Abstract
Abstract. The interpretation of flux measurements in nocturnal conditions is typically fraught with challenges. This paper reports on how the presence of wave-like disturbances in a time series, can lead to an overestimation of turbulence statistics, errors when calculating the stability parameter, erroneous estimation of the friction velocity u* used to screen flux data, and errors in turbulent flux calculations. Using time series of the pressure signal from a microbarograph, wave-like disturbances at an AmeriFlux site are identified. The wave-like disturbances are removed during the calculation of turbulence statistics and turbulent fluxes. Our findings suggest that filtering eddy-covariance data in the presence of wave-like events prevents both an~overestimation of turbulence statistics and errors in turbulent flux calculations. Results show that large-amplitude wave-like events, events surpassing three standard deviations, occurred on 18% of the nights considered in the present study. Remarkably, on flux towers located in a very stably stratified boundary-layer regime, the presence of a gravity wave can enhance turbulence statistics more than 50%. In addition, the presence of the disturbance modulates the calculated turbulent fluxes of CO2 resulting in erroneous turbulent flux calculations of the order of 10% depending on averaging time and pressure perturbation threshold criteria. Furthermore, the friction velocity u* was affected by the presence of the wave, and in at least one case, a 10% increase caused u* to exceed the arbitrary 0.25 m s−1 threshold used in many studies. This results in an unintended bias in the data selected for analysis in the flux calculations. The impact of different averaging periods was also examined and found to be variable specific. These early case study results provide an insight into errors introduced when calculating "purely" turbulent fluxes. These results could contribute to improving modeling efforts by providing more accurate inputs of both turbulent kinetic energy, and isolating the turbulent component of u* for flux selection in the stable nocturnal boundary layer.
Highlights
While the eddy-covariance technique measures fluxes of momentum and scalars accurately in well-mixed convective boundary layer conditions, challenges in the measurement of net ecosystem exchange in the stable nocturnal boundary layer have been reported (Aubinet, 2010; Falge et al, 2001; Goulden et al, 1996; Karipot et al, 2008; Mahrt, 1999, 2010; Mathieu et al, 2005)
After calculating σp the data were assessed to determine a reasonable threshold for events that would be significantly large, a 3σp threshold was chosen to include the events that would have the most impact on turbulence statistics and flux calculations
The error decreases for the longer averaging periods ranging between 30 to 60 min. These results suggest that the wave frequency/period and its relation to the averaging period are important in determining the errors produced
Summary
While the eddy-covariance technique measures fluxes of momentum and scalars accurately in well-mixed convective boundary layer conditions, challenges in the measurement of net ecosystem exchange in the stable nocturnal boundary layer have been reported (Aubinet, 2010; Falge et al, 2001; Goulden et al, 1996; Karipot et al, 2008; Mahrt, 1999, 2010; Mathieu et al, 2005). Ducted waves are bound between the ground surface and some atmospheric reflecting layer above or two reflecting critical layers aloft (Cooper et al, 2006; Fritts et al, 2003; Newsom and Banta, 2003; Rees and Mobbs, 1988), producing a wave guide allowing propagation to occur over long distances and time periods. This implies a large-amplitude wave-like event could have wideranging impacts
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