Abstract
The relaxed eddy accumulation (REA), method based on the conditional sampling concept, has received increasing attention over the past few years as it can be used to measure surface fluxes of a wide variety of trace gases for which fast response analysers are not available. In the REA method, a turbulent flux is simply expressed as the product of the standard deviation of vertical wind velocity, the difference between mean scalar concentration in the updrafts and downdrafts and an empirical coefficient, β (about 0.63 as based on simulations with a Gaussian distribution, and 0.58 as derived from experimental data). A simulation technique is developed here to evaluate the performance of a ground-based REA system. This analysis uses generated series whose internal structure can be controlled to a large extent. They are stationary and their characteristics are similar to those of physical turbulence. In a first step the influence of some statistical characteristics of vertical velocity and scalar concentration series is investigated. The effect of the third- and fourth-order moments can explain to some degree the difference between calculated and measured β values. The impact of a threshold on the vertical velocity is then considered, and its effect on β is quantified. The influence of the time lag between ω and the effective scalar sampling, and the consequences of lowpass filtering of the ω signal are also investigated. The simulation technique presented in this study can be used to develop elaborate algorithms for near real-time conditional sampling, based on the statistical characteristics of the previous sample.
Highlights
In the last decade, increasing concern about global climate change and the enhanced greenhouse effect has required accurate assessment of surface sources and sinks of chemical compounds (e.g., CO2, CH4, O3, N2O, NOx, volatile organic compounds, VOC) and determination of the energy balance over various surfaces
It should be pointed out that the sensitivity of b to higher-order cross-moments is not quantified here, these moments are certainly modified. Up to this point we have studied the sensitivity of the relaxed eddy accumulation (REA) method to statistical characteristics of w and scalar functions
It introduces a delay in w signal detection and an attenuation of the signal amplitude, which increases with a reduction in the cut-off frequency
Summary
In the last decade, increasing concern about global climate change and the enhanced greenhouse effect has required accurate assessment of surface sources and sinks of chemical compounds (e.g., CO2, CH4, O3, N2O, NOx, volatile organic compounds, VOC) and determination of the energy balance over various surfaces. Simulated turbulence series present two advantages: (i) they can be perfectly homogeneous and stationary, as compared with experimental functions and (ii) they allow one to perfectly control and systematically explore a large range of their statistical characteristics (i.e., skewness and kurtosis), presumed to have an impact on the REA measurement. The study has three main objectives: (i) to investigate the sensitivity of b to the correlation between vertical velocity and scalar function, (ii) to quantify the impact of statistical characteristics (third- and fourth-order moments) of w and scalars on a ground-based REA system and (iii) to test the sensitivity of the method to a threshold on w, lowpass filtering of the w signal and time lags between w and the scalar concentration. This analysis is based on a number of criteria defined so as to allow a quality control of the REA measurements
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