UU_* filtering of nighttime net ecosystem CO_2 exchange flux over forest canopy under strong wind in wintertime

摘要

The mechanism of the negative nighttime net CO_2 flux in wintertime and reasonable treatment with it is of great importance in evaluating the carbon metabolism of boreal forest. Results, based on the data obtained with open-path eddy covariance system and CO_2 profile measurement system from Nov. 2002 to Apr. 2003 and that obtained with five sonic anemometers in Nov. 1999, show that (1) the negative net ecosystem CO_2 exchange flux (NEE) always appeared under conditions of strong wind; (2) the pressure fluctuation and horizontal advection flow are dominantly responsible for the negative NEE. Operable upper bound u_* filtering method (UU_* filtering) was introduced since the difficulties in real-time measuring of pressure fluctuation and horizontal advection fluxes under conditions of strong wind. Nighttime upper u_* threshold for the broad-leaved Korean pine mixed forest of the Changbai Mountains is 0.4 ms~(-1) and can be applied to the daytime filtering; and (3) the UU. filtering corrected the nighttime 'problem' of negative NEE under strong wind and made the estimates more ecologically reasonable.