Abstract. Modern eddy covariance (EC) systems collect high-frequency data (10–20 Hz)
via digital outputs of instruments.
This is an important evolution with respect to the traditional and widely
used mixed analog/digital systems, as fully digital systems help overcome
the traditional limitations of transmission reliability, data quality, and
completeness of the datasets. However, fully digital acquisition introduces a new problem for guaranteeing
data synchronicity when the clocks of the involved devices themselves cannot
be synchronized, which is often the case with instruments providing data via
serial or Ethernet connectivity in a streaming mode. In this paper, we
suggest that, when assembling EC systems “in-house”, aspects related to
timing issues need to be carefully considered to avoid significant flux
biases. By means of a simulation study, we found that, in most cases, random timing
errors can safely be neglected, as they do not impact fluxes significantly.
At the same time, systematic timing errors potentially arising in
asynchronous systems can effectively act as filters leading to significant
flux underestimations, as large as 10 %, by means of attenuation of
high-frequency flux contributions. We characterized the transfer function of
such “filters” as a function of the error magnitude and found cutoff
frequencies as low as 1 Hz, implying that synchronization errors can
dominate high-frequency attenuations in open- and enclosed-path EC systems.
In most cases, such timing errors neither be detected nor characterized
a posteriori. Therefore, it is important to test the ability of traditional
and prospective EC data logging systems to assure the required synchronicity
and propose a procedure to implement such a test relying on readily
available equipment.
Eddy Covariance flux errors due to random and systematic timing errors during data acquisition
