairly uniform and night-time respiration was expected to be constant. Wind speed was used as a surrogate for turbulent intensity since the commonly used friction velocity (u Ł ) was not quantified by the Bowen ratio system.…”