Analysis of inadvertent microprocessor lag time on eddy covariance results

Researchers using the eddy covariance approach to measuring trace gas fluxe s are often hoping to measure carbon dioxide and energy fluxes for ecosyste m intercomparisons. This paper demonstrates a systematic microprocessor-cau sed lag of -0.1 to -0.2 s in a commercial sonic anemometer-analog-to-digita l datapacker system operated at 10 Hz. The result of the inadvertent negati ve lag (i.e., the digitized analog concentration signal is received before its corresponding instantaneous wind and temperature signal) is a loss in t he magnitude of the recorded measured flux. Based on raw field data specifi c to the system used in this study (2.6-m sample height; roughness length = 3 cm), errors in flux measurements due to a 0.2-s lag ranged from 10% to 3 1%. Theoretical flux errors, based on ideal near-neutral cospectra, for a 0 .2-s phase shift range from 21% to 55% for neutral-stability wind speeds of 0.5-15 m s(-1). The application of a 0.2-s phase correction improved an ea rly-summer, sage shrubland ecosystem energy balance by 29.5%. Correction eq uations for lag times of 0.1-0.4 s at the sample height of 2.6 m are provid ed.